substrate nutrition

video-Visualizing Soil Properties: Water Infiltration

OSU Extension – Containers and Media for the Nursery

Growing Media for Container Production in a Greenhouse or Nursery

Media Chemical Properties

Table 1. Typical ranges based on a saturated media extract (SME) analysis.
ParameterTypical Range
EC. .500-3,000 umho/cm
NO3–N. .40- 00 ppm
P. .5-50 ppm
K. .50- 00 ppm
Ca. .30-1 0 ppm
Mg . . . . . . . . . . . . . . .0-75 ppm
Na. .4-80 ppm

Media pH is a critical issue because it plays a major role in determining the availability of many nutrients A common problem occurs in organic-based mixes when the pH falls below 5 0 Below this pH, the availability of Zn and Mn increases dramatically and often results in foliar toxicities from these elements While aluminum toxicity is recognized as a common concern in mineral soils at a low pH, this is usually not a problem in organic-based mixes

Cation Exchange Capacity (CEC)

CEC can vary widely depending on the type of component For example, perlite (1 5 meq/100 gm) and sand have very low CEC values relative to peat and vermiculite (125 meq/100 gm) components Some growers have started to use small volumes (2 to 15 percent) of clay-type amendments (e g , zeolites) in their soilless media These clay amendments may increase a medium’s nutrient retention and improve its physical properties It is important to understand which components have a higher CEC to help develop fertility programs and troubleshoot certain nutrient disorders

Soluble Salts

Similar to media pH, the level of soluble salts that may be tolerated is crop specific The extent of injury will be determined by the plant type, stage in production, longevity of exposure, concentration of salt and irriga- tion practices In general terms, fresh media without fertilizers should have a pour-through EC of less than 750 umho/cm The addition of fertilizer may mean this value is much higher and normal for a particular crop at a specific stage of production

Adjusting Media pH

Raising Media pH

In most cases, nursery and greenhouse growers need to be concerned about raising the media pH since most of the organic media components are acidic The most commonly used material is either calcitic (CaCO3) or dolomitic limestone (mixture of CaCO3 and MgCO3) The amount of lime required will depend on the starting pH, the desired pH, the particle size of the limestone (i e , small particles faster acting than large ones), the type of media and the alkalinity of irrigation water used In general, lime rates generally fall in a range between 5 and

15 lbs/yd3 with rates below 8 pounds most common Calcitic or dolomitic limestone is most reactive when incorporated into the media prior to planting Note that many of the pelletized granular limestone materials are actually fine powders that have been glued together with a binder When these granules or prills are exposed to water, they fall apart into a fine powder These fine powders are faster acting than coarser prills but may also wash out of the bottom

of the pot if the media is coarse textured It is absolutely critical that growers know the starting pH of their mix and then monitor the pH over time to see how their fertilizer and irrigation water influence media pH

Other liming materials include calcium oxide (CaO; quick or burned lime, which is very reactive, caustic and more expensive), hydrated lime [Ca(OH)2; also fast acting, caustic and more expensive], marl, egg or oyster shells and wood ash

Greenhouse growers may wish to try one of the following if the pH needs to be raised once the plants are in production The first option is to apply a flow- able limestone drench Start with a 1 quart per

100 gallons rate Avoid getting this mixture on the foliage if possible The second option involves injecting potassium bicarbonate into the irrigation water Continued use of this method may require a grower to switch to a lower potassium source fertilizer

If you are using liquid fertilization (fertigation), you can increase your media pH by switching from an acid-based fertilizer (high percentage of nitrogen in the ammoniacal form) to basic fertilizers that are based on a higher percentage of the nitrogen in the nitrate form

Lowering Media pH

Only rarely do growers using organic mixes express interest in lowering the media pH Generally, the problem develops from using an irrigation water source that has high alkalinity (>100 ppm CaCO3) In those cases, growers typically choose to install an acid injector Others methods are selected if individual blocks of plants require a lower media pH

Materials such as elemental sulfur, ammonium sulfate and ferrous sulfate have all been used Caution must be used when considering using ammo- nium sulfate and ferrous sulfate as you need to account for the nitrogen and iron that accompanies these materials Aluminum sulfate is also an option but is used only for reducing pH around floristhydrangeas (Hydrangea macrophylla) Growers can also affect media pH by selecting specific forms of nitrogen The use of high ammoniacal-nitrogen based fertilizers can lower the media pH over time

Rate recommendations take into consideration the change in pH and type of media and may be obtained from grower manuals or your Cooperative Extension Service

Managing Substrate EC

Electrical conductivity (EC) is a good estimate for the total soluble salts in a media EC does not provide details on the type or amount of individual salts present High ECs can contribute to poor shoot and root growth The first objective is to determine the source for the elevated salts Typically, this will be from the irrigation water source or from the amount or type of fertilizer used Once the source has been identified, you will want to determine if you can reduce or eliminate that source Media salt concentra- tions are directly impacted by what is called the leaching fraction This value represents the percentage of water that leaves a container relative to what is applied High salt conditions can be effec- tively managed by keeping the leaching fraction high (20 to 30 percent) and not allowing pots to dry out The danger in keeping pots wet is that it can contribute to secondary problems with root rot organ- isms (Consult FSA 0 1, Irrigation Water for Greenhouses and Nurseries, for more information on irrigation water quality )

Disinfecting Media

Three methods are primarily considered for sterilization of media Sterilized media is common in plant propagation and greenhouse operations but is not usually considered in an outdoor nursery simply based on the volume of media required and the bene- fits derived Remember that certain amendments

(e g , perlite, vermiculite) are sterile and, therefore, do not require sterilization Composted pine bark and peat contain populations of suppressive microbes that might be eliminated by sterilization techniques

Steam Pasteurization

Steam pasteurization is commonly found in greenhouse or ground bed production The general recommendation is the exposure to steam (212°F) for 30 to 45 minutes The piles should be small enough so all sections reach at least 180°F Piles too big may take too long to achieve uniform heating You must have an appropriate thermometer handy to effectively monitor the temperature at various places in the pile or bed Over-steaming is possible and should be avoided since this kills beneficial organisms and may cause the release of toxic substances, especially when organic components are involved

An alternative pasteurization process, aerated steam, involves blowing a mixture of steam and air through the media Aerated steam (140° to 175°F) uses less energy and fewer beneficial organisms are likely to be harmed  Steam pasteurization SHOULD NOT be used on media that has had slow-release fertilizer blended into it!

Chemical Fumigation

Chemical fumigation is usually limited to ground beds in cut flower production The primary chemicals used were methyl bromide and vapam Methyl bromide uses were phased out in 2005 Consult your Cooperative Extension Service for current recommendations


Solarization is rarely used because the process may take up to one month even under summer condi- tions requiring tremendous planning for future media needs Solarization is accomplished by spreading moist media to a depth of 6 to 10 inches on a clean surface The pile or row is then covered with clear plastic sheets with the edges sealed to the surface to prevent the loss of heat and moisture

Media Physical Properties

Media physical properties may be determined using simple laboratory methods Media samples can be analyzed by commercial laboratories, or you can make the physical measurements yourself using simple tools Procedures for determining physical properties of horticultural substrates are available at hortsublab/pdf/porometer_manual.pdf.

4 Ways to Calculate Porosity – wikiHow

Weight (Bulk Density)

Media weight is kind of a double-edged sword Ideally growers would like a heavy mix when containers are on the ground in an outdoor nursery to minimize blow-over, but during plant movement and shipping, a lightweight mix is desired Weight or bulk density is usually expressed as lbs/ft3 and reported on a dry basis For outdoor container nurseries, dry bulk density of media might range between 12 to 24 lbs/ft3 (wet bulk density of 70 and 90 lbs/ft3) A nursery media that uses a significant percentage of mineral soil will have a dry bulk density of 40 to 50 ft3 For a greenhouse media, the dry bulk density will be lower and in the range of 8 to 18 lbs/ft3 Air-Filled Porosity

When we fill a container with media, the total volume of space in that container is filled with two things: the solid media components and the spaces or voids between all of the solids (Figure 1) Ideally the total volume of empty pores should be in the range of 50 to 70 percent This is referred to as total porosity The remaining container volume would be filled with the solid growing media (i e , 30 to 50 percent) The total porosity of a media is further composed of two parts: air and water Both components are critical for good plant growth, but not enough of either can limit growth

substrates spaces

Figure 1. The container is filled with the solid media components and the spaces or voids between all of the solids.

For one quart and larger containers the air-filled porosity (percentage of pores filled with air) typically ranges from 10 to 20 percent by volume For a 280- plug tray, an acceptable range in air-filled porosity may fall in the range of 3 to 6 percent by volume Obviously, the container volume influences the inter- pretation of the acceptable values The higher the percentage of air-filled porosity, the more frequently watering will be required Propagation media where aeration and drainage are critical may have an air- filled porosity in the range of 15 to 25 percent

 lumetric Moisture Content (sometimes referred to as Water-Holding Capacity)

As described above, the bulk volume of a container will be filled with either solids or pores These pores or voids are then either filled with water or air A typical range in values for the volumetric moisture content (percentage of the pores filled with water after allowing for free drainage) will be between 45 and 65 percent by volume Volumetric moisture content (VMC) gives a grower some indica- tion of how wet or how dry a media will be Sphagnum peat moss that retains water quite well typically has a VMC of 60 percent, while coarse sand, which does not hold water, might have a VMC of

25 percent As was the case with air-filled porosity, the actual values for VMC need to be interpreted relative to the height of the growing container For example, an acceptable VMC for a 6-inch container might be 45 percent, while for a plug tray 68 percent would be a more typical value The effect of media height on the saturated zone is illustrated in

Substrate Containers contain the same media. Notice, saturated zone (textured area at bottom of each container) is the same regardless of the container height.

Figure . Containers contain the same media. Notice, saturated zone (textured area at bottom of each container) is the same regardless of the container height.

All of these physical parameters can be determined in-house with the aid of a scale or balance The measurements can also be determined by an outside commercial laboratory or with the help of the Cooperative Extension Service

EC by substrate

Graph-Electrical conductivity for various soil substrates

Containers and Media for the Nursery

Porosity: Measuring Pore Space

Pore space is expressed in terms of porosity, which is the percentage of pore space volume for a given substrate volume. You can determine the porosity, air space, and water space in your substrate by following the five steps below.

  1. Determine the volume of the pot used to grow plants. You can usually obtain this from the manufacturer.
  2. Add a water-tight liner (such as a polyethylene wrap) to the outer bottom of the pot. Tape the liner to the pot to prevent any material from draining through the holes.
  3. Pour the substrate to the top of the pot. Make sure to compact the substrate as you would normally do during production. At this point, the substrate volume is equal to the volume of pot.
  4. Add water to the top of the substrate, and carefully monitor how much you add. You want to add water until you completely saturate the substrate — that is, when you start to see a shiny layer of water over the substrate. Cover the pot with a lid and keep it aside for an hour. After one hour, check if you need to add a little more water to saturate the substrate. The total volume of water that you added to saturate the substrate

is equivalent to the total pore space volume.

Use this equation to determine porosity:

Porosity (%) = 100 x Volume of water added to fill Pot with substrate / Volume of Pot

Next, place the pot in a water-tight container and make holes at the bottom of the liner you added in step one. Your goal is to drain the water and collect it in the container. Let the pot drain for 10 to 15 minutes so the water can completely drain out. Then, measure the drained volume of water. This volume is equivalent to the air space volume in the substrate.

Use this equation to determine air space:

Air space = 100 x Volume of water drained / Volume of Pot

The water that is left in the substrate after it drains is the water space (also called maximum water-

holding capacity or container capacity). Once you’ve determined air space, you can determine water space with this equation:

Water space (%) = 100 – Air space(%)

Table 1. The following table provides information about the fractional volume of pore space, air space, and water space in two soilless substrates:

Note AFP has different definitions- some have air + water + solid -100

  ComponentPeat + Bark + Perlite + Vermiculite (P-B-P-V)  Peat + Perlite (P-P)
Solids (%)40.938.6
Porosity (%)59.161.4
Water space (as % total pore space)  72  62
Air space (as % total pore space)  28  38
Note AFP has different definitions- some have air + water + solid -100

Soil Electrical Conductivity

Soil electrical conductivity (EC) measures the ability of soil water to carry electrical current. Electrical conductivity is an electrolytic process that takes place principally through water-filled pores. Cations (Ca2+, Mg2+, K+, Na+, and NH +) and anions (SO 2-, Cl-, NO -, and HCO -) from have higher EC than sandy outwash or alluvial deposits. Saline (ECe ≥ 4 dS/m) and sodic (sodium absorption ratio ≥ 13) soils are characterized by high EC. Scientific literature reported a relationship between EC values measured with commercial sensors and depths to claypan, bedrock, and fragipan. Microtopographic depressions in agricultural fields typically are wetter and accumulate  salts dissolved in soil water carry electrical charges and conduct the electrical current. Consequently, the concentration of ions determines the EC of soils. In agriculture, EC has been used principally as a measure of soil salinity (table 1); however, in non-saline soils, EC can be an estimate of other soil properties, such as soil moisture and soil depth. EC is expressed in deciSiemens per meter (dS/m).

Factors Affecting

Inherent – Factors influencing the electrical conductivity of soils include the amount and type of soluble salts in solution, porosity, soil texture (especially clay content and mineralogy), soil moisture, and soil temperature. High levels of precipitation can flush soluble salts out of the soil and reduce EC. Conversely, in arid soils (with low levels of precipitation), soluble salts are more likely to accumulate in soil profiles resulting in high EC. Electrical conductivity decreases sharply when the temperature of soil water is below the freezing point (EC decreases about 2.2% per degree centigrade due to increased viscosity of water and decreased mobility of ions). In general, EC increases as clay content increases. Soils with clay dominated by high cation-exchange capacity (CEC) clay minerals (e.g., smectite) have higher EC than those with clay dominated by low CEC clay minerals (e.g., kaolinite). Arid soils with high content of soluble salt and exchangeable sodium generally exhibit extremely high EC. In soils where the water table is high and saline, water will rise by capillarity and increase salt concentration and EC in the soil surface layers.

It is generally accepted that the higher the porosity (the higher the soil moisture content), the greater the ability of soil to conduct electrical currents; that is, other properties being similar, the wetter the soil the higher the EC. Soil parent materials contribute to EC variability. Granites have lower EC than marine shales and clayey lacustrine deposits organic matter and nutrients and therefore have higher EC than surrounding higher lying, better drained areas.

Dynamic – Mineral soils enriched in organic matter, or with chemical fertilizers (e.g., NH4OH) have higher CEC than non-enriched soils, because OM improves soil water holding capacity, and synthetic fertilizers augment salt content. Continuous application of municipal wastes on soil can increase soil EC in some cases. Electrical

conductivity has been used to infer the relative concentration, extent, and movement of animal wastes in soils. Because of its sensitivity to soluble salts, EC is an effective measure for assessing the contamination of surface and ground water. Although EC does not provide a direct measurement of specific ions or compounds, it has been correlated with concentrations of potassium, sodium, chloride, sulfate, ammonia, and nitrate in soils. Poor water infiltration can lead to poor drainage, waterlogging, and increased EC.

Relationship to Soil Function

Soil EC does not directly affect plant growth but has been used as an indirect indicator of the amount of nutrients available for plant uptake and salinity levels. EC has been used as a surrogate measure of salt concentration, organic

Table 1. Classes of salinity and EC (1 dS/m = 1 mmhos/cm; adapted from NRCS Soil Survey Handbook)

EC (dS/m)Salinity Class
0 < 2Non-saline
2 < 4Very slightly saline
4 < 8Slightly saline
8 < 16Moderately saline
≥ 16Strongly saline

matter, cation-exchange capacity, soil texture, soil thickness, nutrients (e.g., nitrate), water-holding capacity, and drainage conditions. In site-specific management and high-intensity soil surveys, EC is used to partition units of management, differentiate soil types, and predict soil fertility and crop yields. For example, farmers can use EC maps to apply different management strategies (e.g., N fertilizers) to sections of a field that have different types of soil. In some management units, high EC has been associated with high levels of nitrate and other selected soil nutrients (P, K, Ca, Mg, Mn, Zn, and Cu).

Most microorganisms are sensitive to salt (high EC). Actinomycetes and fungi are less sensitive than bacteria, except for halophyte (salt-tolerant) bacteria. Microbial processes, including respiration and nitrification, decline as EC increases (table 2).

Problems with Poor Soil EC Levels

High EC can serve as an indication of salinity (EC > 4 dS/m) problems, which impede crop growth (inability to absorb water even when present) and microbial activity (tables 2 and 3). Soils with high EC resulting from a high concentration of sodium generally have poor structure and drainage, and sodium becomes toxic to plants.

Improving Soil EC

Effective irrigation practices, which wash soluble salts out of soil and beyond the rooting depth, can decrease EC. Excessive irrigation and waterlogging should be avoided since a rising water table may bring soluble salts into the root zone. In arid climates, plant residue and mulch help soils to remain wetter and thus allow seasonal precipitation and irrigation to be more effective in leaching salts from the surface. To avoid the adverse effects of high EC (salinity) in irrigation water, the leaching requirement must be calculated for each crop. Leaching requirement is the fraction of water needed to flush excessive salt below the root zone, that is, the amount of additional water required to maintain a target salinity level. Adding organic matter,

such as manure and compost, increases EC by adding cations and anions and improving the water-holding capacity. In some cases, a combination of irrigation and drainage is necessary to lower salt concentration and EC. An EC water (ECw) ≤ 0.75 dS/m is considered good for irrigation water. Beyond this value, leaching or a combination of leaching and drainage will be necessary if the water is used.

Measuring Soil EC

The EC pocket meter is used to take measurements in the field. The method is described in the Soil Quality Test Kit Guide. Always calibrate the EC meter before use.

The pocket meter can be augmented by a probe that is placed directly into the soil to measure subsoil EC and NO – and make other estimates. NRCS soil scientists and agronomists use electromagnetic induction meters, not pocket EC meters, to map spatial variability of EC and associated soil properties at field scales. Special sensors are used for EC mapping for precision agriculture.

Table 2. Influence of soil EC on microbial process in soils amended with NaCl or nitrate (adapted from Smith and Doran, 1996)
  Microbial process  Salt addedEC Range (dS/m)Relative Decrease (%)Threshold EC (1:1)
RespirationNaCl0.7 – 2.817 – 470.7
DecompositionNaCl + alfalfa0.7 – 2.92 – 250.7
Nitrificationsoil + alfalfa0.7 – 2.910 – 370.7
DenitrificationNO3-N1 – 1.832 – 881
Table 3. Salt tolerance of crops and yield decrease beyond EC threshold (adapted from Smith and Doran, 1996)
  Crop speciesThreshold EC 1:1 (dS/m)*Percent yield decrease per unit EC beyond threshold EC
Alfalfa1.1 – 1.47.3
Barley4.5 – 5.75.0
Cotton4.3 – 5.55.2
Peanut1.4 – 1.829
Potato1.0 – 1.212
Rice1.7 – 2.112
Soybean2.8 – 3.620
Tomato1.4 – 1.89.9
Wheat3.9 – 5.07.1
* Electrical conductivity of a 1:1 soil/water mixture relative to that of a saturated paste extract
Soil Quality Indicators: Soil Electrical ConductivityUSDA
Soil Electrical ConductivityUSDA
Using Soil Electrical Conductivity (EC) to Delineate Field VariationOhio State University
Pour-through Technique of Measuring Electrical Conductivity of the SubstratePurdue University
Commercial Greenhouse Production: pH and Electrical Conductivity Measurements in Soilless SubstratesPurdue University


A Nursery Friendly Method for Measuring Air Filled Porosity of Container Substrates

Bilderback-NC State

  1. Porometer construction: Measuring air-filled porosity requires an apparatus called a porometer. Therefore, the first step is to construct porometers
  2. Pre-moistening Substrate to Be Tested. Pre-moistening 12–24 h before testing is critical for achieving uniform and consistent results.
  3. Packing Porometers with Substrate: After removing the plastic carton tops, individually weigh each porometer and record the weight. The weight of the plastic carton is subtracted from filled cartons as a “tare” weight to provide an accurate mass of substrate in each porometer. Next, overfill each porometer with potting substrate; tap each porometer firmly 3–5 times on a table or bench to eliminate air pockets and establish a bulk density
  4. Saturate Substrate in Porometers: After packing, porometers are set upright in a vessel large enough for all of the test porometers to stand erect and tall enough to add water to the top of the porometers
  5. Collecting and Measuring Drainage: Saturation of each porometer can be observed when water is seen at the surface of the substrate. Drainage from each porometer must be measured individually. This step may require practice. Fingers are used to prevent leaking from the drainage holes while the porometer is lifted from the saturation vessel and a pan is quickly placed under the drain holes. Porometers can be balanced on supports placed in the bottom of the drainage pan and allowed to fully drain. After draining has stopped, the drained volume is measured and recorded for each porometer
  6. Calculating air filled porosity: The drainage volume is divided by the total volume for each porometer to determine a percent air-filled porosity (Table 1). Air-filled porosity measurements are added and divided by the number of porometers to obtain an average AFP for each test substrate. Changes in air filled porosity during a growing season or over a production cycle can be measured by placing porometers packed with substrate in containers which are set in nursery growing beds. Decomposition shrinkage should be measured and marked from the top of the porometer. The volume of the porometer marked at the surface of the substrate would be used as the new total volume and calculations followed as described above. If the important steps for pre- moistening samples and for packing to match the weight of each replicate sample in porometers are followed, consistent results can be accomplished.

Monitoring electrical conductivity in soils and growing media

What is EC? EC is a measure of the salinity (total salt level) of an aqueous solution. Pure, distilled water is a perfect insulator and it’s only because of dissolved ions that it can conduct electricity at all (Figure 1A). An EC meter measures the electrical charge carried by the ions that are dissolved in a solution— the more concentrated the ions, the higher the reading.

In nurseries, dissolved ions come from two sources (Figure 1B). First, all irrigation water contains some salt ions as rain water trickles through the soil and rocks.

The amount of the “background” salinity is a function of the local geology and climate. Soils and parent material  have a major effect. Soils derived from marine sediments will contain high levels of sodium, chloride and sometimes boron. Water running through calcareous rocks or soils picks up calcium, magnesium and bicarbonate ions. Irrigation water from dry climates will have higher salinity than water from a humid climate. This only makes sense because, when water evaporates, the dissolved salts are left behind and the remaining solution would have a higher EC reading.

The second source of salinity in soils or growing media is from added fertilizers (Figure 1B). The release of salts varies considerably depending on how you are fertilizing. When fertigating, the soluble fertilizer that you inject into the irrigation water can be measured immediately. In fact, the best way to check the accuracy of your injector is to measure the EC of the applied fertigation solution. If you are incorporating controlled-release fertilizers into the soil or growing medium, however, then the salts are released according to fertilizer coating, water levels, and temperature. Most solid organic fertilizers release their nutrients very slowly and are less temperature or moisture dependent. Liquid organics release nutrients more rapidly but still much slower than soluble fertilizers.

EC Units. The physics and politics of this subject are complicated but think of it this way. We’re measuring electrical conductance which is the inverse of resistance. The unit of resistance is an ohm, and just to be cute, they call the unit of conductance a mho (pronounced “mow”), which is ohm spelled backwards. The most commonly used EC units in horticulture are micromhos per centimeter (µhos/cm), and the SI units of microsiemens per centimeter (µS/cm) which are equivalent. Because electron activity is strongly dependent on temperature, all EC measurement must be adjusted to a standard temperature of 77 °F (25 °C

Saturated Media Extract (SME). This technique is the laboratory standard that is used by commercial soil and water testing laboratories. If you are interested in absolute EC values, this is the only choice. The SME method uses saturation as the standard soil or media

The use of controlled-release fertilizers (CRF) has complicated the measurement of EC. Because the prills are very fragile, even collecting a sample or squeezing it can damage them. Broken prills will release all their fertilizer salts at once and artificially elevate the EC reading. Thus, some of the EC monitoring procedures should not be used when incorporating CRF

Pour-Through. This is a relatively new technique for measuring EC in containers, and works for all container types except for miniplugs where their short height stops the media solution from freely draining. It would also be impractical for very large containers which are difficult to move (Table 1). The pour-through process consists of 2 steps (Figure 4). First, medium in a container is progressively irrigated until saturated, and then left to stand for about 2 hours. Or, just do the procedure 2 hours after irrigation. Next, pour a volume of distilled water onto the media surface to produce about 100 ml of leachate. Of course, this depends on container volume and type of growing media. Make sure and apply the water slowly enough that it doesn’t run off and down the insides of the container. The idea is to have the applied water force out the solution surrounding the roots. The pour-through technique is ideal for growing media with controlled-release fertilizers because the prills are not squeezed or otherwise damaged (Table 1). Therefore, this method is ideal for outdoor growing compounds where controlled release fertilizers are the standard.

Figure 4 – The pour-through technique works for all containers except miniplugs and larges sizes, and is ideal when using controlled-release fertilizers

EC pour through technique

video-Pour Through Method for pH and EC

video-Hanna Lab – Set Up and Calibrate the Hanna Instruments pH, EC, TDS Combo Tester HI98129

video-Substrate Water Holding Capacity



Dry Bulk Density:

 ρb = Ms / Vt

Where, ρb = Dry Bulk Density,  Ms = Mass of Soil Vt = Volume

Wet Bulk Density:

 Pt = (Msolid+Mliquid)/Vtotal

Where, Pt = Wet Bulk Density Msolid = Mass of Solids Mliquid = Mass of Liquid Vtotal = Total Volume

Moisture Content :

w = (Mw / Ms) * 100

Where, w = Moisture Content Mw = Mass of Water in Soil Ms = Dry Mass of Soil

Soil void ratio (e) is the ratio of the volume of voids to the volume of solids:

e = (V_v) / (V_s)

Where V_v is the volume of the voids (empty or filled with fluid), and V_s is the volume of solids.

soil porosity (n)  is defined as the ratio of the volume of voids to the total volume of the soil. The posoity and the void ratio are inter-related as follows:

e = n /(1-n) ,  and n = e / (1+e)

The value of void ratio depends on the consistence and packing of the soil. It is directly affacted by compaction. Some typical values of void ratio for different soils are given below:

DescriptionUSCSVoid ratio [-]Reference
min maxSpecific value
Well graded gravel, sandy gravel, with little or no finesGW0.260.46 [1],
Poorly graded gravel, sandy gravel, with little or no finesGP0.260.46 [1],
Silty gravels, silty sandy gravelsGM0.180.28 [1],
Gravel(GW-GP)0.300.60 [2], 
Clayey gravels, clayey sandy gravelsGC0.210.37 [1], 
Glatial till, very mixed grained(GC)0.25[4 cited in 5]
Well graded sands, gravelly sands, with little or no finesSW0.290.74 [1], [2], 
Coarse sand(SW)0.350.75 [2], 
Fine sand(SW)0.400.85 [2], 
Poorly graded sands, gravelly sands, with little or no finesSP0.300.75 [1], [2], 
Silty sandsSM0.330.98 [1], [2], 
Clayey sandsSC0.170.59 [1], 
Inorganic silts, silty or clayey fine sands, with slight plasticityML0.261.28 [1], 
Uniform inorganic silt(ML)0.401.10 [3], 
Inorganic clays, silty clays, sandy clays of low plasticity CL0.410.69 [1], 
Organic silts and organic silty clays of low plasticityOL0.742.26 [1], [3], 
Silty or sandy clay (CL-OL)0.251.80 [3], 
Inorganic silts of high plasticity MH1.142.10 [1], 
Inorganic clays of high plasticity CH0.631.45 [1], 
Soft glacial clay1.20[4 cited in 5]
Stiff glacial clay0.60[4 cited in 5]
Organic clays of high plasticity OH1.063.34 [1], [3], 
Soft slightly organic clay(OH-OL)1.90[4] cited in [5]
Peat and other highly organic soilsPt [4 cited in 5]
soft very organic clay(Pt)3.00[4] cited in [5]


Factors Affecting Porosity of Soil:

Wide difference in the total pore space of various soils occurs depending upon the following several factors:

(i) Soil Structure:

A soil having granular and crumb structure contains more pore spaces than that of prismatic and platy soil structure. So well aggregated soil structure has greater pore space as compared to structure less or single grain soil.

(ii) Soil Texture:

In sandy soils the total pore space is small whereas in fine textured clay and clayey loam soils total pore space is high and there is a possibility of more granulation in clay soils.

(iii) Arrangement of Soil Particles:

When the sphere like particles is arrangement in columnar form (i.e. one after another on the surface forming column like shape) it gives the most open packing system resulting very low amount of pore spaces. When such particles are arranged in the pyramidal form it gives the most close packing system resulting high amount of pore spaces.

(iv) Organic Matter:

Soil containing high organic matter possesses high porosity because of well aggregate formation.

(v) Macro-Organisms:

Macro-organisms like earthworm, rodents, and insects etc. increase macro-pores in the soil.

(vi) Depth of Soil:

With the increase in depth of soil, the porosity will decrease because of compactness in the sub-soil.

(vii) Cropping:

Intensive crop cultivation tends to lower the porosity of soil as compared to fallow soils. The decrease in porosity may be due to reduction in organic matter content.

(viii) Puddling:

Due to puddling under sufficient soil moisture, the soil surface layer is made dense and compact. Eventually, the porosity of this surface soil is reduced by the infiltration of muddy surface materials.


Although it may seem counter-intuitive, the small pore spaces of clay add up to more total void space than the fewer number of large pore spaces in sand. Consequently, in light rain or slow snowmelt, clay may be able to hold more water than sand.

pore space

However, water drains from clay soil more slowly than from sandy soils. So in successive rain events, clay soils may remain saturated between storms and therefore produce more runoff in the later rain events.

infiltration, percolation
High permeability. Low permeability.

More References

CuttingsNutrient supply during cutting propagation.pdf
Hydroponic Nutrients
hydro-Hochmuth Hydroponic Tomato Fertilizer.pdf
Nutrients-recirculating solutions.pdf
planting substrates

soil reactions

Ion Exchange in Soil: Cation and Anion

Cation Exchange:

In a near neutral soil, calcium remains adsorbed on colloidal particle. Hydrogen ion (H+ ) generated as organic and mineral acids formed due to decomposition of organic matter. In colloid, hydrogen ion is adsorbed more strongly than is the calcium (Ca++). The reaction takes place rapidly and the interchange of calcium and hydrogen is Chemically equivalent.

The reaction is as follows:

calcium hydrogen reaction

This phenomenon of the exchange of cations between soil and salt solution is known as Cation exchange or Base exchange and the cations that take part in this reaction are called exchangeable cations. Cation exchange reactions are reversible.

Hence, if some form of limestone or other basic calcium compound is applied to an acid soil, the reverse of the replacement just given above occurs. The active calcium ions replace the hydrogen and other cations by mass action. As a result, the clay becomes higher in exchangeable calcium and lower in adsorbed hydrogen and aluminium.


Cation preferences

When the valence of the cations are equal (i.e. both +1 charge) the cation with the smallest hydrated radius is more strongly adsorbed. In the case of the monovalent cations of potassium and sodium, the potassium ion is more strongly adsorbed since it has a smaller hydrated radius and hence is more strongly adsorbed to the site of the negative charge. In comparison the sodium ion is so loosely held and so ready to hydrate that sodium rich soil will disperse.


This is similarly the case with the divalent cations of calcium and magnesium. Because the hydrated magnesium ion is larger than that of calcium, the magnesium ion is held more weakly and behaves in some instances in soil (i.e. when calcium is low) like sodium.

The charge of the cation and the size of the hydrated cation essentially govern the preferences of cation exchange equilibria. In summary, highly charged cations tend to be held more tightly than cations with less charge and secondly, cations with a small hydrated radius are bound more tightly and are less likely to be removed from the exchange complex. The combined influence of these two criteria can be summarized generally by the lysotrpoic series.

cation order preference soil

aluminium > calcium > magnesium > potassium, ammonium-NH4+ > sodium > hydrogen

It indicates, from left to right, the decreasing strength of adsorption of the various cations. As such, the less tightly held cations are located furthest from the surface of colloids and are most likely to be leached away or further down the profile most quickly. Conversely, the most strongly adsorbed cations will tend to move the slowest down through the profile.

The proportion and kinds of cations adsorbed on soil mineral particles and organic colloids is also a function of the concentration of cations in the soil solution. If the concentration of a cation in soil solution is high, there is an increased chance or tendency for that cation to be adsorbed.

This is the reason that dissolved gypsum (CaSO4) is added to ameliorate sodic soil. In this case, the addition of dissolved gypsum increases the concentration of calcium in the soil solution and this leads to an increase in calcium ions on the exchange complex at the expense of exchangeable sodium.

The major source of cations in soil solution are from mineral weathering (i.e. primary minerals), mineralization of organic matter and addition of soil ameliorants (i.e. lime, gypsum, etc).

Soil Properties: Exchangeable Cations


If a soil is treated with a liberal application of a fertilizer containing potassium chloride, following reaction may occur:

Some of the added potassium pushes its way into the colloidal complex and forces out equivalent quantities of calcium, hydrogen and other elements (e.g., M) which appear in the soil solution. The adsorption of the added potassium largely in an available condition. Hence, cation exchange is an important consideration for making already present nutrients in soils available to plants. Cation exchange also makes available the nutrients, applied in commercial fertilizers form.

Cation Exchange Capacity (C.E.C.):

The cation exchange capacity of a soil represents the capacity of the colloidal complex to exchange all its cations with the cations of the electrolyte solution (surrounding liquid). It also represents the total cation adsorbing capacity of a soil. Cation exchange in most soils increases with pH. At a very low pH value, C.E.C. is higher and at high pH, C.E.C. is relatively lower.

Factors affecting the Cation Exchange Capacity:

The following factors affect the cation exchange capacity:

(i) Soil texture:

Fine-textured (clay) soils tend to have higher cation exchange capacity (CEC) than sandy soils. Cation exchange capacity for clay soils usually exceeds 30 me/100 gm. while the value ranges from 0 to 5 for sandy soils.

(ii) Organic matter content:

Organic matter content of a soil affects the CEC. Higher organic matter content in a soil have higher CEC.

(iii) Amount and kind of clay:

Montmorillonite has higher CEC in comparison to illite or kaolinite clay.

(iv) pH:

The cation exchange capacity of most soils increases with pH. At very low pH value, the cation exchange capacity is also generally low. As the pH is raised, the negative charges on some 1 : 1 type silicate clay (Kaolinite), humus and Fe, Al oxides increases, thereby increasing the cation exchange capacity.


The cation exchange capacity (C.E.C.) is expressed in terms of equivalents or more specifically, as milliequivalents per 100 grams. The term equivalent is defined as one gram atomic weight of hydrogen (or the amount of any other ion) that will combine with or displace this amount of hydrogen. For monovalent ions such as Na+, K+, NH4+ and CI, the equivalent weight and atomic weight are the same since they can replace one H+ ion. Divalent cations such as Ca++ and Mg++ can take the place of two H+ ions.

The milliequivalent weight of a substance is one thousandth of its atomic weight. Since the equivalent weight of hydrogen is about 1 gm., the term milliequivalent (meq) may be defined as 1 milligram of hydrogen. It indicates that other ions also may be expressed in terms of milliequivalents.

Consider calcium, for example. Ca has an atomic weight of 40 compared to 1 for hydrogen. Each Ca++ ion has two charges and is thus, equivalent to two H+ ions. Therefore, the amount of calcium required to displace 1 mg of hydrogen is 40/2 = 20 mg (atomic wt. divided by 2 to obtain the equivalent wt.). This is the weight of 1 meq of calcium.

If 100 grams of a certain clay is capable of exchanging a total of 250 meq of calcium, the cation exchange capacity is 250/20 = 12.5 meq per 100 gm. The milliequivalent method of expression can be converted easily to practical field terms. For example, 1 meq of hydrogen can be replaced on the colloids by 1 meq of CaCO3(limestone). The molecular weight of CaCO3 is 100, it contains 2 equivalent weights (divalent).

Since the amount of CaCC3 needed is only 1 meq wt., 100/2 = 50 mg will be needed to replace 1 mg of hydrogen (or 1 meq). In view of fact that 1 meq of H+ per 100 grams can be expressed as 20 pounds of hydrogen per grams of soil.

Expressed in the metric system this figure is 1100 kilograms per hectare. In general, the more clay there is in a soil, the higher the C.E.C. Sandy soils have, on the average 0.5 m.e. of C.E.C. per 100 gm. of soil, while for clay soils, it usually exceeds 30 m.e./100 gm.

Percentage Base Saturation of Soils:

Hydrogen and aluminium tend to dominate acid soils, both contributing to the concentration of H+ ions in the soil solution. Adsorbed hydrogen contributes directly to the H+ ion concentration in the soil. A+++ions do so indirectly through hydrolysis.


Base Saturation – Calculating Cation Exchange Capacity, Base Saturation, and Calcium Saturation

Base saturation is calculated as the percentage of CEC occupied by base cations. Figure 2 shows two soils with the same CEC, but the soil on the right has more base cations (in blue). Therefore, it has a higher base saturation. Base saturation is closely related to pH; as base saturation increases, pH increases.

Base Saturation (%) = (Base cations/CEC) x 100

Similarly, we can calculate the base saturation for each individual base cation. Calcium base saturation is calculated as the percentage of CEC occupied by calcium cations. In Figure 2, the soil on the right has twice as many calcium cations (Ca2+), thus a higher calcium saturation.

Calcium Saturation (%) = (Calcium cations/CEC) x 100


Reactions are as follows:

calcium reaction saturation

Most of the other cations (Ca++, Mg++), called exchangeable bases, neutralize soil acidity. The proportion of the cation exchange capacity (C.E.C.) occupied by these bases is called the percentage base saturation. Thus, if the % base saturation is 80 in clay loam soil, 4/5th of the cation exchange capacity (20 meq) is satisfied by bases, the other by hydrogen and aluminium. Same as, 50% base saturation in clay soil having 20 meq C.E.C. x 1/2 (10 meq C.E.C.).of the C.E.C. is satisfied by bases likewise in sandy loam soil with a C.E.C. of only 10 meq, 80% base saturation satisfied, 4/5 of C.E.C.

A definite correlation exists between the percentage base saturation of a soil and its pH. As the base saturation is reduced as a result of loss of calcium in drainage, the pH is also lowered (more acidity)in a definite proportion.

Within the range pH 5 to 6, the ratio for humid temperate region mineral soils is roughly at 5% base saturation, change for every 0.10 change in pH. thus, if the percentage base saturation is 50% at pH 5.5, it should be 25% and 75% at pH 5.0 and 6.0.

Role of Cation Exchange:

Importance of exchangeable cations on plant nutrients is discussed below:

Cation exchange reactions are very important chemical reactions for the availability of plant nutrients in the soil. The capacity of soil to exchange cations is the best single index of soil fertility. Plant roots, when they come in contact with colloidal particles, absorb exchangeable cations directly by inter-exchange or contact exchange between the root hairs and colloidal complex.

(a) Nature and content of exchangeable bases:

The nature and content of exchangeable bases in a soil have an important bearing on its general properties. In all normal fertile soils the total exchangeable bases (Ca, Mg, K, Na) constitute about 80 to 90% of the cation adsorbing capacity. Exchangeable hydrogen is usually under 20%. In these soils, calcium forms the predominant exchangeable base, constituting 60 to 80% of the total exchangeable cation.

The predominance of exchangeable calcium give rise to Ca- clay which imparts a neutral reaction to the soil. The pH value varies from 6.5 to 7.5. When the proportion of exchangeable hydrogen (H) is high it gives rise to acid soil. In such soils, exchangeable calcium is correspondingly low, and in highly acid soils it is almost absent. In such cases the clay is saturated with hydrogen cations (H+) and forms H-clay. Acid soils are less fertile. It is called base unsaturated soil.

When exchangeable sodium form more than 10 to 15% of the total exchangeable cation it gives rise to alkaline soils. The pH value of such soils is usually greater than 8.0. When the proportion of exchangeable sodium exceeds these limits (or saturates the colloidal complex), the clay is turned into a Na-clay.

The soil is now highly alkaline and the pH value ranges from 9 to 12 Alkaline soils are also Jess fertile. Soils with a high calcium base saturation are in the most satisfactory physical and nutritional condition. A calcium dominated soil is granular in structure and ensure good drainage and aeration.

(b) Type of colloid:

Type of colloid affects the cation exchange. Montmorillonite colloid hold the calcium ion with greater tenacity than Kaolinite at a given base saturation. As a result, Kaolinite will liberate calcium much more readily than Montmorillonite.

(c) Associated ions:

Presence of exchangeable calcium in excessive quantities in a soil will limit the availability of potassium to plants. In same manner, high-exchangeable potassium may depress the availability of potassium to plants. In same manner, high- exchangeable potassium may depress the availability of magnesium.

(d) Adsorption of cations:

Colloidal clay (humus) hold in varying amount of plant nutrients (calcium, magnesium, potassium, nitrogen, phosphorus and most of the micronutrients) which are available to plant.

(e) Property of base exchange:

Base exchange (cation exchange) property checks leaching losses of available nutrients. On application of potassium sulphate fertilizer in the soil, potassium ions are held on the surface of colloids by cation exchange process. Subsequently, exchangeable potassium ions are directly available to plants.

Cation Exchange and Soil Fertility:

Cation exchange capacity is the best-index of soil fertility. By cation exchange, hydrogen ions from the root hairs and microorganisms replace nutrient cations from the exchange complex. The nutrient cations are forced into the soil solution where they can be assimilated by the adsorptive surface of roots and soil organisms, or they may be removed by drainage water.

(i) Cation saturation and Soil fertility:

Soil with a high calcium base saturation are the most satisfactory physical and nutritional condition. A calcium-dominated soil is granular in structure and porous. Calcium-dominated clay ensures good aeration and good drainage, thus increases fertility of the soils.

Base unsaturated soils are acidic in nature due to exchangeable hydrogen. These soils are less fertile. Base saturated soils with dominant sodium cations are alkaline in nature. Alkaline soils are not fertile due to de-flocculation, stickiness, hard to work, poor drainage and poor aeration.

(ii) Cation exchange and Soil fertility:

Due to the property of cation exchange (base exchange) the soluble inorganic fertilizer nutrients are not washed away from the soil. For example, ammonium sulphate fertilizer is added to the soil, ammonium ions are held on the surface of colloids by cation exchange. Ammonium ions are taken up by plants. This process checks nutrient losses by leaching and make the soil fertile. The cations Ca, Mg, K, and NH4 are held on the colloidal surfaces and are readily available to plants.

(iii) Influence of complementary adsorbed cations and soil fertility:

The order of strength of adsorption, when the ions are present in equivalent quantities, is as follow:

Al3+> Ca2+> Mg2+> K+ = NH4+> Na+

Consequently, a nutrient cation such as K+ is less tightly held by the colloids if the complementary ions are Al3+and H+ (acid soils) than if they are Mg++ +Na+ (neutral to alkaline soils). The loosely held K+ ions are more readily available for absorption by plants or for leaching in acid soils.

There are also some nutrient “antagonisms”, which in certain soil cause inhibition of uptake of some cations by plants. Thus, potassium uptake by plants is limited by high levels of calcium in some soils. Likewise, high potassium levels are known to limit the uptake of magnesium even when significant quantities of magnesium are present in the soil.

Anion Exchange:

The process of anion exchange is similar to that of cation exchange. Under certain conditions hydrous oxides of iron and aluminium show evidence of having positive charges on their crystal surfaces. The positive charge of colloids are due to addition of hydrogen (H+) in hydroxyl group (OH) resulted in net positive charge (OH2+). This + charge will attract anions (—).

The capacity for holding anions increases with the increase in acidity. The lower the pH the greater is the adsorption. All anions are not adsorbed equally readily. Some anions such as H2 PO4– are adsorbed very readily (quickly) at all pH values in the acid as well as alkaline range. Cl and SO4– ions are adsorbed slightly at low pH but none at neutral soil, while NO3– ions are not adsorbed at all. Hence, at the pH commonly prevailing in cultivated soils—nitrate (NO3), chloride (Cl) and sulphate (SO4) ions are easily lost by leaching.

In general, the relative order of anion exchange is:

OH > H2PO4–>SO4–>NO3–

Importance of Anion Exchange:

The phenomenon of anion exchange assumes importance in relation to phosphate ions and their fixation. The exchange is brought about mainly by the replacement of OH ions of the clay mineral.

The reaction is very similar to cation exchange:

anion order

The adsorption of phosphate ions by clay particles from soil solution reduces its availability to plants. This is known as phosphate-fixation. As the reaction is reversible, the phosphate ions again become available when they are replaced by OH ions released by substances like lime applied to soil to correct soil acidity.

Hence, the fixation is temporary. The whole of the phosphate adsorbed by clay is, however, not exchangeable, as even at pH, 7.0 and above. So, substantial quantities of phosphate ions are still retained by clay particles.

The OH ions originate not only from silicate clay minerals but also from hydrous oxides of iron and aluminium present in the soil. The phosphate ions, therefore, react with the hydrous oxides also and get fixed as in the case of silicate clay, forming insoluble hydroxy-phosphates of iron and aluminium.

phosphate reaction

If this reaction takes place under conditions of slight acidity it is reversible, and soluble phosphate is again liberated when hydroxy-phosphate comes in contact with ions. If the reaction takes place at a low pH under strongly acid conditions, the phosphate (ions) are irreversibly fixed and the totally unavailable for the use of plants.



Chemistry and Behaviour of Phosphorus Present in Soil

Chemistry of Phosphorus:

1. Sorption Reactions:

The surfaces on which phosphate ions enter into sorption reactions of two types-surfaces of constant charge e.g. crystalline clay minerals and surfaces of variable charge including Fe3+ and Al—oxides and organic matter where H+ and OH ions determine the surface charge and calcite (CaCO3) in which Ca2+ and CO ions involve the charge development.

Besides, some other clay minerals including amorphous such as allophane also involves in the phosphate sorption.

Hydrated Fe and Al oxides are the most important surfaces of variable charge in most soils excepting peats and highly calcareous soils. These oxides have surfaces of negatively charged OH groups which take up and dissociate protons (H+) and hence they are amphoteric having either negative, zero or positive charge depending on pH.

The pH at which there are equal numbers of positive and negative charges on the surface is known as point of zero charge (PZC). At pH levels below the PZC, phosphorus and other anions like SO42- and H3SiO4 are attracted to the positively charged oxide surfaces.

2. Precipitation Reactions:

Precipitation reactions mainly govern by the solubility product principles which are controlled by the pH of the system.

When some common phosphatic fertilizers like super phosphate, mono ammonium phosphate, Di-ammonium phosphate, some poly phosphates etc. are applied to the soil, within a very short time the released soluble phosphorus converts into very less soluble forms rendering unavailable and with time passes the strong insoluble phosphate fertilizer reaction products will form depending on the nature and type of soil as well as soil reaction.

In acid soils mono-calcium phosphate produces a number of substances like di-calcium phosphate (dihydrate and anhydrate), CaFe2 (HPO4)4. 8H2O; CaAl H(PO4)2.6H2O etc. whereas in calcareous soils, di-calcium phosphate (CaHPO4) is the dominant initial reaction product and in presence of excess amounts of calcium carbonate (CaCO3), octacalcium phosphate may also form.

Further, when di-ammonium phosphate is applied to soils, the following reaction products viz. Ca4 (PO4)3.3H2O; Ca2 (NH4)2 (NPO4)2.2H2O, CaHPO4-2H2O; CaNH4PO4.H2O; CaxH2 (PO4)6-5H2O etc. will form. Dicalcium phosphate dihydrate is one of the most dominant reaction products formed in high-calcium soils followed by octacalcium phosphate.

When polyphosphate fertilizers are applied to soils it undergoes precipitation and adsorption reactions. In addition the orthophosphate present initially plus which formed by the hydrolysis of polyphosphates react with the soil components similar to that happened in orthophosphate compounds.

Hydrolysis of polyphosphates results in a stepwise breakdown forming orthophosphates and different short chain polyphosphate fragments. Then such short chain polyphosphates undergo further hydrolysis. However, reactions of polyphosphates in soil and the nature of substances produced are dependent upon the rate of their reversion back to orthophosphates.

Slow rate of hydrolysis permits condensed phosphates to sequester or form soluble complexes with soil cations and hence reduce phosphate retention in soils. Two mechanisms namely chemical and biological are involved in the hydrolysis of polyphosphates. In soils, where both mechanisms can function, the rate of hydrolysis will be rapid.

precipitation reactions

Enzymatic activity is the most important factor which controls the rate of hydrolysis. Phosphatases associated with plant roots and rhizosphere organisms are believed to be responsible for biological hydrolysis of pyro-and polyphosphates. Various factors like, temperature, soil pH, moisture, organic carbon content etc. can affect the transformation of polyphosphates.

Behaviour of Phosphorus:

Both organic and inorganic forms of phosphorus undergo transformation in soils leading to either release or retention of phosphorus. It is evident that decomposition of organic phosphorus substances gives both active and inactive substances.

The active substances are primarily the portions of the residues that have not yet been transformed into microbial products, whereas the inactive forms of phosphorus behave similarly to the resistant forms of nitrogen in humic acid.

1. Organic Phosphorus:

During mineralisation of organic phosphorus substances, the release of inorganic phosphorus takes place in the soil solution and such released phosphorus reacts very quickly with various soil components forming insoluble complex phosphatic compounds and there by unavailable to the plants.

Mineralisation of organic phosphorus is of three types:

(i) Based on the lowering of organic phosphorus level in soils due to long term cultivation.

(ii) Based on the results of short laboratory investigations decreasing the level of organic phosphorus with simultaneous increase in the amount of inorganic phosphorus in the soil and

(iii) Based on monitoring levels of soil organic phosphorus in the presence and absence of plants considering seasonal variation.

Mineralisation of organic phosphorus is carried by phosphatase enzymes and these enzymes are broad group of enzymes which catalyze the hydrolysis of both esters and anhydrides of phosphoric acid. However, there are a wide range of micro-organisms that are capable of mineralising (dephosphorylating) organic phosphorus on soils through their phosphatases activities.

Phosphatase activity of a soil is due to the combined functioning of the soil micro-organisms and any free enzymes present. Mineralisation of organic phosphorus is not entirely similar to that of organic carbon and nitrogen mineralisation and the mineralisation of organic phosphorus increases with an increase in soil pH but organic carbon and nitrogen mineralisation did not.

Most of the organic soil phosphates are present as inositol phosphate esters and these are prone to adsorption resulting less available in soils having higher adsorption capacity. The ultimate process by which organic phosphates are rendered available is by cleavage of inorganic phosphate by means of a phosphatase reaction.

The principle of this reaction is hydrolysis which is shown below:

phosphate hydrolysis

For carrying out the mineralisation of organic phosphatic substances in soils it is essential to have some idea about C: N: P ratios in the soil. A carbon: nitrogen: phosphorus (C: N: P) ratio of 100: 10: 1 for soil organic matter has been advocated, but its values ranges from 229: 10: 0.39 to 71: 10: 3.05—depending on nature and type of soils.

C: P inorganic ratio – Process Operates

200: 1 or less – Mineralisation

Above 200: 1 but – Neither net mineralisation nor

Less than 300: 1 – Net immobilisation

300: 1 and above Immobilisation

A concentration of about 0.2% phosphorus is critical in the mineralisation of organic phosphorus substances. If the system contains less than this, net immobilisation takes place, as both the plant and the native soil phosphorus are utilised by micro-organisms. The transformation of P takes place both in upland (aerobic) and low land submerged (anaerobic) soils.

2. Inorganic Phosphorus:

It is evident that most of the soluble inorganic phosphorus either released from the mineralisation of organic phosphorus or applied as soluble phosphatic fertilizers are rendered unavailable to the plants and hardly 20% of the applied phosphatic fertilizers are available to the plant.

The reasons for such recovery are the conversions of soluble form of phosphorus to a form which is very less soluble through reactions with various soil components involving different mechanisms.

Such mechanism for the removal of phosphorus from the solution phase in the soil is known as “retention or fixation”. However, the retention of phosphorus in the soil involves various mechanisms namely, sorption and precipitation reactions.



3 Main Forms of Potassium in Soils

Form # 1. Soil Solution Potassium:

It is recognised as the readily available form of potassium to the plants. The potassium availability in soils is controlled not only by the soil solution potassium but also by its buffering capacity (ability of a soil to maintain potassium intensity). The soil solution potassium (intensity, I) is maintained by the exchangeable potassium (quantity, Q) in a dynamic equilibrium.

The higher dQ/dl or the higher potassium buffer capacity indicates that during active period of crop growth, the potassium concentration in the soil solution will be depleted very rapidly. Soil solution potassium content usually higher in arid region and saline soil ranging from 3 to 156 ppm whereas the content of the same is lower in humid region soils ranging from 1 to 80 ppm.

Concentration of water soluble potassium may be as low as 8 ppm in deficient soils. However, under actual field conditions, the potassium concentration of soil solution varies with concentration and dilution processes brought about by evaporation and rainfall respectively.

The potentiality of soil solution K for plant growth and nutrition is influenced by the presence of other cations like Ca, Mg, and Al in acid soils and Na in salt affected soils. The activity ratio of potassium at equilibrium (AReK) with respect to these above cations is a measure of the “intensity” of labile potassium in the soil indicating instantly available to plant roots.

Soils having same AReK values may have different capacity in maintaining AReK during depletion of K by crop uptake or leaching and hence for the K status of soils it is necessary to specify not only the status of potassium in the labile pool but also the way in which the intensity depends on the amount (quantity) of labile potassium present.

However, the detail discussion about the Q/I relationship of K is presented in the following section.

Buffer capacity indicates how intensity varies with quantity. A simple relationship between K intensity and K quantity for two soils (Soil X and Soil Y) having differential K adsorbing capacity is being depicted in Fig. 21.9. From the figure it is found that for both soils increasing intensity is accompanied by an increase in quantity.

quantity intensity K in soils potassium

Soil X, however, shows a steeper rise in the slope than that of soil Y. Where an equal amount of K is removed from both soils by plants a similar decrease in the quantity of (∆Q) takes place. The consequent decrease in intensity (∆l), however, varies greatly for both soils (∆Ix and ∆IY).

This example shows that the two soils differ in their capacity of replenishing the soil solution with K. Soil X is better able to maintain the K concentration in the soil solution. Soil X is, therefore, more buffered than soil Y.

In quantitative terms the buffer capacity is expressed as the ratio ∆Q/∆I as follows:

BK = ∆Q/∆I,

where BK = buffer capacity of K in soils

The higher the ratio of ∆Q/∆I, the more the soil is buffered. Usually, the rate of K uptake by plant roots is higher than the diffusive flux of K towards the roots. The K concentration at the root surface may decrease during the period of plant uptake.

Such decrease in K concentration is dependent on the K buffering capacity of the soil. If the buffer capacity is high, the decrease may be low because of efficient K replenishment of the soil solution.

Again, for spoils having poor K buffer capacity, the concentration of K at the root surface may decrease appreciably throughout the plant growth period. For optimum growth of the plant, the concentration of nutrients in soil solution should be maintained above a certain level.

This concentration is termed as the critical nutrient concentration (CNC) below which the yield of crop is decreased. The critical level of K in the bulk soil solution is related to the buffer capacity of K (buffer power). The critical concentration is higher, the lower K buffer capacity.

In addition to AReK in assessing soil solution K, electro-ultra filtration (EUF) technique, a process of combination of electro dialysis and ultrafiltration, is used most satisfactorily for the characterisation of soil solution K (intensity) particularly in upland soils. The principle of EUF technique consists of utilizing the acceleration imposed upon ions by an electrical field for the separation of ions from soil colloids.

By adequate variation of voltage (50, 200 or 400 V) and timing (0-35 minutes), the total extractable K or any other nutrients can be separated into their water soluble and exchangeable forms with varying bonding energies.

Extraction and fractionation is done automatically. For the EUF-K fraction I (potassium in the extract obtained after 10 min of EUF—the first 5 min at 50 V and the next 5 min at 200V) and total EUF-K fractions (sum of all potassium fractions obtained after 35 min of EUF— the first 5 min at 50 V, the next 25 min at 200 V, and the last 5 min at 400 V).

The EUF-K fraction I considered as soil solution K (intensity factor) whiles the total EUF-K fractions as total amount of effectively available K (quantity factor). This electro- ultra filtration technique is better suited than that of AReK in distinguishing soils of varying K availabilities.

The amount of K in the soil solution is very low to meet the demand of the crop throughout the growing period and therefore it is necessary for satisfactory potassium nutrition of crops the soil solution K must be continuously replenished from the exchangeable, non-exchangeable and mineral forms of K.

Form # 2. Exchangeable Potassium:

Potassium ion (K+) is held by soil colloids through electrostatic attraction similar to other cations. However, potassium held by soil colloids is easily displaced or exchanged when extracting the soil with neutral salt solutions. The amount of K exchanged varies with cations and usually neutral normal ammonium acetate solution is used for the purpose.

A small amount of potassium in this fraction occurs in soils (<1.0% of the total potassium). The distribution of potassium on soil colloids as well as soil solution depends upon nature and amounts of complementary cations, anion concentration and nature and characteristics of clay minerals.

As for an example, if a soil colloid is saturated with potassium and in that condition a neutral salt like calcium sulphate is applied then the following exchange reaction takes place:

clay reaction

Besides, if a soil is saturated with Al and Ca and in that conditions the application of muriate of potash gives the following exchange reaction:

Al Ca soil

When muriate of potash is applied to soils containing adsorbed calcium and aluminium, calcium is more easily replaced than aluminium by potassium. Coarse textured sandy soils having a greater base saturation lose very little of their exchangeable potassium by leaching as compared to soils containing low basic cations.

Liming is considered as the most common method of increasing the base saturation of soils which results the decrease in the loss of exchangeable potassium.

Sites for K Exchange:

It is evident that the exchangeable potassium on soil colloids is not homogeneous. Usually potassium is held at three binding sites of soil colloids namely p-(planar) position (outer surface of colloids, non-specific), e-(edge) position and i-(inner or inter layer) position (specific for K).

The amount of K held on p-position is in equilibrium with the soil solution K, while the amount of soil solution K in equilibrium with K held on e and i positions of soil colloids is low. However, under actual field situations, potassium concentrations in the soil pollution are probably the net result of three possible equilibria.

It is evident that the exchangeable form of potassium plays an important role in replenishing soil solution potassium removed by either intensive cropping or leaching losses.

In view of the above fact, it is very much essential to establish the quantity relationship between exchangeable K (Q quantity) and the activity of potassium in the soil solution (I intensity) in order to assess the availability of more labile potassium in soils to plants (Fig. 21.10).

potassium availability

The Q/I concept has been developed by Beckett which is used for predicting the status of potassium in soils. Different parameters of the above curve have some practical implications in relation to potassium in soils and plants.

∆K = Amount through which the soil gains or loses potassium in bringing equilib­rium (Q, quantity factor).

ARK = Activity ratio of potassium (I, intensity factor).

AReK = Activity ratio of potassium at equilibrium

∆Kex = Exchangeable or labile pool of potassium

KSP = Specific sites for potassium

PBCK = Potential buffering capacity

ARK (Intensity Factor, I):

It is calculated from the determined concentration of calcium, magnesium, potassium and sodium correcting to the appropriate activities with the help of extended Debye-Huckel theory.

AReK (Activity Ratio of K at Equilibrium):

It is a measure of availability or intensity of labile pool of potassium in soil and can be modified by potassium fertilization, being increased due to application of K fertilizers. However, the availability of potassium in soils can either be increased or decreased due to liming which modifies the AReK values either favorably or adversely.


It is used more successfully for the estimation of labile soil potassium held in plannar (p) positions. Greater values of labile potassium i.e. more negative (-Kex) indicate a higher potassium release into the soil solution which results greater amount of potassium in the labile pool. However

the application of potassic fertilizers and lime in the cropped field have been found to be increased the amount of potassium in the labile pool.

Ksp (Specific Sites for Potassium):

It is a curved portion of the Q/I relationship while the linear portion of the curve (Q/I) is attributed to non-specific sites for potassium. Specific sites having high affinity for potassium are believed to exist on edges of clay minerals (e-positions) and in interlayer or wedge zones of weathered micas (i-positions).

Whereas non-specific sites for potassium are associated with planar surfaces of clay minerals (p-positions). The i-position has the greatest specificity for K+ which largely account for K+ fixation in soils.

PBCK (Potential Buffering Capacity of Potassium):

It is a measure of ability of a soil to maintain potassium concentration in the soil solution. The potential buffering capacity for potassium is proportional to the cation exchange capacity (CEC) of the soil that means, with an increase in CEC the value of PBCK increases and vice-versa resulting from changes in ARK values.

A high PBCK value indicates a good potassium supplying power of soils whereas a low PBCK value signifies very low potassium supplying power of soils indicating frequent potassium fertilization. However, the higher PBCK value may be obtained due to time application which probably as a result of increase in pH-dependent cation-exchange capacity.

If PBCK is low, small changes in exchangeable potassium produce large differences of potassium content in the soil solution. This value is very small coarse textured sandy soils where mainly organic matter is contributed to the CEC value. In such soils, extensive leaching, rapid plant growth etc. deplete available potassium within a few days.

In general, the relation between exchangeable and soil solution potassium is a good measure of the availability of the labile pool of potassium in soils to plants. The ability of a soil to maintain the activity ratio of K against depletion by crop uptake, leaching etc. is controlled by nature of the labile pool potassium as well as the rate of release of fixed potassium and diffusion and transport of K+ ions in the soil solution.

Form # 3. Non-Exchangeable and Mineral Form of Potassium:

Potassium in these forms is not readily available to the plants. However, non-exchangeable potassium pools not instantly available to plants, can contribute significantly to the maintenance of the labile pool of potassium in the soil. On the other hand, in some soils these fractions of potassium may become available as water-soluble and exchangeable forms are removed by leaching, crop uptake etc.

These forms of potassium are consisting of different K-bearing minerals namely primary minerals (K-feldspars) and micas (muscovites, biotites etc.), originating from the parent rock and secondary minerals (clays of the illitic group) formed by alteration of micas.

The main source of K+ for plants growing under natural conditions is from the weathering of K containing minerals mentioned above. In potash feldspars, potassium occurs in the interstices of the Si, Al—O framework of the crystal lattice and held rigidly by covalent bonds. The weathering of feldspars starts at the surface of the particle.

Initially potassium is released by water and weak acids at a more rapid rate, However, with the progress of weathering, a Si—Al—O residue envelope is formed surrounding the un-weathered core. This layer reduces the rate of potassium loss from the mineral and hence protects K from further degradation.

Minerals of the mica type and also the secondary minerals of 2: 1 layer silicates vary in structure from feldspars and thereby these minerals also differ in their properties of releasing and binding potassium.

The micas consist of unit layers each containing two Si, Al—O tetrahedral sheets between which is an M (Al, Fe, Mg)—O, OH octahedral sheet potassium (K+) ions occupy the approximately hexagonal spaces between the unit layers and as a result the distance between unit layers is relatively small i.e. 1.0 nm in micas.

The replacement of un-hydrated interlayer K+ by hydrated cations like Na+, Ca2+ or Mg2+ expands the mineral with an increase in the distance between the unit layers i.e., 1.4 nm in vermiculite (Fig. 21.11 and 21.12).

mica vermuculite

Al or Fe layer in soil

Usually K+ of the lattice is vulnerable to weathering and can diffuse out of the mineral in exchange for other cations. High H+ concentrations and low K+ concentrations in the soil favour the net release of non-exchangeable, inter layer K+.

This K+ release may be an exchange process associated with diffusion in which K+ adsorbed to i-positions of the inter layer zone is replaced by other large cations like Na+, Ca2+ and Mg2+ resulting an expansion of clay lattice and the formation of wedge zones (See in above figure).

“Frayed edge” or “wedge” zone formation is typical of weathering micas which results release of interlayer K+. The rate of release of K+ by weathering not only depends on the K content of the mineral, but also affected by structural variation between minerals.

The gradual release of potassium from positions of mica lattice results in the formation of illite (hydrous mica) and eventually vermiculite with accompanying gain of water or H3O+ and swelling of the lattice (Fig. 21.13).

potassium ion dehydrated

There is also an increase in specific surface charge and CEC of clay minerals formed during the weathering of K containing minerals as well as transformation of mica. However, the applied soluble potassic fertilizers are converted to fixed or non- exchangeable forms of K and such conversions are affected by various factors. Besides fixation, the applied potassic fertilizers also undergo leaching loss from soils.

It is evident that a substantial amount of potassium can be lost through leaching in soils containing more amounts of sands due to flooding. However, in case of silty loam and clay loam soils, the loss of K through leaching is less because of fairly higher rate of adsorption of potassium by soil colloids. Again, in organic soils e.g. muck soils have high exchange capacities.

The bonding strength for cations like potassium is not great and the amount of exchangeable K tends to vary with the intensity of rainfall. Therefore, care should be taken for the supply of potassium to crops through its annual application.

Leaching losses can be reduced with the application of lime to the soil by maintaining a favourable pH level. Leaching losses of potassium frequently occurs in coarse textured sandy or organic soils particularly in areas of high rainfall.

Cation and Anion Exchange Capacity

CEC and Soil Nutrient Availability

Soil pH

Soil pH defines the relative acidity or alkalinity of the soil solution (Table 1.). The pH scale in natural systems ranges from 0 to 14. A pH value of 7.0 is neutral. Values below 7.0 are acid and those above 7.0 are alkaline, or basic. Many agricultural soils have a soil pH between 5.5 and 6.5.

Soil pH is a measurement of hydrogen ion (H+) activity, or effective concentration, in a soil and water solution. Soil pH is expressed in logarithmic terms, which means that each unit change in soil pH amounts to a tenfold change in acidity or alkalinity. For example, a soil with a pH of 6.0 has 10 times as much active H+ as one with a pH of 7.0.

acidity table

Descriptive terms commonly associated with certain ranges in soil pH are:

Cation and anion exchange capacity (CEC)

Cation-exchange capacity is defined as the degree to which a soil can adsorb and exchange cations.on surface with negative charge.

Cation exchange capacity ec

Sources of negative charge:

isomorphous substiution clay charge
soil clay layers
cations on clay layers

The main source of charge on clay minerals is isomorphous substitution which confers permanent charge on the surface of most layer silicates.

Ionization of hydroxyl groups on the surface of other soil colloids and organic matter can result in what is describes as pH dependent charges-mainly due to the dependent on the pH of the soil environment. Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.

edge charge pH
organic cation retention

Presence of surface and broken – edge -OH groups gives the kaolinite clay particles their electronegativity and their capacity to absorb cations. In most soils there is a combination of constant and variable charge. Cation-a positively charged ion There are two types of cations, acidic or acid-forming cations, and basic, or alkaline-forming cations. The Hydrogen cation H+ and the Aluminum cation Al+++ are acid-forming.

ckay humus complex

The positively charged nutrients that we are mainly concerned with here are Calcium, Magnesium, Potassium and Sodium. These are all alkaline cations, also called basic cations or bases. Both types of cations may be adsorbed onto either a clay particle or soil organic matter (SOM). All of the nutrients in the soil need to be held there somehow, or they will just wash away when you water the garden or get a good rainstorm. Clay particles almost always have a negative (-) charge, so they attract and hold positively (+) charged nutrients and non-nutrients. Soil organic matter (SOM) has both positive and negative charges, so it can hold on to both cations and anions.(

soil groups

Anion-a negatively charged ion (NO3, PO42-, SO42-, etc…)

Soil particles and organic matter have negative charges on their surfaces. Mineral cations can adsorb to the negative surface charges or the inorganic and organic soil particles. Once adsorbed, these minerals are not easily lost when the soil is leached by water and they also provide a nutrient reserve available to plant roots.

These minerals can then be replaced or exchanged by other cations (i.e., cation exchange)

vThe exchage processes (Figure 23) are REVERSIBLE (unless something precipitates, volatilizes, or is strongly adsorbed).

cation exchange examples

CEC is highly dependent upon soil texture and organic matter content Table 3, 4.). In general, the more clay and organic matter in the soil, the higher the CEC. Clay content is important because these small particles have a high ration of surface area to volume. Different types of clays also vary in CEC. Smectites have the highest CEC (80-100 millequivalents 100 g-1), followed by illites (15-40 meq 100 g-1) and kaolinites (3-15 meq 100 g-1).

soil texture
Mineral types and CEC

Measurement of CEC.

The CEC of soil is usually measured by saturating the soil with an index cation such as Na+, removal of the excess salts of the index cation with a dilute solution, and then displacing the Na+ with another cation. The amount of Na+ displaced is then measured and the CEC is calculated.

In general, the CEC of most soils increases with an increase in soil pH. Two factors determine the relative proportions of the different cations adsorbed by clays. First, cations are not held equally tight by the soil colloids. When the cations are present in equivalent amounts, the order of strength of adsorption is Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+.

The relative concentrations of the cations in soil solution helps determine the degree of adsorption. Very acid soils will have high concentrations of H+ and Al3+. In neutral to moderately alkaline soils, Ca2+ and Mg2+ dominate. Poorly drained arid soils may adsorb Na in very high quantities.

cation exchange schematic

Base saturation

The proportion of CEC satisfied by basic cations (Ca, Mg, K, and Na) is termed percentage base saturation (BS%). This property is inversely related to soil acidity. As the BS% increases, the pH increases. High base saturation is preferred but not essential for tree fruit production. The availability of nutrient cations such as Ca, Mg, and K to plants increases with increasing BS%.

Vase saturation and arid soils, pH

Base saturation is usually close to 100% in arid region soils. Base saturation below 100% indicates that part of the CEC is occupied by hydrogen and/or aluminum ions. Base saturation above 100% indicates that soluble salts or lime may be present, or that there is a procedural problem with the analysis.

CEC and availability of nutrients

Exchangeable cations, may become available to plants. Plant roots also possess cation exchange capacity. Hydrogen ions from the root hairs and microorganisms may replace nutrient cations from the exchange complex on soil colloids. The nutrient cations are then released into the soil solution where they can be taken up by the adsorptive surfaces of roots and soil organisms. They may however, be lost from the system by drainage water.

moving nutrients from soils to plants

Additionally, high levels of one nutrient may influence uptake of another (antagonistic relationship). For example, K uptake by plants is limited by high levels of Ca in some soils. High levels of K can in turn, limit Mg uptake even if Mg levels in soil are high.

Anion-exchange capacity (AEC)

Sources of anion exchange capacity

Anion exchange arise from the protonation of hydroxyl groups on the edges of silicate clays and on the surfaces of metal oxide clays Anion exchange is inversely related with pH is greatest in soils dominated by the sesquioxides. The anions Cl, NO3, and SeO42- and to some extent HS ands SO42-, HCO3, and CO3 adsorb mainly by ion exchange. Borate, phospahate and carboxylate adsorb principally by specific adsorption mechanisms. (

The total exchangeable anions that a soil can adsorb, measured as milliequivalents per 100 grams of soil. ( )

In contrast to CEC, AEC is the degree to which a soil can adsorb and exchange anions. AEC increases as soil pH decreases. The pH of most productive soils is usually too high (exceptions are for volcanic soils) for full development of AEC and thus it generally plays a minor role in supplying plants with anions.

negatively charged ions

Because the AEC of most agricultural soils is small compared to their CEC, mineral anions such as nitrate (NO3 and Cl) are repelled by the negative charge on soil colloids. These ions remain mobile in the soil solution and thus are susceptible to leaching. (

Phosphate anions are relativelly bounded on the positivelly charged places (iron, aluminium, calcium compounds etc.) (Figure 28).

phosphate retention in soil

Nitrate is weakly bounded and the nitrate compounds are well soluble soluble in water, that why nitrate can easily be leahed out (Figure 29).

Nitrate retention


NH4+ NO3 Some of the ammonium produced by decomposition is converted to nitrate via a process called nitrification. The bacteria that carry out this reaction gain energy from it. Nitrification requires the presence of oxygen, so nitrification can happen only in oxygen-rich environments like circulating or flowing waters and the very surface layers of soils and sediments. The process of nitrification has some important consequences. Ammonium ions are positively charged and therefore stick (are sorbed) to negatively charged clay particles and soil organic matter. The positive charge prevents ammonium nitrogen from being washed out of the soil (or leached) by rainfall. In contrast, the negatively charged nitrate ion is not held by soil particles and so can be washed down the soil profile, leading to decreased soil fertility and nitrate enrichment of downstream surface and groundwaters.


NO3 N2+ N2O Through denitrification, oxidized forms of nitrogen such as nitrate and nitrite (NO2) are converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas. Denitrification is an anaerobic process that is carried out by denitrifying bacteria, which convert nitrate to dinitrogen in the following sequence:

NO3 NO2 NO N2O N2.

Nitric oxide and nitrous oxide are both environmentally important gases. Nitric oxide (NO) contributes to smog, and nitrous oxide (N2O) is an important greenhouse gas, thereby contributing to global climate change.

Once converted to dinitrogen, nitrogen is unlikely to be reconverted to a biologically available form because it is a gas and is rapidly lost to the atmosphere. Denitrification is the only nitrogen transformation that removes nitrogen from ecosystems (essentially irreversibly), and it roughly balances the amount of nitrogen fixed by the nitrogen fixers described above.

Denitrification of Nitrate-N

Certain soil bacteria that thrive in saturated (anaerobic) soil conditions will convert nitrate-N to oxygen and nitrogen gases. Volatilization of the nitrogen gas can result in N losses of as much as 5% of the available nitrate-N per day. Soils at greatest risk to denitrification N loss are those that are naturally heavy and poorly drained, plus fields with significant levels of soil compaction that restricts natural drainage. Because denitrification affects nitrate-N, the relative risk of N fertilizer products is identical to that for leaching N loss (Fig.16.).

Nitrogen immobilization

A fourth N loss mechanism is more temporary in nature. Soil microbes that decompose high carbon-content plant residues to organic matter use soil N during the decomposition process. Consequently, the nitrogen from the surface-applied fertilizer is “tied up” in the resulting organic matter and is temporarily unavailable for plant uptake until mineralization of the organic matter occurs at a later date. Such immobilization of soil N can be especially prevalent in high-residue no-till cropping systems. Unfortunately, applying N fertilizer in the fall to corn residues has not been shown to reduce N immobilization or speed residue decomposition. (

phosporous cycle

Phosphorus cycle

Phosphorus is an essential nutrient for plants and animals in the form of ions PO43- and HPO42-. It is a part of DNA-molecules, of molecules that store energy (ATP and ADP) and of fats of cell membranes. Phosphorus is also a building block of certain parts of the human and animal body, such as the bones and teeth.

Phosphorus can be found on earth in water, soil and sediments. Unlike the compounds of other matter cycles phosphorus cannot be found in air in the gaseous state. This is because phosphorus is usually liquid at normal temperatures and pressures. It is mainly cycling through water, soil and sediments. In the atmosphere phosphorus can mainly be found as very small dust particles.

Phosphorus moves slowly from deposits on land and in sediments, to living organisms, and than much more slowly back into the soil and water sediment. The phosphorus cycle is the slowest one of the matter cycles that are described here.

Phosphorus is most commonly found in rock formations and ocean sediments as phosphate salts. Phosphate salts that are released from rocks through weathering usually dissolve in soil water and will be absorbed by plants. Because the quantities of phosphorus in soil are generally small, it is often the limiting factor for plant growth. That is why humans often apply phosphate fertilizers on farmland. Phosphates are also limiting factors for plant-growth in marine ecosystems, because they are not very water-soluble. Animals absorb phosphates by eating plants or plant-eating animals.

Phosphorus cycles through plants and animals much faster than it does through rocks and sediments. When animals and plants die, phosphates will return to the soils or oceans again during decay. After that, phosphorus will end up in sediments or rock formations again, remaining there for millions of years. Eventually, phosphorus is released again through weathering and the cycle starts over (Fig. 17).

potassium cycle

Potassium cycle

Potassium is taken up by plants in large quantities and is necessary to many plant functions, including carbohydrates metabolism, enzyme activation, osmotic regulation, and protein synthesis. Potassium is essential for photosynthesis, for nitrogen fixation in legumes, starch formation, and translocation of sugars. As a result of several of these functions, a good supply of potassium promotes production of plump grains and large tubers.

Potassium is important in helping plants adapt to environmental stresses (e.g. improved drought tolerance and winter hardiness, better resistance to fungal diseases and insect pests (Fig. 64).

potassium cycle


It means the gradual increase in the concentration of phosphorus, nitrogen, and other plant nutrients in an aging aquatic ecosystem such as a lake. The productivity or fertility of such an ecosystem increases as the amount of organic material that can be broken down into nutrients increases. This material enters the ecosystem primarily by runoff from land that carries debris and products of the reproduction and death of terrestrial organisms. Blooms, or great concentrations of algae and microscopic organisms, often develop on the surface, preventing the light penetration and oxygen absorption necessary for underwater life.

Soil science

Prof. Blaskó Lajos (2008), Soil Science


Figure 2.10 Cation uptake can have an acidic effect

Ammonium uptake acidity

Figure 2.12 Anion uptake can have a basic effect

anion uptake leads to basicity

Figure 2.11 Co-absorption of cations and anions at the same rate does not lead to a change in pH.

co-absorption is neeutral to pH

UF-AG Extension


INTRODUCTION Iron is an essential nutrient for plants and serves as a cofactor for a wide variety of cellular processes, such as oxygen transport, cellular respiration, chlorophyll biosynthesis, thylakoid biogenesis and chloroplast development (Kobayashi and Nishizawa, 2012; Tanuja Poonia et al, 2018). The availability of Fe is severely limited in calcareous soils due to their low solubility at high pH and bicarbonate concentration which reduces the Fe uptake by inactivating the Fe in plants (Mortvedt, 1991; Najafi-Ghiri et al., 2013). Hence, Fe-deficiency induced chlorosis is a serious problem resulted in the yield loss and quality of crop produces in many crops particularly in the calcareous soils (Kim and Guerinot, 2007; Zheng, 2010). It is also closely related to the prevalence of Fe-deficiency-induced anemia in human beings (Murgia et al., 2012). Amelioration of Fe deficiency in soils and plants was generally achieved through the use of inorganic Fe salts, Fe chelates, organic manures, etc. either through soil application or as a foliar spray which differs significantly in maintaining soil Fe availability. Addition of Fe chelates to calcareous soils was proved to be very effective in maintaining soil solution Fe and the efficacy was better with Fe- EDDHA, Fe EDTA and Fe-DTPA but the recovery of Fe from ferrous sulphate was negligible under high soil pH and calcareousness (Jaloud et al., 2013; Faraz et al., 2014; Sedigheh Safarzadeh et al., 2018). Inclusion of organic manures proved to be beneficial in increasing the availability of Fe in soils and it was widely reported by many researchers (Ali et al., 2007; Yunchen Zhao, 2009; Amin, 2018). Further the Fe availability significantly correlated with many soil properties particularly pH, carbonate and bicarbonate ions and organic carbon content which majorly controls the availability (Obrador et al., 2007; Wang et al., 2009; Wu et al., 2010; Canasveras et al., 2014; Mahendra Kumar et al., 2017). Hence the present study was taken up to test the effectiveness of various levels and sources of Fe on Fe availability in calcareous and noncalcareous soils with and without amendments.

CONCLUSION To conclude, a linear increase in DTPA Fe extractability was observed with incubation period in all three soils and the highest availability was associated with the addition of 10 kg Fe EDTA ha-1 followed by 50 kg FeSO4 ha-1. Inclusion of FYM at 12.5 t ha-1 and 0.25 % Acetic acid considerably improved the Fe availability in soils and the better effect was registered with FYM. Higher Fe extractability was noted up to 30 days in the red calcareous soil while in black calcareous soils the release was linear up to 45 days. A negative correlation between soil pH and calcareousness was observed on the Fe availability in the soil

A Iron Availability in Calcareous and Non Calcareous Soils as Influenced by, Various Sources and Levels of Iron

Calcareous soils (containing free lime) are common in many arid and semi-arid re­ gions af North America and occur as inclusions in mare humid regions. Phosphorus

(P) is very reactive with lime. Fallowing fertilizer application, P undergoes a series af reactions that gradually reduce its solubility. In mast calcareous sails, there does not appear ta be a strong agronomic advantage af any particular P source when managed properly. Organic matter can inhibit P fixation reactions ta same extent. SOILS fertilizer recommendations call far additional P ta be added when the sail contains high amounts af free lime.  a lc ar eous soils are common in arid and semi-arid climates and occur as inclusions in more humid regions, af­ fecting over 1.5 billion acres of soil world­ wide and comprising more than 17% of the soils in the U.S. Calcareous soils are identi­ fied by the presence of the mineral calcium carbonate (CaC0 3 or lime) in the parent material and an accumulation of lime. This is most easily recognized by the efferves­

cence (fizzing) that occurs when these soils are treated with dilute acid. The pH of these soils is usually above 7 and may be as high as 8.5. When these soils contain sodium carbonate, the pH may exceed 9. In some soils, CaC0 3 can concentrate into  very hard layers, termed caliche, that are impermeable to water and plant roots. Calcareous soils can be extremely pro­

 surfaces, and pre­ cipit a ti onof various calcium phosphate min­ erals. While the total lime con­ tent of a soil is important for predicting P re­ actions, the lime particle size (and its effect on reac­ tive surface area) is often a better predictor of P behavior. Al­ though a calcar­ eous soil may be dominated  by  Effervescence (fizzing) occurs when colcoreous soils ore treoted with dilute ocid. Regulor soil testing is importont to monitor ovoilobility of P in colcoreous soils.   ductive for agricultural use when they are managed properly. Since they are most fre­ quently found in semi-arid and arid re­ gions, supplemental irrigation water is of­ ten the frrst barrier for crop production. Limited availability of Pis often the next most limiting factor for plant growth. When P fertilizer is added to calcare­ ous soils, a series of fixation reactions oc­ cur that gradually decrease its solubility and eventually its availability to plants. Phosphorus “fixation” is a combination of surface adsorption on both clay and lime  free lime, it may also contain significant amounts of iron (Fe), aluminum (Al), and manganese (Mn)… either as discrete min­ erals, as coatings on soil particles, or complexed with soil organic matter. These metals provide strong sorption sites for P and are frequently more significant in con­ trolling P solubility in calcareous soils than lime itself. Their importance should not be ignored. As fertilizer P reacts in calcareous soils, it is converted to less soluble compounds such as dicalcium phosphate dihydrate or

fertilizer P availability

Figure 1. Fertilizer P undergoes a reduction in solubility following addition to three calcareous soils (Sharpley et al., 1989).

 octacalcium phosphate. In some cases it may eventually convert to hydroxyapatite. A variety of management practices can he used to slow these natural fixation pro­ cesses and increase the efficiency of applied fertilizer for crop growth. A number of the factors controlling P availability will he briefly covered. Time—ln soluhle rock Pis treated af­ ter mining from geologic deposits to en­ hance its solubility and usefulness for plants. Fertilizer P is most soluble imme­ diately after addition to soil, then it un­ dergoes many chemical reactions that re­ sult in gradually diminished solubility (Figure 1).

Residual fertilizer P continues to he available for plant uptake for many years, but freshly applied P is generally most soluble and available for plant uptake. The   common practice of building soil P concentrations to appropriate agronomic ranges provides a long-term source of this nutrient to crops. Phosphorus Fertilizer Source—Many studies have demonstrated that there are no consistent agronomic differences in most commercially available P fertilizers added to calcareous soils. The selection of a specific P source should he based on other factors such as application equipment, suitability of fluids or granules, and price. However, considerable work is cur­ rently underway to improve P availability with new P products and fertilizer addi­ tives. This topic will he explored in greater detail in future articles. For example, re­ cent work from Australia in extremely cal­ careous soils has suggested that fluid P sources may have somewhat greater solu­ bility and enhanced plant availability than granular fertilizers. It has been hypoth­ esized that granule dissolution may he sup­ pressed in these soil conditions. Additional work is underway in the U.S. to see if these results hold for soil conditions more typi­ cal of North America There is large variability in the solu­ bility and availability of P from various materials added to calcareous soil (Figure 2). These large differences are largely due to the unique properties of the materials, rather than any unique character associ­ ated with a specific soil. For example, the polymer-coated, slow release P source has very low apparent solubility, but is able to sup­ port high levels of plant P ac­ cumula tio n. The soluble P sources and liquid manures have very high solubility and also are able to maintain high P recovery by barley. Organic Matt e1′- l n the soil solution, there are several chemical components that will   delay or prevent the reaction of P with lime. Organic mat­ ter has been found to interfere in the fixation reactions of P with lime. This inhibition of P

Figure 2. Extractability and P uptake by barley from various sources following incubation in a 12% lime soil. Sources initially added at a rate of 60 mg P/kg; extractions are average of 2 and 6-weeks sampling dates. (Leytem and Westerman, 2005).

Figure 3. The effect of soil temperature on fertilizer P extractability in a calcareous soil µavid and Rowell, 2003).

fixation may account for the observation that P availability is frequently greater in manured soils and with the addition of humic substances in lime-rich soil. Higher levels of soluble Fe, Al, and Mn are also related to increased P fixation in calcare­ ous soils. Temperature Soil temperature has two opposing effects on soil P availability. When fertilizer P is added to soil, it con­ tinually reacts and forms increasingly stable compounds for many months after application. The kinetics of the conversion of P to less soluble forms is more rapid under warmer conditions than incooler soil (Figure 3). An opposite effect occurs as increased soil temperature raises the solubility of soil P forms (both adsorbed or precipitated P). This well-known phenomenon accounts for frequent crop responses from added P in cool soils in the spring. In addition to im­ proved solubility, higher soil temperature increases P diffusion to plant roots and en­ hances overall root activity and prolifera­ tion. When planting early in the season, or in high-residue conditions, cold soil tem­ peratures can induce an early-stage P de­ ficiency in many types of soil. A starter P fertilizer application may help overcome these limitations. Adj11sting for Calcareous Soil s—Since the presence of lime in soils can reduce P availability to crops, fertilizer recommen­ dations are frequently adjusted to account for this condition. For example, the Uni­ versity of Idaho recommendations for potat rtilization state that an additional 10 lb Pp/A needs to he applied for every 1% increase in soil lime (Figure 4).

 Figure 4. University of Idaho P fertilizer recom­ mendations for potatoes grown in calcareous soil take into account the free lime content of the soil (Tindall and Stark, 1997).

Calcareous soils can he extremely productive when managed properly. Maintaining an adequate supply of plant­ available P is essential to profitable and sustainable crop production. Since a vari­ ety of soil reactions tend to decrease the plant-availability of added fertilizer P in calcareous soil, regular soil testing should he conducted to avoid crop loss due to plant nutrient deficiency. ‘!ffl

The Nature of Phosphorus in Calcareous Soils, By A.B. Leytem and R.L. Mikkelsen, pdf

Fertilizer Chemistry

Agricultural Salts

Ammonium Nitrate –  NH4NO3.3400000
Ammonium Phosphate(NH4)3PO4.or ADP-MAP (NH4)(H2PO4).12610000
Ammonium Sulfate – (NH4)2SO421000024
Calcium Nitrate – Ca(NO3)2 or CaN2O615001900
Magnesium Nitrate – Mg(NO3)2(1100090
Magnesium Sulfate – MgSO 4,00001013
Potassium NitrateKNO 3.13046000
Urea – CO(NH2)2. carbamide4600000
Ammonium Ion

Lewis Dot Structure for the Ammonium Ion

Nitrate Ion

Nitrate Ion Lewis Structure: How to Draw the Lewis Structure for Nitrate Ion

How to Draw the Lewis Dot Structure for NH4NO3: Ammonium nitrate

How to Write the Formula for Ammonium nitrate

PO4 3- Lewis Structure: How to Draw the Lewis Structure for PO4 3-

ammonium nitrate
phosphate ion

How to Write the Formula for Ammonium phosphate

How to draw the (NH4)3PO4 Lewis Dot Structure (Ammonium Phosphate)

How to Draw the Lewis Structure for the Sulfate Ion

How to Write the Formula for Ammonium sulfate

sulfate anion so4 -2

How to draw the (NH4)2SO4 Lewis Dot Structure (Ammonium Sulfate)

ammonium sulfate

How to Draw the Lewis Dot Structure for Mg(NO3)2 : Magnesium nitrate

How to Draw the Lewis Dot Structure for KNO3 (Potassium Nitrate)

How to Draw the Lewis Dot Structure for CH4N2O / CO(NH2)2 : Urea

urea lewis structure



Salts- In chemistry, a salt is a chemical compound consisting of an ionic assembly of cations and anions.[1] Salts are composed of related numbers of cations (positively charged ions) and anions (negatively charged ions) so that the product is electrically neutral (without a net charge). Often a salt is an ionic compound in which the cation is a metal and anion is a nonmetal or group of nonmetals.

An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce the H+ cation and the anion of the acid.[1]

Element groupElement (central atom)Oxidation stateAcid formulaAcid name[8][9]Anion formulaAnion name
7Manganese+7HMnO 4Permanganic acidMnO4Permanganate
+6H 2MnO 4Manganic acidMnO2− 4Manganate
8Iron+6H2FeO4Ferric acidFeO42–Ferrate
13Boron+3H 3BO 3Boric acid (formerly orthoboric acid)[10]BO3− 3Borate (formerly orthoborate)
14Carbon+4H 2CO 3Carbonic acidCO2−3Carbonate
Silicon+4H 4SiO 4Silicic acid (formerly orthosilicic acid)[10]SiO4−4Silicate (formerly orthosilicate)
14, 15Carbon, nitrogen+4, −3HOCNCyanic acidOCNCyanate
15Nitrogen+5HNO 3Nitric acidNO3Nitrate
+3HNO 2Nitrous acidNO2Nitrite
Phosphorus+5H 3PO 4Phosphoric acid (formerly orthophosphoric acid)[10]PO3−4Phosphate (orthophosphate)
H 3PO 5Peroxomonophosphoric acidPO3−3Peroxomonophosphate
+5, +3(HO) 2POPO(OH) 2Diphosphoric(III,V) acidO 2POPOO2− 2Diphosphate(III,V)
16Sulfur+6H 2SO 4Sulfuric acidSO2− 4Sulfate
H 2S 2O 7Disulfuric acidS 2O2− 7Disulfate
17Chlorine+7HClO 4Perchloric acidClO 4Perchlorate
+5HClO 3Chloric acidClO 3Chlorate

Hydroxyl Group Definition

A hydroxyl group is a functional group that attaches to some molecules containing an oxygen and hydrogen atom, bonded together. Also spelled hydroxy, this functional group provides important functions to both alcohols and carboxylic acids. Alcohols are chains of carbon molecules with a functional hydroxyl group side chain. The electronegativity of the oxygen adds a slight polarity to alcohols, which is why they are able to interact with other polar molecules such as water and some solutes. Below is a general alcohol which contains a hydroxyl group. The oxygen is the red atom, while the hydrogen is represented by the grey atom. The R represent any generic carbon chain.

Carboxylic acids contain a hydroxyl group within their functional carboxyl group. A carboxyl group consists of a carbonyl group bonded to a hydroxyl group. A carbonyl group is simply a carbon double bonded to an oxygen. These two functional groups together create an extremely reactive molecule, which is prone to forming new carbon-carbon bonds. Along with alcohols, carboxylic acids are commonly seen in nature. A generic carboxylic acid with its hydroxyl group can be seen below.

Besides these two large classes of molecules that are functionally dependent on the hydroxyl group, many other molecules contain hydroxyl groups. As mentioned, a large part of the action caused by the hydroxyl group is due to the electronegativity of the oxygen. Because oxygen has a stronger attraction with the electrons bonding hydrogen to the molecule, the hydroxyl group can easily lose the hydrogen to an atom that will share electrons more equally. When this happens, the oxygen takes on a much more negative electrical energy, and can donate the extra electrons it has to a number of reactions. Biological organisms use this property of oxygen to help connect and disconnect chains of carbon molecules, which hold energy the organism can use to power cellular functions.

Related Biology Terms

Carboxyl group – A carbon doubled bonded to an oxygen and also bonded to a hydroxyl group.

Carbonyl group – A carbon double bonded to an oxygen and any other molecules, including more carbons.

Electronegativity – The attraction that an atom has for electrons, compared to the other types of atoms that it shares electrons with in covalent bonds.

Polarity – The property of a molecule that arises from the stable differentiation of electrical poles across a molecule or part of a molecule.

Nitric Acid- HNO3

Ammonium nitrate is the ammonium salt of nitric acid. It has a role as a fertilizer, an explosive and an oxidising agent. It is an inorganic molecular entity, an ammonium salt and an inorganic nitrate salt.

Phosphoric acid, H3PO4 or H3O4P ,  is a phosphorus oxoacid that consists of one oxo and three hydroxy groups joined covalently to a central phosphorus atom. It has a role as a solvent, a human metabolite, an algal metabolite and a fertilizer. It is a conjugate acid of a dihydrogenphosphate and a phosphate ion.

Ammonium dihydrogen phosphate is the ammonium salt of phosphoric acid (molar ratio 1:1). It has a role as a fertilizer. It contains a dihydrogenphosphate.

Sulfuric acid, H2SO4 or H2O4S ,  is a sulfur oxoacid that consists of two oxo and two hydroxy groups joined covalently to a central sulfur atom. It has a role as a catalyst. It is a conjugate acid of a hydrogensulfate

Ammonium sulfate, (NH4)2SO4,  is an inorganic sulfate salt obtained by reaction of sulfuric acid with two equivalents of ammonia. A high-melting (decomposes above 280℃) white solid which is very soluble in water

Calcium nitrate,  is inorganic nitrate salt of calcium. It has a role as a fertilizer. It is an inorganic nitrate salt and a calcium salt. It contains a calcium(2+).

            NO3 Nitrate is a nitrogen oxoanion formed by loss of a proton from nitric acid.

Magnesium sulfate is a magnesium salt having sulfate as the counterion.. It is a magnesium salt and a metal sulfate.

Potassium nitrate is the inorganic nitrate salt of potassium. It has a role as a fertilizer. It is a potassium salt and an inorganic nitrate salt.

Urea is a carbonyl group with two C-bound amine groups. It has a role as a flour treatment agent, a human metabolite, a Daphnia magna metabolite, a Saccharomyces cerevisiae metabolite, an Escherichia coli metabolite, a mouse metabolite and a fertilizer. It is a monocarboxylic acid amide and a one-carbon compound. It derives from a carbonic acid. It is a tautomer of a carbamimidic acid.

Al2(SO4)3.  Aluminum Sulfate Anhydrous is an aluminum salt

Agricultural lime, CaCO3. ,also called aglime, agricultural limestone, garden lime or liming, is a soil additive made from pulverized limestone or chalk. The primary active component is calcium carbonate.


Calcareous SoilsPlant Nutrition
Cation and Anion Exchange Capacityplant videos 1 chemistry
Chemistry pHplant videos 2
Fertilizer Chemistryplant videos 3 physiology
greenhouse techplant water and transport
MicronutrientsSoil and Roots
nutrient tablesoil science
Plant Cell Biologyspring-lake
Plant Cell PhysiologyStylistics
plant nutrients

Calcareous Soils

What is Calcareous Clay?

posted by Karen Orlandi on July 1, 2016 in Wine IQ

Many vineyard sites, especially the top tier french chateaux and domaines, describe their soil type as calcareous clay. Calcareous is actually an adjective meaning “mostly or partly composed of calcium carbonate.” Other synonyms for calcium carbonate include lime or chalk. Due to the low acidity level of calcium carbonate, the calcareous clay soil is quite alkaline compared to other soil types. The biggest benefits of calcareous soil are the special nutrients it supplies to the grapes, which make them grow better and sweeter. It also has the effect of keeping the soil at a cooler temperature, which can be helpful on hot days, but it also usually delays ripening, yielding a more acidic wine. The clay portion of the soil retains moisture, which helps the grapes during dry weather.

wine clay calcareous soil
Oamaru Shallow Clay LoamSouthlandTypic Rendzic Melanic SoilLithic Rendoll0 – 5 cm: Black calcareous clay loam, fine spheroidal structure, friable, non-sticky.
Bluff ClayHawkes BayCalcareous Orthic Melanic SoilTypic Rendoll0 – 30 cm : Very dark brown clay, strongly pedal, fine polyhedral macrofabric.

Excerpts- papers on nutrition in calcareous soils

Food Availability in Alkaline-Calcareous Soils (pdf)

The alkaline-calcareous soils of Arizona contain large reserves of phosphate, potassium, and calcium.   The availability rating of the phosphate reserve is very low under alkaline and calcareous conditions. The potassium reserve has a high availability rating. A large reserve of calcium is present as in the form of caliche, with appreciable amounts present as replaceable calcium, and as soluble calcium salts in the soil solution.


Before the question of efficient use of fertilizers can be ap- proached intelligently it is necessary to have            regard- ing the availability of fertilizer materials, their fixation in soils, and the influence of the many growth limiting or growth promot- ing factors which affect crop growth on irrigated lands. This investigation has added considerably to our knowledge of the solubility of the phosphate, potassium, and calcium compounds in alkaline-calcareous    their physiological availability, and some of the factors influencing this, and the comparative quantitative value of the Neubauer and carbonic acid methods for estimating this availability. Due to the difference in chemical composition of the phosphate,   and calcium compounds and minerals, as well as the fixation of these elements, their avail- ability is affected by a complex set of factors. The most tant single physiological affecting all is the buffer capacity of the          for this materially influences the H-ion concentration in the root-soil contact film. It is within this           boundary that plant food elements are converted from insoluble forms to physiological availability. This condition is magnified under alkaline-calcareous soil conditions, for all these soils are highly buffered.

has been frequently mentioned phosphate availability is the major plant food availability problem in alkaline-calcareous soils. In most part, the reserve phosphate in these soils is present as calcium carbonate-phosphate,

The availability of this compound, which is naturally very low, is further reduced by solid phase   (caliche) and sodium and calcium salts which are present in the soil solution, factors which are abundantly present in southwestern soils. All these factors reduce both the solubility and the absorption of phosphate ions by plant roots. A small part of the soil phosphate is probably present in replaceable         as shown by its solubility in alkali and the abundant soluble phosphate which exists in black alkali soils where hydroxyl ions exist in excess in the soil solution. On the other hand, since plant roots cannot assimilate phosphate ions in the presence of an excess of OH ions, this soluble phosphate in black alkali soils is of little use to crops unless [the pH] of the soil is lowered.

In view of the above, soluble phosphate fertilizers, at least nothing less soluble than single superphosphate, are recommended for these calcareous soils. Even the soluble phosphates are im- mediately fixed largely as tricalcium phosphate. According to Naftel (15) monobasic calcium phosphate, exists only within the pH range of 3.0 to 5.0; dibasic calcium phosphate, at pH 5,0 to 6.4; and tribasic, above pH 6.4.

It seems reasonable to suppose then that most of the soluble phos- phate used on Arizona soils is rapidly fixed as tribasic calcium phosphate. Since this fixed phosphate has been shown to possess contact availability and to maintain this state of availability over an extended period, it is apparent that the final conversion to calcium carbonate-phosphate is a very slow process. The evidence indicates then that soluble phosphates are fixed as tribasic cal- cium phosphate and that the reserve naturally in the soil is largely calcium present in the soil minerals largely in nonreplaceable and insoluble forms. Replaceable potassium is however present in sufficient abundance to meet present needs, andthe potassium problem is, as yet, not a serious one on Arizona soils.

Calcium is present in greatest part as with lesser amounts as replaceable calcium and soluble calcium salts in the soil solution. It has been generally assumed that calcium nutrition is not a problem of any consequence in calcareous soils.  However, due to the great difference in solubility of and         soluble calcium is very low in the more alkaline soils where bicarbonates do not exist. The investigations in this laboratory have shown a wide variation in the amount of calcium taken up by plants from    calcareous                   defin- itely proving a variant physiological availability. In many of these calcareous      notably the most alkaline       negative absorption of calcium by plants was observed for the seedlings grown in the Neubauer test.

For estimating the availability of these three elements a study involving a comparison of numerous plant cultures and the car- bonic acid method, for chemical soil analysis, has been made. While the absorption of phosphate, potassium, and calcium from soils is different for          plants, where a large number of soils are compared, and the data graphically presented, the rel- ative absorption is in good agreement. The Neubauer method, using the rye seedlings, can therefore be used as a measure of availability for many crops. Criticism of the method, on this basis, does not appear to hold valid for alkaline-calcareous Experiments do, however, show that if the carbonic acid method is, like other chemical methods, to be classed as empirical, then the same empiricism must be true for the Neubauer method. The carbonic acid method and the Neubauer method agree quite well in availability tests on calcareous soils.

It is shown that the Neubauer plant test is not strictly a method for determining the amount  of available plant food in the soil, in all plant tests on irrigated soils other growth limiting will affect the results. It may be better referred to as a measure of physiological availability. For interpreting the  value in terms of other crops some consideration must be given to their tolerance the growth limiting which commonly exist in irrigated soils.

The carbonic acid method is specifically a measure of solubility or availability without consideration        the            which deter- mine physiological availability or absorption.   With a knowledge of the tolerance of crops for these factors the carbonic acid method has considerable intrinsic merit for alkaline-calcareous soils and appears to be at least on a par with the Neubauer test.

The empirical nature of the Neubauer test is shown by the re- sults obtained from the experiments where the number of plants was kept constant at 100 and the weight of soil in each culture was On the whole most of the soils showed closely constant

values above 100 plants per 80 grams of soil. One hundred plants per 100 grams of soil appears therefore to be a fair arbitrary proportion.

There are certain advantages in both the Neubauer and carbonic acid methods measuring available plant in soils. With the Neubauer method affecting physiological absorption come into     while the carbonic acid method makes no measure of these measures only the available plant food and consideration must be given to growth limiting factors. A


study of some of these factors showed that pH of the soil and its related influence has a major effect on absorption of phosphate, and calcium by the plants. All increased as the pH of

soil decreased. Of the soluble salts present in irrigated soils the calcium salts have the greatest influence on            and this is even true for calcium carbonate which is only very slightly soluble.

The empirical nature of the Neubauer method was also shown by experiments in which the weight of soil and number of plants per culture were varied. The magnitude of the value, on a 100 plant           decreases with increase in weight of soil and number of rye plants used in the cultures. In other words the extraction of phosphate, potassium, and calcium per plant, from the soil, is greater the smaller the number of plants grown. For alkaline- calcareous soils the ratio of 100 plants to 100 grams of soil may be accepted as useful, although not the best ratio. Using weights of soil varying from 10 grams to 200 grams per 100 seedlings the extraction reaches a maximum at 80 to 100 grams soil. The phosphate extraction remains practically constant above this. No sat- isfactory explanation of this constancy can be offered at this time, although several suggestions have been presented in the text. The minimum Neubauer value for soils is 10.6  yet few Arizona soils except those      with phosphate reach this minimum. This is true regardless of the fact that in these experiments as high as 200 grams of soil per 100 plants were used, and still the values not only failed to reach the value of 10.6 but even showed no increase above the value obtained for 100 grams of soil. This indicates that smaller weights of soil might be more desirable for the Neubauer   test   on   alkaline-calcareous possibly 50 grams soil per 100 plants. This ratio of weight of soil to number of plants would be on a point well below the 100/100 ratio and within the range where the curve is increasing with in- crease in weight of soil rather than at the point where the value has become constant.

The above condition does not hold true in entirety potassium. The magnitude of the      Neubauer values, like the         values, does decrease with increase in number of plants and weight of soil per culture. Also on maintaining the number of plants constant at 100 and increasing the weight of soil from 10 grams to 200 grams the K values become practically constant at 100 grams of soil. However, the values do not become constant until they have extended well beyond the minimum K value of namely, 19.9 mgm. K. Potassium availability then in alkaline- calcareous soils is          from that of phosphate where regard- less of weight of soil above a certain soil-plant ratio few soils will reach the minimum                      value established by Neubauer himself.

The addition of soluble phosphate fertilizer to the cultures showed that phosphate fertilization will increase the absorption  f potassium by rye plants and decrease the absorption of calcium, depending somewhat upon the amount of phosphate added.

On the basis of previous phosphate investigations only soluble phosphate fertilizers are recommended for Arizona soils. Even these phosphates are immediately fixed and do not move far from the place of application. For this reason a part of investiga- tion involved a study of the availability of this fixed phosphate. It appears to be a precipitation in the form of tribasic calcium phosphate with some sorption by the soil colloids. The data show definitely that while the fixed phosphate is not mobile in the soil solution it is readily available when in contact with feeding roots. Both the Neubauer method and the carbonic acid extraction method are           in measuring this fixed phosphate in calcareous soils.


1.         A comparison of the quantitative removal of phosphate, and calcium from                       soils by rye, wheat, cowpeas,         tomatoes, and corn show a good

relative agreement for rye, barley, wheat, and tomatoes and a fair relative agreement of rye with           corn, and hegari.

2.         There is a good correlation between the availability of phos- phate, calcium, and potassium as measured by the carbonic acid method and the availability values obtained from the Neubauer test.

3.         Some factors which are related to or which influence the ab- sorption of these three ions are as

a.         Phosphate absorption increases with decrease in pH, decrease in water soluble calcium, carbonic acid soluble calcium, and calcium carbonate.

b.         Potassium absorption increases with decrease in pH value.

c.         Calcium absorption increases with decrease in pH value.

d.         Phosphate Neubauer values correlate with the carbonic acid soluble phosphate and with the Ca Neubauer values.

e.         Potassium Neubauer values correlate with increase in Ca Neubauer values, carbonic acid soluble potassium, and replace- able potassium.

f.          Calcium Neubauer values correlate with carbonic acid sol- uble calcium, and calcium carbonate but inversely with replace- able calcium.

4.         In varying the weight of soil in each 100 plant Neubauer culture there is a steady increase in         value or           per 100 plants up to 80 to 100 grams of soil, above which the value is closely constant.

5.         For potassium the Neubauer K value is closely constant above 100 grams of soil.

6.         For calcium the Neubauer value curve shows less constancy, reaching maximum values at 40 to 90 grams soil, beyond which it decreases.


7.         When both number of plants and weight of soil are varied in the Neubauer test, the value on the basis of 100 plants de- creases as the number of plants and weight of soil are increased.

8.         When weight of soil is held constant and number of plants increased, there is a decrease in Neubauer value on a basis with increase in weight of soil.

9.         The suggestion is offered on the basis of this investigation that even though the results obtained with a 100 plant 100 grams of soil ratio are useful and arbitrary, using lesser amount of soil might enhance its value. The curves obtained, both    phosphate and potassium, on plotting values obtained by maintaining the number of plants constant at 100 and increasing the weight of soil per culture, show practically horizontal curves above 80 to 100 grams of soil. In other words the absorption is at a maximum and practically constant above this. It is believed that some point below 100 where the values are steadily increasing, possibly 50 grams of soil, would give more satisfactory values.

Phosphorus Availability with Alkaline/Calcareous Soil (pdf)

Phosphorus (P) is an essential nutrient required by plants for normal growth and development. The availability of P to plants for uptake and utilization is impaired in alkaline and calcareous soil due to the formation of poorly soluble calcium phosphate minerals. Adding fertilizer P at “normal” rates and with conventional methods may not result in optimal yield and crop quality in these soils common in arid and semi-arid regions. Several fertilizer P management strategies have been found to improve P nutrition for plants grown in alkaline and calcareous soil. Research results show that relatively high P fertilizer rates are required for crops grown in alkaline soil, with increasing rates needed as lime content in these soils increases. Concentrated P fertilizer bands improve P solubility with resulting yield increases, even when applied to crops grown in soil with relatively high soil test P concentrations. Applying organically complexed P in the form of biosolids or as a mixture of liquid P and humic substances can also enhance P nutrition and result in yield increases. Application of slow release and cation complexing specialty fertilizer P materials has also been shown to effectively increase yields in calcareous soil. In-season applied P through the irrigation water can deliver P to plant roots when deficiencies are observed, but the effectiveness and results are less than with P incorporated into the soil. Finally, it is important to maintain a proper balance of P with other nutrients for general plant health and to avoid excess nutrient induced deficiencies of other nutrients. In some cases, these methods are relatively new and need further refinement with regard to rates, timing, and technique; but all are potential methods for improving P supply to plants grown in alkaline and calcareous soil.


Substantial research on nitrogen in potato production shows the value of split applying this nutrient, with a majority applied during the peak uptake rates during the season (Stark and Westermann, 2003). Nitrogen fertilizer quickly converts to the nitrate form, which is easily leached or volatilized under conditions with ample soil water. Furthermore, nitrate does not form poorly soluble minerals and, thus, is not subject to solubility problems. Alternatively, P is not typically subject to leaching loss and does not volatilize. And, as mentioned previously, P readily forms poorly soluble soil minerals. Therefore, the chemistry of P does not lend itself to movement in the soil and, as such, it is reasonable to assume that P should be incorporated into the soil zone where roots grow and take up nutrients.

At times, tissue analysis shows that a crop is potentially P deficient despite pre-season fertilization efforts. Can P be applied in-season to some benefit? Recent research in potatoes shows that, although not as efficient as pre-plant broadcast/incorporated fertilizer, P can be applied with some benefit during the season as an injection in the irrigation water (Hopkins and Ellsworth, 2003). However, it is recommended that every effort should be made to supply adequate P with other methods and use the in-season P application as a last resort if tissue analysis shows deficiency. Foliar applications of P may also be beneficial, but should not take the place of a good pre-plant fertilizer P program (data not shown).


Adding P in combination with ammonium tends to enhance availability of both nutrients. Ammonium and other acidifying fertilizer materials can enhance P solubility and uptake by roots. In general, it is also wise to have generally sufficient nutrient levels of other nutrients to promote overall good root development, which is so important for P interception and uptake.

Excessively high levels of certain nutrients can induce deficiencies of others. High rates of zinc, iron, manganese, and copper can induce deficiencies of P and visa-versa. This effect has been observed in many crops. Recent work in Idaho shows that excess P can result in potato yield and quality loss, which may be overcome with addition of zinc fertilizer (Hopkins and Ellsworth, 2003). Although this effect was observed by other researchers and in one year of this trial, no yield loss was observed at rates as high as 600 lbs P2O5/acre in two other years of the trial. There are also many claims with regard to optimum P-micronutrient ratios, but these purported ratios are not generally based on experimental data collected under field conditions. In fact, it is common to see a variety of soil P-micronutrient ratios in fields with exceptionally high yields, leaving the concept of optimal ratios in doubt. More work is needed to determine if optimum ratios exist and, if so, the width of this range.


Phosphorus is an important and essential nutrient for all plants. Availability of P in high pH soils, especially those with excess lime, is relatively poor. Lowering pH is not an economical option for most crops and, as such, other strategies must be employed to enhance P uptake by roots, including: 1) relatively high P fertilizer rates, 2) concentrated P fertilizer bands, 3) complexed P fertilizer, 4) slow release fertilizer P, 5) cation complexing P fertilizer, 6) in-season P fertilizer application, and 7) balancing P with other nutrients.

Plant-soil-nutrient status of vegetables and wheat grown oncalcareous soil (pdf)

Calcareous soil is extremely important in determining nutrient status and availability when applying different fertilizers to different crops associated with or without irrigation. The objective of this study was to investigate the nutritional status (nutrient availability) of different plants grown in two different ecosystems dominated with calcareous soil : Ghor Alsafi (eggplants, tomato and beans) as an example of irrigated area and the Karak Mountain Area (wheat) as an example for non-irrigated area. Physiochemical properties in soils and nutrients concentrations (C, Fe, Mn, Zn, available P, available and non-available K) in soil and plant leaves were analyzed. This study showed that N concentration in soil from both Ghor Alsafi (ranged from 4.4-5.1 mg/g) and the Karak Mountain Area (1.9-3.2 mg/g) was relatively low. This study showed that Fe content in vegetables grown in Ghor Alsafi was about 3 to 5 fold higher than the recommended maximum Fe concentration. The concentration of other nutrients (N, K, Zn and Mn) in investigated vegetables fell within the recommended concentrations range (38.3-39.7 mg/g, 36.6-39.5 mg/g, 21.7-27.3 ppm and 49.2-102.6 ppm, respectively). Phosphorus content in vegetables grown in Ghor Alsafi and in wheat grown in different locations in the Karak Mountain Area was about 5?6 fold higher than the recommended P concentration. Our results indicated that the high levels of P (ranged from 3.28-3.43 mg/g) and Fe (ranged from 340.4-551.3 ppm) in vegetables grown in Ghor Alsafi and high levels of P (ranged from 2.2-2.52 mg/g) in wheat in the Karak Mountain Area can be attributed to high levels of fertilizer application.


It can be concluded from this study that farmers in both the areas studied (Ghor Alsafi and the Karak Mountain Area) used improper cultivation practices, which was more noticeable in Ghor Alsafi. The continual addition of P rich fertilizer increased the P content in soil above the optimum level required by crops. This study revealed that farmers in the Karak Mountain Area did not use enough K rich fertilizers. Crop rotation could be an effective approach to maintain soil fertility in the mountain area. Further research should be conducted to minimize nitrogen losses through the volatilization of NH3 from calcareous soil

Nitrogen Management in Calcareous Soils: Problems and Solutions (pdf)

Nitrogen (N) is the most widely applied plant nutrient and is a key constituent of animal manures. Improvement in crop yields and high economic returns have been made possible through supplementation of crops with organic and inorganic fertilizers. The movement of N in the environment has been extensively studied and documented. The fate of N ranges from nitrification, denitrification, nitrous oxide formation, leaching of nitrate, and volatilization of ammonia. Even with this knowledge the variation in N movement within the soil, air and water varies greatly with changed edaphic factors. For instance the fate of N fertilizer under calcareous soil systems still has not been investigated widely. We reviewed the sporadic information available on N fate and management in calcareous soils especially in Pakistan. We discussed the sources and fate of N in calcareous soils of Pakistan and also the currently adopted and newly developed method to reduce the N losses.


 Even though extensive research has been conducted on N dynamics and management worldwide, still a lot of gaps are present in information regarding fate of N fertilizers in soil and its impact on crop growth and environment in Pakistan’s agricultural systems. Most of work found was conducted under controlled conditions in pot and lab trials that lack field validation. According the published data available, nitrogen use efficiency (NUE) for major crops including wheat, maize and rice seldom exceeds 40%. In Pakistan, about 22 to 53% NH3 is predisposed to atmosphere due to high soil pH and hot climatic conditions. Studies have also shown that soil moisture content and salinity have a combined effect on increasing NH3 volatilization. The second major source of N losses in Pakistan’s agro-climatic conditions is de-nitrification. The ammonium fertilizers are quickly nitrified because of warm climate and NO – is vulnerable to denitrification following reduced conditions or flooding.

Nitrogen losses through volatization may be reduced by upto 80% by adopting a few management practices such as application of irrigation after 8 hours of urea application, placing the urea below 3-5 cm below the surface under moist soil conditions and urea co-applied with potash. Use of urease inhibitors such as N-(n-butyl) thiophosphoric triamide (NBPT) or phenyl phosphoro diamidate (PPD) and polymer coating of N fertilizers have shown significant reduction in volatilization. However, information is lacking on their effects under various cropping systems in Pakistan. More recently the use of cheap natural plant products or byproducts including neem (Azadirachta indica), Karanj (Pongomia glabra), vegetable tannins, waste products of tea, mint oil, Japanese mint and Mustard (Brassica juncea L.) are being used as nitrification inhibitors and to increase the NUE (Personnel Communication). However, no extensive research has been conducted on use of these natural substances in the field which is direly needed. Research is still lacking in understanding and quantification of N budgets in different cropping zones and scientist should conduct research on this aspect as well as test new and cheap means to reduce the losses of N in calcareous soils of Pakistan.

Nitrogen Application in Warm, Dry Weather

e have been seeing relative warm, dry, windy weather in many parts of the state over the last week or so, which has raised some questions about higher potential for ammonia-N volatilization loss from surface applied urea containing fertilizers. Applying any urea containing fertilizer to the soil surface during warm, dry, windy conditions will maximize the potential for N volatilization losses. This loss occurs quickly, starting within hours following application with most of the loss occurring within 2 days following application. If the N is going to be incorporated by tillage or rainfall, it is critical that tillage be done or rainfall occurs as soon as possible following application. To minimize losses this should happen within a day following application. One strategy is to delay application a few days if rain is in the forecast. However, an important consideration is, that under generally dry conditions with occasional small showers, a little bit of rain, a few tenths of an inch, can actually make the loss worse because it is just enough to dissolve the urea and activate the volatilization process, but not enough to soak the urea into the soil to minimize the loss. Management options under these conditions include immediate incorporation of the urea by tillage. In no-till or where immediate tillage is not possible, there are other options.

Nitrogen in the Environment: Denitrification

oil microorganisms need oxygen for fuel. When the soil is very wet, water fills in the spaces between soil particles. This leaves very little room for oxygen. Some soil microorganisms can get the oxygen they need from the oxygen portion of the nitrite (NO2-) and nitrate (NO3-) forms of nitrogen. When this happens, nitrogen (N2) and nitrous oxide (N2O) gas are formed. These gases return to the atmosphere, and there is a net cycle in the soil. This is called denitrification

Two main factors influence denitrification:

  • The oxygen supply in the soil.
  • The soil microorganisms.

Anything that changes these two factors will change how much nitrogen is lost and how fast this happens. These factors include the amount of organic matter, soil water content, soil oxygen supply, soil temperature, soil nitrate levels and soil pH.

A small amount of denitrification can be found taking place in soils all the time. Denitrification becomes significant when the soil is waterlogged for 36 hours or more. The longer the soil is waterlogged, the greater the potential loss of nitrogen from the soil system.

Once nitrates get into the groundwater, the greatest concerns are for infants less than one year old and for young or pregnant animals. High levels of nitrates can be toxic to newborns, causing anoxia, or internal suffocation. Seek alternative water sources if nitrate levels exceed the health standard of 10 ppm nitrate-N. Do not boil water to eliminate nitrates. It increases nitrate levels rather than decreases them. The most common symptom of nitrate poisoning in babies is a bluish color to the skin, particularly around the baby’s eyes and mouth. These symptoms of nitrate toxicity are commonly referred to as the “blue-baby” syndrome.

Soil–Plant Nutrient Interactions on Manure-Enriched Calcareous Soils (pdf)

As there are a limited number of ion carrier sites on root plasma membranes, ions with similar diameters and ion strength can outcompete each other for space on these sites (Marschner, 2011). Potassium is known to be a strong competitor against Mg, Ca, and Na, and can restrict uptake of these nutrients when in abundant supply (Mengel, 2007). In addition to P, dairy manures also contain high quantities of K, which is a common dairy feed additive. In relation to fertilizer sources, excessive quantities of K has decreased Ca and Mg tissue concentrations in forage sorghum [Sorghum bicolor (L.) Moench] (Reneau et al., 1983) and in corn (Claassen and Wilcox, 1974), decreased Mg tissue concentrations in sorghum (Ologunde and Sorensen, 1982), and triggered Mg deficiencies in orange trees (Citrus spp.) (McColloch et al., 1957). Parsons et al. (2007) found significant reductions in Ca uptake by wheat receiving manure applications in comparison to fertilizer or an unamended control. The authors noted that Ca uptake may have been hindered by competition with K. Leytem et al. (2011) also found reduced Ca uptake in corn with increasing dairy manure application rates. In addition to cation competition, the authors suggested that the reduction in Ca uptake on manure-treated soils may also be related to the formation of Ca-phosphate precipitates.

Excess K application may also increase the potential for lu xury K consumption by plants. When K supply is abundant, luxury K consumption is known to be an issue in many crop plants (Marschner, 2011). Unfortunately, dairy cows that consume forage tissue with K concentrations of 1.5% or greater are at risk of milk fever (Penn State University Extension, 2013), explaining why many animal producers are concerned with luxury consumption of K by forage crops. As described above, K is known to accumulate in manure-treated soils, and could therefore be a potential issue for growers who use their crop as an animal feed.


From this research, we found correlations between soil nutrient levels and tissue nutrient levels for corn silage grown on nutrient-enriched calcareous soils with reported dairy manure application histories.

 Specifically, we found positive linear correlations between

 soil test K and tissue N,

soil test B and tissue N, and

soil test K and tissue K

concentrations, and found a significant negative inverse relationship between

 soil test Fe and tissue Mn concentration.

These findings suggest that growers producing corn silage on alkaline soils receiving dairy manure applications should consider monitoring plant tissue for N, K, and Mn concentrations to avoid reaching toxic (N or K) or deficient (Mn) levels. Our findings also suggest that interactions such as P–Zn and cation competition between K, Ca, and Mg may not be a major issue on nutrient-enriched alkaline and calcareous soils with dairy manure application histories. Finally,

there were significant soil accumulations of [N], K, Fe, Zn, and B associated with increasing Olsen P accumulations, suggesting that dairy manures are significant sources of these five nutrients in the Snake River Region of southern Idaho. Controlled studies are needed to further validate the interactions found in our study.

Legumes: Model Plants for Sustainable Agriculture in Calcareous Soils (pdf)

The most typical root responses of plants in increasing the solubilisation of soil P is the decrease in rhizosphere pH by the release of H+ and phosphatases acids to the rhizosphere [12]. Enhancement of acid phosphatase activity (APases) under P starvation conditions has been demonstrated for N2- fixing legumes including Vicia faba [12], Lupinus albus and Glycine max [13, 14]. Studies in Vicia faba and Vicia sativa showed that P deficient plants increased both extracellular APases, which are involved in hydrolysis of soil’s various organic phosphate monoesters, and intracellular enzymes acting in the remobilization of Pi from rich P components inside the plant cell [12].

With P deficiency, the concentration of phenols increases to improve the availability of the sparingly soluble soil P for plants [15]. In Vicia faba, the exudation of phenolic compounds have been reported a higher concentrations in P-deficient plants [12]. Likewise, it has been demonstrated that Lupines albus and Brassica napus released large amounts of phenolics into the rhizosphere in response to P deficiency [16]. The maintenance of the P-homeostasis in nodules is considered as a main adaptive strategy for rhizobia- legume symbiosis under P starvation by increasing P allocation to nodules, formation of a strong P sink in nodules and direct P acquisition and remobilization [15].

In the face of Fe deficiency widespread in calcareous soils, legumes have evolved adaptive strategies in order to tolerate the stress and increase Fe bioavailability [15]. The main adaptive responses are the rhizosphere acidification by the activation of plasma membrane H+-ATPase, the stimulation of the reduction of Fe3+ to Fe2+ by a NADPH-dependent Fe (III)-chelate reductase (FCR) and the stimulation of root exudates [17]. Several research have been described the genetic variability among legumes regarding their tolerance to Fe starvation. In fact, there are numerous studies aimed to identify and select legume species and genotypes tolerant to Fe deficiency with the potential to increase agriculture production in calcareous soils [18]. assessed the genetic variability among 12 cultivars of peanut (Arachis hypogaea L.) in tolerance to iron deficiency based on spectral and photosynthetic parameters. These authors selected three groups according to their tolerance to Fe deficiency:


In arid and semiarid regions, alkalinity is an important environmental constraint for nutrients bioavailability especially Fe and P, which leads to lower crop production. Factors that contribute to Fe and P deficiencies in calcareous soils are high pH values and bicarbonate concentrations. Improving crop production in such soils demand adoption of special management practices which aims to ensure sustainable agriculture that respects the environment. The introduction of legumes in calcareous soils sustain productive agriculture. Hence, it is primordial to select rhizobia strains and legumes genotypes with enhanced tolerance able to thrive under nutrient limiting conditions and subsequently the use of these selected genotypes in crop rotation.

Iron deficiency and chlorosis in orchard and vineyard ecosystems (pdf)

A significant part of the fruit tree industry in Europe and especially in the Mediterranean area is located on calcareous or alkaline soils, which favour the occurrence of Fe chlorosis. Fruit trees and grape species differ as to their susceptibility to Fe chlorosis, but it is widely accepted that  peach, pear, and and kiwifruit are among the most susceptible to Fe chlorosis (Korcak, 1987). Vitis spp. also differ as to their susceptibility, some being very tolerant (e.g. V. _inifera and V. rupestris) others being susceptible (e.g. V. riparia).


Several perennial, deciduous, as well as evergreen fruit crops develop symptoms of iron deficiency— interveinal chlorosis of apical leaves— when cultivated in calcareous and alkaline soils. Under these conditions fruit yield and quality is depressed in the current year and fruit buds poorly develop for following year fruiting. This paper reviews the main fundamental and applied aspects of iron (Fe) nutrition of deciduous fruit crops and grapevine and discusses the possible development of sustainable Fe nutrition management in orchard and vineyard ecosystems. Cultivated grapevines and most deciduous fruit trees are made up of two separate genotypes the cultivar and the rootstock, providing the root system to the tree. The effect of the rootstock on scion tolerance of Fe chlorosis is discussed in terms of biochemical responses of the roots to acquire iron from the soil. Symptoms of iron chlorosis in orchards and vineyards are usually more frequent in spring when shoot growth is rapid and bicarbonate concentration in the soil solution buffers soil pH in the rhizosphere and root apoplast. Since the solubility of Fe-oxides is pH dependent, under alkaline and calcareous soils inorganic Fe availability is far below that required to satisfy plant demand, so major role on Fe nutrition of trees is likely played by the iron chelated by microbial siderophores, chelated by phytosiderophores (released into the soil by graminaceous species) and complexed by organic matter. As most fruit tree species belong to Strategy I-based plants (which do not produce phytosiderophores in their roots) Fe uptake is  preceded  by  a reduction step from Fe3+ to Fe2+. The role of ferric chelate reductase and proton pump activities in Fe uptake and the possible adoption of these measurements for screening procedure in selecting Fe chlorosis tolerant rootstocks are discussed. In a chlorotic leaf the existence of Fe pools which are somehow inactivated has been demonstrated, suggesting that part of the Fe coming from the roots does not pass the leaf plasmamembrane and may be confined to the apoplast; the reasons and the importance for inactivation of Fe in the apoplast are discussed. The use of Fe chlorosis tolerant genotypes as rootstocks in orchards and vineyards represents a reliable solution to prevent iron chlorosis; in some species, however, available Fe chlorosis resistant rootstocks are not very attractive from an agronomic point of view since they often induce excessive growth of the scion and reduce fruit yields. As most fruit tree crops and grapes are high value commodities, in many countries growers are often willing to apply synthetic Fe chelates to cure or to prevent the occurrence of Fe deficiency. The application of iron chelates does not represent a

Iron chlorosis is a more complex phenomenon in fruit trees than in annual crops (Tagliavini et al., 2000a). Deciduous fruit trees and grapes ex- hibit reproductive cycles starting with bud forma- tion in one year and ending with flowering, fruit set and fruit maturity the following year. Esti- mates of chlorosis severity among pear orchards in the Po Valley (Northern  Italy)  (Scudellari, 1999, personal communication) suggest that trees bearing a large amount of fruits in one year are more prone to show a severe chlorosis development the following year. This phenomenon, in accordance with findings by Pouget (1974), has been explained considering the fact that fruits represent a strong sink for  carbohydrates,  of which storage at root level might be insufficient to sustain root growth and activity during growth resumption in spring. In this context it is  of interest that Fe is only taken up by root tips (Clarkson and Hanson, 1980) and, therefore, the number of root tips produced by a rootstock in spring may have an influence on Fe uptake.

Another peculiar aspect of Fe nutrition in trees is related to their size and to the fact that, after absorption, Fe has to be transported for a long distance to reach the tree canopy. Problems in Fe transport through the xylem are, therefore, more likely in mature trees. Symptoms of iron chloro- sis-typical interveinal yellowing or sometimes atypical uniform chlorosis as in pear-in orchards and vineyards often start as soon as buds open, likely as a result of insufficient storage of Fe, or develop throughout the vegetative season as a consequence of plant demand being excessive in respect to Fe availability. In general, however, chlorosis occurs more frequently in spring when rainfalls cause a raise in soil bicarbonate concen- tration (Boxma, 1982) in a period of intense Fe demand. If soil conditions then improve, new leaves appear green, but  those  previously chlorotic unlikely re-green. Fruit yield losses caused by leaf chlorosis also depend on the degree and the period the chlorosis develop and, in gen- eral, critical periods coincide with blooming and fruit set: this particularly applies to fruits not easy to set, like pears (Pyrus communis) or those like kiwifruit (Actinidia deliciosa), of which the final size strongly depends on seed number. A rela- tively low level of chlorosis is likely more accept- able at other phenological phases, especially if it is confined to parts of the canopy where only vege- tative and not reproductive buds are present.

Chlorotic symptoms also vary  from  year  to year as a result of several tree and environmental variables, like yields, temperatures, rains. In soils where shallow layers are less rich in CaCO3 than deeper layers, it is likely that trees and vines develops chlorosis only when they age and roots explore layers with poor conditions for Fe uptake.

Studying the characteristics of soil profile pro- vides a useful tool for understanding such prob- lems. To our experience, soils which had been for many years subjected to ploughing before the plantation may present layers of fine texture, just below the ploughing depth, which could be rich in CaCO3 because of leaching from more shallow layers.

Symptoms of iron chlorosis are not always uniform within a single tree, where chlorotic parts of the canopy are often present  together  with green branches. Little attention has been given to explaining this phenomenon, but few hypotheses can be made: (1) tree root systems exploit a relatively large volume of soil with heterogeneous characteristics, some roots developing in mi- crosites favourable to Fe uptake while others are located in poor areas. Due to the poor phloem mobility of the iron absorbed by one root, redis- tribution of iron from green canopy parts to chlorotic ones is unlikely; (2) since iron is mainly transported in a non-ionic form in the xylem (Tiffin, 1970) its transport, driven by the transpi- ration stream, is likely not uniform. It has been reported (Tagliavini et al., 2000a) that within the same tree or vine, leaf Fe chlorosis is often more severe on second or third order branches than on those directly branching off the trunk.

Problems in transport of Fe  may  also  arise from the fact that in some species (e.g. pear), a certain degree of scion/rootstock (usually quince) incompatibility occurs and is sometimes desirable as it allows a control of the tree size. Graft incompatibility, however, impairs both upward nutrient transportation through the xylem and downward carbohydrates replenishment to  the roots through the phloem (Breen, 1975). Under these conditions, root reactions to Fe deficiency, such as the increase of Fe-reduction activity and proton extrusion (Mengel and Malissovas, 1982), may be impaired because of a lack of organic carbon supplied from the shoots.

Iron demands by mature orchards and vine- yards are in the range of 650 – 1100 g Fe ha−1 per year  (Ga¨rtel,  1993):  net  removals  are  mainly  ac- counted by amounts recovered in yields and prun- ing wood, if not left in the ground and chopped, while the Fe amounts in the perennial framework,

 with a relatively constant biomass do not signifi- cantly vary from year to year and are, therefore, negligible. For kiwifruit, fruit Fe concentrations of 33 µg g−1 (DW) were estimated, resulting in a total removal of Fe in fruits of around 160 g ha−1 for a fruit production of 30 t ha−1. For several flesh fruit crops (Tagliavini et al., 2000b) removals are in the range of 1 – 10 g t−1 of harvested yield. Annual removal of Fe by pruning wood were estimated by Abadia et al. (personal communica- tion, 2000) for peach trees in Northern Spain being in the order of 150 g Fe ha−1 and similar Fe amounts were estimated returning to the soil through the leaves after their abscission.

2.2.         Soil iron availability

As previously described, annual removals  of iron in orchards and vineyards are relatively low. Total amounts of iron in cultivated soils, in the- ory would not justify the development of iron deficiency, which, nevertheless, often occurs as a result of poor availability of iron for plants. Sev- eral soil-related characteristics may lead to devel- opment of iron chlorosis (Table 1).

The prediction of risks of future development of iron chlorosis in a plantation is of great impor- tance in fruit tree and grape industry and should lead to the correct choice of the rootstock to be used. Mistakes at this stage would make unlikely the achievement of satisfactory yields without agronomic and chemical means for correcting the chlorosis throughout the life span of the orchard or vineyard. Due to the number  of  soil  factors that impair Fe nutrition, it is not always easy to predict the possible chlorosis development of a perennial crop on the basis of a single soil parameter. Soil pH is often a useful but not sufficient parameter: it is well known that fruit crops adapted to acidic soils quickly develop chlorosis at sub-alkaline or alkaline  conditions (e.g. blueberry, raspberry, kiwifruit) while other genotypes are more able to cope with high soil pH (likely through an inherent ability to lower root apoplastic pH), unless the soil is also calcareous and, therefore, buffered in the range of 7.5 – 8.5 (Loeppert et al., 1994). Total lime, however, is not particularly useful for predicting the development  of iron chlorosis, while the fine, clay-sized, frac- tion of CaCO3, active carbonate or active lime (Drouineau, 1942), is more reactive and, there \ fore, able to build and maintain high levels of HCO− in the soil solution (Inskeep and Bloom, 1986), and is, therefore, often a more reliable (than High soil pH) indicator. Species are ranked according to the level of active lime at which they start to develop chlorotic  symptoms:  very  susceptible  species  or

Other iron containing compounds

Ferrous sulphate, either applied to the soil or to the tree (leaf treatments,  trunk  injection),  has been the major therapy against Fe chlorosis from the first description of this nutritional  disorder until the introduction of Fe synthetic chelates and is still widely used by fruit growers especially in the developing countries due to its low costs. If supplied alone, however, soil applied Fe(II)sulphate is of little or no agronomic value in calcareous soils where the Fe2+ is subject to rapid oxidation and insolubilisation as hydroxide. For example, Fe sulphate was not effective for curing Fe chlorosis in A. deliciosa in a soil with a high CaCO3 content (32%), while a quite complete recovery was achieved by Fe– EDDHA (Loupas- saki et al., 1997).

The effectiveness of soil applied Fe sulfate may be improved by combining iron sulphate with organic substrates able to complex the Fe (e.g. animal manures, sewage sludges, compost, peat, plant residues). Canopy applied Fe sulphate also represents a valuable, inexpensive, alternative to foliar applied synthetic Fe chelates (Tagliavini et al., 2000a) to cure iron chlorosis.

soil injection of vivianite was slightly less effec- tive, but showed a more lasting impact. Vivianite application has the advantage of being relatively inexpensive and is directly prepared by growers simply mixing ferrous sulphate with di-ammo- nium (or  mono-ammonium)  phosphate  (Rosado et al., 2000). The effectiveness of soil applied Fe amorphous minerals is presumably due to the fact that they are more easily mobilised by plants and microorganisms as compared with crystalline Fe forms (Loeppert et al., 1994).

Use of vivianite (Fe3(PO4)2.8H2O) to prevent iron chlorosis in calcareous soils

For various reasons, iron phosphate might be effective in correcting Fe chlorosis in calcareous soils. To test this hypothesis, several pot experiments were conducted using an Fe chlorosis-sensitive chickpea (Cicer arietinum L.) cultivar cropped in soils to which partially oxidized vivianites (Fe3(PO4)2.8H2O) and Fe(III) phosphates with different characteristics had been added. Vivianites mixed with the soil at a rate of 1 g kg−1 were as effective in preventing chlorosis as Fe chelate (FeEDDHA). However, the effectiveness of Fe(III) phosphates was less, suggesting that the presence of Fe(II) in the phosphates used was a key factor in their Fe-supplying value to plants. The effectiveness of vivianites, however, seemed to be largely independent of their Fe(II) content.

Citrus Fertilizer Management on Calcareous Soils (pdf)

Citrus fertilizer management on calcareous soils differs from that on noncalcareous soils because of the effect of soil pH on soil nutrient availability and chemical reactions that affect the loss or fixation of some nutrients. The presence of CaCO3 directly or indirectly affects the chemistry and availability of nitrogen (N), phosphorus (P), magnesium (Mg), potassium (K), manganese (Mn), zinc (Zn), and iron (Fe). The availability of soil copper (Cu) is also affected; however, since the citrus Cu requirement is normally satisfied through foliar sprays of Cu fungicides, it is not discussed in this fact sheet. concentration, unless a significant quantity of sodium (Na) is present.

Many Florida flatwoods soils contain one or more calcareous horizons, or layers (see Table 1). A typical characteristic is an alkaline, loamy horizon less than 40 inches deep, which can be brought to the surface during land prepara- tion for citrus planting. These soils are important for citrus production in the Indian River area (east coast) and, to

 Soil pH affects the rates of several reactions involving N and can influence the efficiency of N use by plants. Nitrifica- tion, or the conversion of ammonium (NH +) to nitrate (NO -) by soil bacteria, is most rapid in soils with pH values between 7 and 8. Nitrification approaches zero below pH

Ammonium-N fertilizers applied to calcareous soils are converted within a few days to nitrate, which moves freely with soil water. The acidity produced during nitrification is quickly neutralized in highly calcareous soils but may lower the pH value in soils containing low levels of CaCO3.

Ammonia volatilization is the loss of N to the atmosphere through conversion of the ammonium ion to ammonia gas (NH3). Volatilization of ammoniacal-N fertilizer is signifi- cant only if the soil surface pH value is greater than 7 where the fertilizer is applied. This condition occurs in calcareous soils, or where the breakdown of the N fertilizer produces alkaline conditions (e.g., urea decomposition). Nitrogen loss through ammonia volatilization on calcareous soils is

a concern when ammoniacal N is applied to the grove floor and remains there without moving into the soil. Following an application of dry fertilizer containing ammoniacal N, the fertilizer should be moved into the root zone through irrigation or mechanical incorporation if rainfall is not imminent. Since urea breakdown creates alkaline condi- tions near the fertilizer particle, surface application of urea can cause N loss if the urea is not incorporated or irrigated into the soil, regardless of initial soil pH.

The Effect of CaCO3 On Magnesium and Potassium

Although low concentrations of Mg and K in citrus leaves are not uncommon in groves planted on calcareous soils, there is not necessarily a concurrent reduction in fruit yield or quality. If a low concentration of leaf K or Mg is found in a grove that produces satisfactory yield and quality, attempts to increase leaf levels with fertilizer are not neces- sary. However, if a detrimental condition such as low yield, small fruit, or creasing is observed, an attempt to raise the leaf K or Mg concentration with fertilizer is justified.

It is often difficult to increase leaf Mg and K levels with fertilizer applied directly to calcareous soils, which contain tremendous quantities of both exchangeable and nonexchangeable Ca. Leaf Mg and K concentrations are strongly influenced by soil conditions that control leaf Ca

concentration, including high soil Ca levels. High Ca levels suppress Mg and K uptake by citrus trees in part, presum- ably, through the competition of Ca2+, Mg2+, and K+. Citrus growing on soils high in Ca often requires above normal levels of Mg and K fertilizer for satisfactory tree nutrition. In cases where soil-applied fertilizer is ineffective, the only means of increasing leaf Mg or K concentration may be through foliar application of water-soluble fertilizers, such as magnesium nitrate [Mg(NO3)2] or potassium nitrate (KNO3).

The Effect of CaCO3 On Phosphorus

Phosphorus availability in calcareous soils is almost always limited. The P concentration in the soil solution is the factor most closely related to P availability to plants. The sustainable concentration is related to the solid forms of

P that dissolve to replenish soil solution P following its depletion by crop uptake. Many different solid forms of phosphorus exist in combination with Ca in calcareous soils. After P fertilizer is added to a calcareous soil, it undergoes a series of chemical reactions with Ca that decrease its solubility with time (a process referred to as P fixation). Consequently, the long-term availability of P to plants is controlled by the application rate of soluble P and the dissolution of fixed P. Applied P is available to replenish the soil solution for only a relatively short time before it converts to less soluble forms of P.

The Effect of CaCO3 On Zinc and Manganese

Soil pH is the most important factor regulating Zn and Mn supply in alkaline soils. At alkaline (high) pH values, Zn and Mn form precipitous compounds with low water solubility, markedly decreasing their availability to plants. A soil pH value of less than 7 is preferred to ensure that Zn

chlorosis can be corrected by using organic chelates, a method discussed in detail in a later section.


  1. Calcareous soils are alkaline because they contain CaCO3. They are commonly found in south Florida citrus groves, especially in the Indian River area.
  2. The availability of N, P, K, Mg, Mn, Zn, and Fe to citrus decreases when soil CaCO3 concentration increases to more than about 3% by weight. These soils generally have a pH value in the range of 7.6 to 8.3.
  3. To avoid ammonia volatilization, fertilizers containing ammonium-N or urea should be moved into the root
  4. where soil applications are not.
  5. The least expensive and more efficient way to correct Zn and Mn deficiencies of citrus in calcareous soils is through foliar application of inorganic or organically chelated forms.
  6. The easiest way to avoid lime-induced Fe chlorosis on calcareous soils is to plant trees budded on tolerant rootstocks.
  7. The most effective remedy for lime-induced Fe chlorosis on nontolerant rootstocks involves the use of organically chelated Fe.
  8. Sulfur products that act as soil acidulents can potentially improve nutrient availability in calcareous soils.


Iron deficiency and chlorosis in orchard and vineyard ecosystems (pdf)
Citrus Fertilizer Management on Calcareous Soils (pdf)
Iron deficiency and chlorosis in orchard and vineyard ecosystems (pdf)
Legumes: Model Plants for Sustainable Agriculture in Calcareous Soils (pdf)
Soil–Plant Nutrient Interactions on Manure-Enriched Calcareous Soils (pdf)
Nitrogen Management in Calcareous Soils: Problems and Solutions (pdf)
Plant-soil-nutrient status of vegetables and wheat grown oncalcareous soil (pdf)
Phosphorus Availability with Alkaline/Calcareous Soil (pdf)
Food Availability in Alkaline-Calcareous Soils (pdf)

The Real Dirt on Austin Area Soils

The Austin area is home to three ecoregions that have very different types of soil; the Edwards Plateau, the Blackland Prairies, and the Post Oak Savannah Floodplains. All of them are somewhat alkaline, have challenging clay issues, and are low in organic matter. We’ll describe each region to help you identify which one you are gardening in and give you some tips for success. Here is a link to Soil Survey of Travis County Texas if you want to deep dive even further to see the variability in the Austin area soils.

Travis County Soil Survey showing three ecoregions - Edwards Plateau, Blackland Prairies, Post Oak Savannah

Edwards Plateau Description and Gardening Strategies

The Edwards Plateau is characterized by thin soils on top of exposed limestone

I-35 helps to physically separate the western Edwards Plateau from the eastern Blackland Prairies region here in Austin. This is the reason that garden center employees and the Travis County Master Gardeners ask you which side of the freeway you live on!

You know you’re on the Edwards Plateau if you can observe the following:

  • Exposed Limestone
  • Soil contains large amounts of crumbled limestone, referred to as calcareous rubble
  • Clay soil prevalent

The biggest challenge is not having enough soil. Sometimes builders will bring in soil for new housing developments, but it’s usually not very much and sometimes can be from poor materials like sandy clay. Your best bet is to prioritize what you want to grow and think of your landscape as planting islands. Planting islands allow you to concentrate on soil improvement or find affordable ways to purchase more soil.

Slow Down Runoff

The thin soils and elevation changes contained on the Edwards Plateau make this area prone to runoff. Evaluate your gardening site for ways to slow down the runoff. You can do this with physical barriers, tiers, raised beds, and retaining areas like rain gardens. Utilize materials like larger stones and coarsely ground mulch for transition zones to help prevent decomposed granite or soil mixes from washing away. Summer and Winter cover crops also help stabilize things and keep bare soil in place. An added benefit is that roots help to break down limestone. The clumping and fibrous nature of native prairie grass roots are also good plant choices to help break down limestone.

Add Organic Matter for Soil Health

All clay soils are deficient in organic matter, and especially so on the Edwards Plateau. But the trick is not to go overboard. Your garden soil should contain at least 30% by volume of real soil. This mineralized content is vital for plant health. If you need to purchase soil, make sure that it is not a soilless mixture like a potting mix. Look for words like “top soil” and ask to see the ingredients listing.

Improve the soil that you have with one to two inches of compost added to the topsoil each spring and fall. You’ll also want to retain soil moisture by using three inches of mulch. These thin soil layers leave plants susceptible to summer drought when the root zone can quickly deplete the soils limited moisture reserves. The mulch will eventually break down and become compost, so you may need to add more each planting season. When applying mulch, be sure to give your plants some space by allowing a gap between the mulch and the main stems of the plant.

Chemistry pH

Calcareous SoilsPlant Nutrition
Cation and Anion Exchange Capacityplant videos 1 chemistry
Chemistry pHplant videos 2
Fertilizer Chemistryplant videos 3 physiology
greenhouse techplant water and transport
MicronutrientsSoil and Roots
nutrient tablesoil science
Plant Cell Biologyspring-lake
Plant Cell PhysiologyStylistics
plant nutrients

Table 6.2

Names of the Monatomic Anions


The names of monatomic cations always start with the name of the metal, sometimes followed by a Roman numeral to indicate the charge of the ion. For example, Cu+ is copper(I), and Cu2+ is copper(II).

Table 6.3

Common Polyatomic Ions

NH4+ammonium3- PO4phosphate2- SO4sulfate
2- CO3carbonate    

Some polyatomic anions are formed by the attachment of one or more hydrogen atoms. In fact, it is common for hydrogen atoms to be transferred from one ion or molecule to another ion or molecule. When this happens, the hydrogen atom is usually transferred without its electron, as H+. If an anion has a charge of -2 or -3, it can gain one or two H+ ions and still retain a negative charge. For example, carbonate, CO 2-, can gain an H+ ion to form HCO3-, which is found in baking soda. The sulfide ion, S2-, can gain one H+ ion to form HS-. Phosphate, PO 3-, can gain one H+ ion and form HPO 2-, or it can gain two H+ ions to form H PO -. Both HPO 2- and H PO – are found in flame retardants. These polyatomic ions are named with the word hydrogen in front of the name of the anion if there is one H+ ion attached and dihydrogen in front of the name of the anion if two H+ ions are attached.

Arrhenius recognized that when ionic compounds dissolve, they form ions in solution. (For example, when sodium chloride dissolves, it forms sodium ions and chloride ions. He postulated that acids dissolve in a similar way to form H+ ions and some kind

of anion. For example, he predicted that when HCl is added to water, H+ ions and Cl- ions form. We now know that H+ ions do not persist in water; they combine with water molecules to form hydronium ions, H3O+. Therefore, according to the modern

form of the Arrhenius theory, an acid is a substance that produces hydronium ions, H3O+, when it is added to water. On the basis of this definition, an acidic solution is a solution with a significant concentration of H3O+.

To get an understanding of how hydronium ions are formed when Arrhenius acids are added to water, let’s consider the dissolving of gaseous hydrogen chloride, HCl( g ), in water. The solution that forms is called hydrochloric acid. When HCl molecules dissolve in water, a chemical change takes place in which water molecules pull hydrogen atoms away from HCl molecules. In each case, the hydrogen atom is transferred without its electron, that is, as an H ion, and because most uncharged hydrogen atoms contain only one proton and one electron, most hydrogen atoms without their electrons are just protons. For this reason, the hydrogen ion, H, is often called a proton. We say that the HCl donates a proton, H, to water, forming hydronium ion, H3O, and chloride ion, Cl (Figure 6.1).

Because HCl produces hydronium ions when added to water, it is an acid according to the Arrhenius definition of acids. Once the chloride ion and the hydronium ion are formed, the negatively charged oxygen atoms of the water molecules surround the hydronium ion, and the positively charged hydrogen atoms of the water molecules surround the chloride ion. Figure 6.2 shows how you can picture this solution.

HCl Water
acid solution

Types of Arrhenius Acids

In terms of chemical structure, Arrhenius acids can be divided into several different subcategories. We will look at three of them here: binary acids, oxyacids, and organic acids. The binary acids are HF(aq), HCl(aq), HBr(aq), and HI(aq); all have the general formula of HX(aq), where X is one of the first four halogens. The formulas for the binary acids will be followed by (aq) in this text to show that they are dissolved in water. The most common binary acid is hydrochloric acid, HCl(aq).

Oxyacids (often called oxoacids) are molecular substances that have the general formula HaXbOc. In other words, they contain hydrogen, oxygen, and one other element represented by X; the a, b, and c represent subscripts. The most common oxyacids in the chemical laboratory are nitric acid, HNO3, and sulfuric acid, H2SO4. Acetic acid, the acid responsible for the properties of vinegar, contains hydrogen, oxygen, and carbon and therefore fits the criteria for classification as an oxyacid, but it is more commonly described as an organic (or carbon-based) acid. It can also be called a carboxylic acid.

Acids can have more than one acidic hydrogen. If each molecule of an acid can donate one hydrogen ion, the acid is called a monoprotic acid. If each molecule can donate two or more hydrogen ions, the acid is a polyprotic acid. A diprotic acid, such as sulfuric acid, H2SO4, has two acidic hydrogen atoms. Some acids, such as phosphoric acid, H3PO4, are triprotic acids. Most of the phosphoric acid produced by the chemical industry is used to make fertilizers and detergents, but it is also used to make pharmaceuticals, to refine sugar, and in water treatment. The tartness of some foods and beverages comes from acidifying them by adding phosphoric acid. The space- filling model in Figure 6.4 shows the three acidic hydrogen atoms of phosphoric acid.

Acid Review

according to the modern form of the Arrhenius theory, an acid is a substance that produces hydronium ions, H3O+, when it is added to water, and an acidic solution is a solution with a significant concentration of H3O+. Acids can be binary acids—such as HF(aq), HCl(aq), HBr(aq), and HI(aq)—oxyacids, which have the general formula HaXbOc, and organic acids, such as acetic acid, HC2H3O2.

An acid, such as hydrofluoric acid, HF(aq), whose molecules can each donate one proton, H+, to a water molecule is called a monoprotic acid. The acids, such as sulfuric acid, H2SO4, that can donate two protons are called diprotic, and some acids, such as phosphoric acid, H3PO4, are triprotic acids.

A strong acid, such as hydrochloric acid, HCl(aq), is a substance that undergoes a completion reaction with water such that each acid particle reacts to form a hydronium ion, H3O+. Thus strong acids form nearly one H3O+ ion in solution for each acid molecule dissolved in water.

completion reaction

Sulfuric acid, H2SO4, is a strong diprotic acid. When added to water, each H2SO4 molecule loses its first hydrogen ion completely.

H2SO4(aq) + H2O(l )   ®    H3O+(aq) + HSO4-(aq)

The hydrogen sulfate ion, HSO4– that forms is a weak acid. It reacts with water in a reversible reaction to form a hydronium ion and a sulfate ion.

reversible reaction

HSO4-(aq) + H2O(l )   H3O+(aq) + SO 2-(aq)

A weak acid is a substance that is incompletely ionized in water because of the reversibility of its reaction with water that forms hydronium ion, H3O+. Weak acids yield significantly less than one H3O+ ion in solution for each acid molecule dissolved in water.

ammonia water 8-1
ammonia water solution

When ammonia, NH3, dissolves in water, some hydrogen ions, H+, are transferred from water molecules to ammonia molecules, NH3, producing ammonium ions, NH4+, and hydroxide ions, OH-. The reaction is reversible, so when an ammonium ion and a hydroxide ion meet in solution, the H+ ion can be passed back to the OH- to reform an NH3 molecule and a water molecule (Figure 8.1).

Ammonia is an Arrhenius base because it produces OH- ions when added to water. Because the reaction is reversible, however, only some ammonia molecules have acquired protons (creating OH-) at any given time, so an ammonia solution contains fewer hydroxide ions than would be found in a solution made using an equivalent amount of a strong base. Therefore, we classify ammonia as a weak base, which is a base that produces fewer hydroxide ions in water solution than there are particles of base dissolved.


nitric solution

Text Box: oText Box: BjeCtiveText Box: 10Text Box: aThe reaction between the strong acid nitric acid and the strong base sodium hydroxide is our first example. Figure 8.5 shows the behavior of nitric acid in solution. As a strong acid, virtually every HNO3 molecule donates an H+ ion to water to form a hydronium ion, H3O+, and a nitrate ion, NO3-. Because the reaction goes essentially to completion, you can picture the solution as containing H2O, NO3-, and H3O+, with no HNO3 remaining. The negatively charged oxygen ends of the water molecules surround the positive hydronium ions, and the positively charged hydrogen ends of water molecules surround the nitrate ions.

Like a water solution of any ionic compound, a solution of sodium hydroxide (NaOH) consists of ions separated and surrounded by water molecules. At the instant that the solution of sodium hydroxide is added to the aqueous nitric acid, there are four different ions in solution surrounded by water molecules: H3O+, NO3-, Na+, and OH-

The ions in solution move in a random way, like any particles in a liquid, so they will constantly collide with other ions. When two cations or two anions collide, they repel each other and move apart. When a hydronium ion and a nitrate ion collide, it

is possible that the H3O+ ion will return an H+ ion to the NO3– ion, but nitrate ions

are stable enough in water to make this unlikely. When a sodium ion collides with a hydroxide ion, they may stay together for a short time, but their attraction is too

weak and water molecules collide with them and push them apart. When hydronium ions and hydroxide ions collide, however, they react to form water (Figure 8.7), so more water molecules are shown in Figure 8.8 than in Figure 8.6.

The sodium and nitrate ions are unchanged in the reaction. They were separate and surrounded by water molecules at the beginning of the reaction, and they are still separate and surrounded by water molecules after the reaction. They were important in delivering the hydroxide and hydronium ions to solution, but they did not actively participate in the reaction. In other words, they are spectator ions, so they are left

out of the net ionic chemical equation. The net ionic equation for the reaction is therefore

H3O+(aq) + OH-(aq)      ®    2H2O(l )

nitric acid solution
niitric acid

Most chemists are in the habit of describing reactions such as this one in terms of H+ rather than H3O+, even though hydrogen ions do not exist in a water solution in the same sense that sodium ions do. When an acid loses a hydrogen atom as H+, the proton immediately forms a covalent bond to some other atom. In water, it forms a covalent bond to a water molecule to produce the hydronium ion. Although H3O+ is a better description of what is found in acid solutions, it is still convenient and conventional to write H+ in equations instead. You can think of H+ as a shorthand notation for H3O+. Therefore, the following net ionic equation is a common way to describe the net ionic equation above.

H+(aq) + OH-(aq)   ®    H2O(l )


The scientific term pH has crept into our everyday language. Advertisements encourage us to choose products that are “pH balanced,” while environmentalists point to the lower pH of rain in certain parts of the country as a cause of ecological damage (Figure 8.3). The term was originated by chemists to describe the acidic and basic strengths of solutions.

We know that an Arrhenius acid donates H+ ions to water to create H3O+ ions. The resulting solution is called an acidic solution. We also know that when you add a certain amount of a strong acid to one sample of water—say the water’s volume is a liter—and add the same amount of a weak acid to another sample of water whose volume is also a liter, the strong acid generates more H3O+ ions in solution. Because the concentration of H3O+ ions in the strong acid solution is higher (there are more H3O+ ions per liter of solution), we say it is more acidic than the weak acid solution. A solution can also be made more acidic by the addition of more acid (while the amount of water remains the same). The pH scale can be used to describe the relative acidity of solutions.

If you take other chemistry courses, you will probably learn how pH is defined and how the pH values of solutions are determined. For now, all you need to remember is that acidic solutions have pH values less than 7, and that the more acidic a solution is, the lower its pH. A change of one pH unit reflects a ten‑fold change in H3O+ ion concentration. For example, a solution with a pH of 5 has ten times the concentration of H3O+ ions as a solution with a pH of 6. The pH of some common solutions are listed in Figure 8.4. Note that gastric juice in our stomach has a pH of about 1.4, and orange juice has a pH of about 2.8. Thus gastric juice is more than ten times more concentrated in H3O+ ions than orange juice.

The pH scale is also used to describe basic solutions, which are formed when an Arrhenius base is added to water, generating OH- ions. When you add a certain amount of a strong base to one sample of water—again, let’s say a liter—and add the same amount of a weak base to another sample of water whose volume is the same, the strong base generates more OH- ions in solution. Because the concentration of OH- ions in the strong base solution is higher (there are more OH- ions per liter of solution), we say it is more basic than the weak base solution. A solution can also be made more basic by the addition of more base while the amount of water is held constant.

ph scale solutions chemistry

Basic solutions have pH values greater than 7, and the more basic the solution is, the higher its pH. A change of one pH unit reflects a ten‑fold change in OH- ion concentration. For example, a solution with a pH of 12 has ten times the concentration of OH- ions as does a solution with a pH of 11. The pH difference of about 4 between household ammonia solutions (pH about 11.9) and seawater (pH about 7.9) shows that household ammonia has about ten thousand (104) times the hydroxide ion concentration of seawater.

In nature, water contains dissolved substances that make it slightly acidic, but pure water is neutral and has a pH of 7 (Figure 8.4).

Active and exchangeable acidity

The pH of a soil is a measure of the hydrogen ion concentration in the soil solution. pH is a the negative logarithm of H+ concentration in moles / liter:

pH = – log [H+]

and is therefore a solution measurement which only reflects the presence of acid cations adsorbed on soil colloids. A pH scale is shown below along with some reference points.

Hydrogen ion concentration is acidic soils is largely determined by the number of hydrogen ions that disassociate from the cation exchange complex. Dissociation of hydrogen is directly related to the fraction of the exchange complex that is occupied by hydrogen and aluminum ions. pH decreases (or acidity increases) as percentage saturation of H+ and Al3+ increases. Hydrogen ion in soil solution is termed active acidity and is the acidity measured by common pH tests. Hydrogen and aluminum ions adsorbed on soil colloids are termed exchangeable (or sometimes reserve) acidity. Exchangeable acidity is much larger than active acidity.

titrate acidity

Soil pH and Salt Concentration

Acidic cations on soil colloids will exchange with cations in the soil solution. The amount of exchange is proportional to the concentration of all cations in solution, since equilibrium conditions exist. Consequently, pH of a soil solution decreases as the concentration of neutral salts (eg. NaCl, CaSO4, etc.) increases.

This phenomenon has considerable influence on measurements of pH. Measurements of pH in a soil that has been dried will be lower than those measured in the same soil when wet. Measurements of soil pH in water will be higher in situ. Further fertilizer salts will lower pH measurements.

Several methodologies have been proposed for measuring pH. Measurement in distilled water is common, but its limitations in replicating field conditions must be recognized. Measurement in 0.01 M CaCl2 has advantages in that it replicates “typical” soil solution concentrations at “average moisture contents”.

soil pH scale


Single nutrient (“straight”) fertilizers

The main nitrogen-based straight fertilizer is ammonia or its solutions. Ammonium nitrate (NH4NO3) is also widely used. Urea is another popular source of nitrogen, having the advantage that it is solid and non-explosive, unlike ammonia and ammonium nitrate, respectively. A few percent of the nitrogen fertilizer market (4% in 2007)[23] has been met by calcium ammonium nitrate (Ca(NO3)2 • NH4 • 10H2O).

The main straight phosphate fertilizers are the superphosphates. “Single superphosphate” (SSP) consists of 14–18% P2O5, again in the form of Ca(H2PO4)2, but also phosphogypsum (CaSO4 • 2H2O). Triple superphosphate (TSP) typically consists of 44–48% of P2O5 and no gypsum. A mixture of single superphosphate and triple superphosphate is called double superphosphate. More than 90% of a typical superphosphate fertilizer is water-soluble.

The main potassium-based straight fertilizer is muriate of potash (MOP). Muriate of potash consists of 95–99% KCl, and is typically available as 0-0-60 or 0-0-62 fertilizer.

Multinutrient fertilizers

These fertilizers are common. They consist of two or more nutrient components.

Binary (NP, NK, PK) fertilizers

Major two-component fertilizers provide both nitrogen and phosphorus to the plants. These are called NP fertilizers. The main NP fertilizers are monoammonium phosphate (MAP) and diammonium phosphate (DAP). The active ingredient in MAP is NH4H2PO4. The active ingredient in DAP is (NH4)2HPO4. About 85% of MAP and DAP fertilizers are soluble in water.

NPK fertilizers are three-component fertilizers providing nitrogen, phosphorus, and potassium. There exist two types of NPK fertilizers: compound and blends. Compound NPK fertilizers contain chemically bound ingredients, while blended NPK fertilizers are physical mixtures of single nutrient components.


Soil pH

DenominationpH range
Strongly acidic5.1–5.5
Moderately acidic5.6–6.0
Slightly acidic6.1–6.5
Slightly alkaline7.4–7.8
Moderately alkaline7.9–8.4
Strongly alkaline8.5–9.0

Sources of Soil acidity

  • Root respiration and decomposition of organic matter by microorganisms releases CO2 which increases the carbonic acid (H2CO 3) concentration and subsequent leaching.
  • Plant growth: Plants take up nutrients in the form of ions (e.g. NO3, NH+ 4, Ca2+ , H 2PO4), and they often take up more cations than anions. However plants must maintain a neutral charge in their roots. In order to compensate for the extra positive charge, they will release H+
    ions from the root. Some plants also exude organic acids into the soil to acidify the zone around their roots to help solubilize metal nutrients that are insoluble at neutral pH, such as iron (Fe).
  • Fertilizer use: Ammonium (NH+4) fertilizers react in the soil by the process of nitrification to form nitrate (NO3), and in the process release H+ ions.  Acid rain: The burning of fossil fuels releases oxides of sulfur and nitrogen into the atmosphere. These react with water in the atmosphere to form sulfuric and nitric acid in rain.
  • Oxidative weathering: Oxidation of some primary minerals, especially sulfides and those containing Fe2+ , generate acidity.

Sources of Soil alkalinity

Total soil alkalinity increases with:[13][14]

  • Weathering of silicate, aluminosilicate and carbonate minerals containing Na+ , Ca2+ , Mg2+ and K+ ;
  • Addition of silicate, aluminosilicate and carbonate minerals to soils; this may happen by deposition of material eroded elsewhere by wind or water, or by mixing of the soil with less weathered material (such as the addition of limestone to acid soils);
  • Addition of water containing dissolved bicarbonates (as occurs when irrigating with high-bicarbonate waters).

The accumulation of alkalinity in a soil (as carbonates and bicarbonates of Na, K, Ca and Mg) occurs when there is insufficient water flowing through the soils to leach soluble salts. This may be due to arid conditions, or poor internal soil drainage; in these situations most of the water that enters the soil is transpired (taken up by plants) or evaporates, rather than flowing through the soil.[13]

The soil pH usually increases when the total alkalinity increases, but the balance of the added cations also has a marked effect on the soil pH. For example, increasing the amount of sodium in an alkaline soil tends to induce dissolution of calcium carbonate, which increases the pH. Calcareous soils may vary in pH from 7.0 to 9.5, depending on the degree to which Ca2+ or Na+ dominate the soluble cations.[13]

Treatment of High pH Soil

Fertilizers and chelates can be added to soil to increase concentrations of plant nutrients. It is important to note that addition of phosphate fertilizer alone will further reduce the availability of other nutrients.

Lowering the pH of alkaline soils, or acidifying the soil, is an option. Elemental sulfur can be added to soil as it forms sulfuric acid when it reacts with water and oxygen in the presence of sulfur-oxidizing bacteria. Iron and aluminum compounds can be added to soil, as they cause the release of hydrogen when they react with water. Sulfuric acid may also be added directly.

Additions of appreciable amounts of organic matter will help to acidify the soil as microbes decompose the material, releasing CO2 which then forms carbonic acid. Organic acids are also released during humus decomposition. Peat and peat moss are highly acidic forms of organic matter but can be costly.

Application of acidifying fertilizers, such as ammonium sulfate [(NH4)2SO4,], can help lower soil pH. Ammonium is nitrified by soil bacteria into nitrate and hydrogen ions.

Soils naturally containing carbonates, or lime [Calcium Carbonate CaCO3.] , are very difficult to acidify, and it may take years before a significant change in soil pH is seen. Even then, the carbonatic parent material will continue to weather, producing more soluble carbonate and buffering the soil solution pH.

Many plants can tolerate pH values between 7 and 8, and some actually thrive at these higher pH values. Choosing plants that grow well in mildly alkaline soils can be selected. This is the most reasonable “treatment” option for soils that have developed from carbonatic parent material.

Vegetable garden plants such as asparagus, beets, cabbage, cauliflower, celery, carrots, lettuce, parsley and spinach grow well in soils whose pH is between 7 and 8.


A chemical change or chemical reaction is a process in which one or more pure substances are converted into one or more different pure substances. Chemical changes lead to the formation of substances that help grow our food, make our lives more productive, cure our heartburn, and much, much more. For example, nitric acid, HNO3, which is used to make fertilizers and explosives, is formed in the chemical reaction of the gases ammonia, NH3, and oxygen, O2. Silicon dioxide, SiO2, reacts with carbon, C, at high temperature to yield silicon, Si—which can be used to make computers—and carbon monoxide, CO. An antacid tablet might contain calcium carbonate, CaCO3, which combines with the hydrochloric acid in your stomach to yield calcium chloride, CaCl2, water, and carbon dioxide. The chemical equations for these three chemical reactions are below.

Once you know how to read these chemical equations, they will tell you many details about the reactions that take place.

Bishop chemical reactions

Chemical changes lead to the formation of substances that help grow our food, make our lives more productive, and cure our heartburn.

Interpreting a Chemical Equation

In chemical reactions, atoms are rearranged and regrouped through the breaking and making of chemical bonds. For example, when hydrogen gas, H2(g), is burned in the presence of gaseous oxygen, O2(g), a new substance, liquid water, H2O(l), forms. The covalent bonds within the H2 molecules and O2 molecules break, and new covalent bonds form between oxygen atoms and hydrogen atoms (Figure 7.1).

H2 gas burn
Gas burn 7-2

chemical equation is a shorthand description of a chemical reaction. The following equation describes the burning of hydrogen gas to form liquid water.

2H2(g) + O2(g) → 2H2O(l)

Chemical equations give the following information about chemical reactions.

  • Chemical equations show the formulas for the substances that take part in the reaction. The formulas on the left side of the arrow represent the reactants, the substances that change in the reaction. The formulas on the right side of the arrow represent the products, the substances that are formed in the reaction. If there are more than one reactant or more than one product, they are separated by plus signs. The arrow separating the reactants from the products can be read as “goes to” or “yields” or “produces.”
  • The physical states of the reactants and products are provided in the equation. A (g) following a formula tells us the substance is a gas. Solids are described with (s). Liquids are described with (l). When a substance is dissolved in water, it is described with (aq) for aqueous, which means “mixed with water.”
  • The relative numbers of particles of each reactant and product are indicated by numbers placed in front of the formulas. These numbers are called coefficients. An equation containing correct coefficients is called a balanced equation. For example, the 2’s in front of H2 and H2O in the equation we saw above are coefficients. If a formula in a balanced equation has no stated coefficient, its coefficient is understood to be 1, as is the case for oxygen in the equation above (Figure 7.2).
  • If special conditions are necessary for a reaction to take place, they are often specified above the arrow. Some examples of special conditions are electric current, high temperature, high pressure, and light.


Soil fertility testing is a valuable tool to make informed nutrient management decisions. Three main philosophies exist to interpret soil test reports and provide appropriate fertilizer recommendations. Those include (i) sufficiency level of available nutrients (SLAN), (ii) buildup and maintenance, and (iii) basic cation saturation ratio (BCSR) concepts (Black, 1993). The SLAN approach works on the principle that there are certain critical levels of individual nutrients. If a soil tests above the critical level, the crop will not likely respond to fertilization but if the soil tests below the critical level, the crop will respond to fertilization (Eckert, 1987). The buildup-and-maintenance approach calls for a gradual buildup of soil nutrient levels above the critical levels over time, and then to maintain these levels by replacing the amounts of each nutrient removed by the crop at harvest (Olson et al., 1987; Black, 1993; Voss, 1998). The focus here is to always maintain the soil fertility status at a high level with constant fertilizer applications, so that yields are sustained. The third approach is the BCSR, which recommends an optimum or ideal calcium (Ca), magnesium (Mg), and potassium (K) saturation ratio on the soil exchange complex to achieve maximum crop yields (McLean, 1977; Voss, 1998). With BCSR, fertilizer recommendations are made to adjust the cation saturation ratios to an optimum or ideal level, ir­respective of actual soil nutrient levels. A soil with such an optimal saturation ratio of base cations is considered to be a balanced soil and the practice of adding fertilizer/amendments to achieve a desired ratio is called “soil balancing.” Generally, the SLAN approach recommends fertilizing based on plant needs, the buildup-and-maintenance approach focuses on fertilizing the soil, and BCSR targets on countering the mineral imbalances in soil (Black, 1993; Eckert, 1987). In contrast to sufficiency level and buildup-and-maintenance philosophies, BCSR focuses only on Ca, Mg, and K and does not directly relate to the availability of nitrogen (N), phosphorus (P), sulfur (S), or micronutrients (Eckert, 1987).

cations and acidity
acid cations soil space

There is an apparent discrepancy in using differ­ent philosophies for soil fertilizer recommendation programs. However, over the past three decades, ongoing research by soil fertility scientists on soil testing, inter­pretation, and calibration has resulted in the adoption of SLAN and buildup-and-maintenance philosophies or a hy­brid of these two, as a standard fertilizer recommendation practice by land-grant universities. On the other hand, some commercial soil-testing laboratories employ BCSR and buildup-and-maintenance approaches in their lime and fertilizer recommendation programs (Voss, 1998). To date, there is little published research that substantiates the BCSR theory and the concept of a balanced soil for maximizing yields, and only a few studies have tested its efficacy. Still, some agronomists, consultants, commercial soil-testing labs, and farmers strongly subscribe to this practice and continue to use it to guide their soil man­agement decisions and nutrient recommendations. This review (i) presents a brief history of the BCSR theory, (ii) provides an overview of the research that has been con­ducted on BCSR and crop production, and (iii) identifies knowledge gaps that need attention.

What is basic cation saturation?

Soil nutrients can exist in many forms, but the nutrients that plants take up are mostly positively (cations) or nega­tively (anions) charged ions. Cations and anions are avail­able in soil solution or on the soil exchange sites. Those that are in soil solution are readily available to plants. Cation exchange sites are negatively charged surfaces of clay and organic matter that attract and hold cations in the soil. Soil tests measure the sum of these exchange sites and report them as cation exchange capacity (CEC). The CEC is a defining feature of soils, and the greater the CEC of a soil, the more cations it can hold (Hazelton and Murphy, 2007). Cations are generally classified as “basic” and “acidic” based on their influence on soil pH through various soil reactions. Basic cations (also called as non­acidic cations) include calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+). Acidic cations consist of hydrogen (H+) and aluminum (Al3+). Figure 1 shows different basic and acidic cations on soil exchange sites. Base saturation indicates the proportion of these basic cations that occupy the soil exchange sites (CEC). In other words, if a soil has a 50% base saturation of Ca, then Ca occupies 50% of the exchange sites. Figure 2 gives a pictorial representation of the base saturations of Ca2+ at approximately 60%, Mg2+ at approximately 15%, and K+ at approximately 5%, making up to approximately 80% of the total CEC of the soil. Remaining sites (approximately 20% of CEC) could be occupied by other basic cations, such as Na+, or acidic cations such as H+ and Al3+. How­ever, BCSR is primarily concerned with the percent satura­tion of only Ca2+, Mg2+, and K+ ions on the exchange sites.


 Chapter 5: Chemical Compounds

Chapter 6: More on Chemical Compounds

Chapter 7: An Introduction to Chemical Reactions

Chapter 8: Acids, Bases, and Acid-Base Reactions 

Lab 9 – Soil pH and Soil Testing – Crop and Soil Science

Historical Perspective of Soil Balancing Theory and Identifying Knowledge Gaps: A Review

Soil and Roots

Clay and CEC


Molecular FormulaAl2H2O12Si4
silicon tetrahedron
aluminum octahedron
clay layers
Montmorillonite structure

The basic structural unit of montmorillonite mineral consists of one alumina sheet sandwiched between two silica sheets. The oxygen atoms at the tip of each silica sheet combine with the OH ions of both sides of the alumina sheet by hydrogen bond. As the bond is fairly strong, the basic structural unit of montmorillonite mineral is also stable. The thickness of the basic structural unit of montmorillonite is 9.2 Å.

Montmorillonite mineral is formed by stacking of several structural units one over the other. The interface between the two structural units during this stacking is through the silica sheet of one structural unit and the silica sheet of the other unit.

This bond is by van der Waals’ forces, which is rather weak. In dry condition, the van der Waals’ forces are rather strong so that it is difficult to break a dry lump of montmorillonite clay. However, water can easily enter between the layers, often dividing the montmorillonite particle into indi­vidual structural units.

One characteristic of van der Waals’ forces is that their magnitude decreases rapidly with distance. As water enters between the structural units, the distance between them increases, causing reduction in attractive forces. Thus, a montmorillonite clay particle is stable as long as it is dry but breaks into individual struc­tural units in the presence of water.


Cation Exchange:

cation exchange CEC 1
CEC clay

In a near neutral soil, calcium remains adsorbed on colloidal particle. Hydrogen ion (H+ ) generated as organic and mineral acids formed due to decomposition of organic matter. In colloid, hydrogen ion is adsorbed more strongly than is the calcium (Ca++). The reaction takes place rapidly and the interchange of calcium and hydrogen is Chemically equivalent.

This phenomenon of the exchange of cations between soil and salt solution is known as Cation exchange or Base exchange and the cations that take part in this reaction are called exchangeable cations. Cation exchange reactions are reversible.

Hence, if some form of limestone or other basic calcium compound is applied to an acid soil, the reverse of the replacement just given above occurs. The active calcium ions replace the hydrogen and other cations by mass action. As a result, the clay becomes higher in exchangeable calcium and lower in adsorbed hydrogen and aluminium.

If a soil is treated with a liberal application of a fertilizer containing potassium chloride, following reaction may occur: ; then:

Some of the added potassium pushes its way into the colloidal complex and forces out equivalent quantities of calcium, hydrogen and other elements (e.g., M) which appear in the soil solution. The adsorption of the added potassiumis  largely in an available condition. Hence, cation exchange is an important consideration for making already present nutrients in soils available to plants. Cation exchange also makes available the nutrients, applied in commercial fertilizers.

Cation Exchange Capacity (C.E.C.):

The cation exchange capacity of a soil represents the capacity of the colloidal complex to exchange all its cations with the cations of the electrolyte solution (surrounding liquid). It also represents the total cation adsorbing capacity of a soil. Cation exchange in most soils increases with pH. At a very low pH value, C.E.C. is higher and at high pH, C.E.C. is relatively lower.



The soil is a complex physical, chemical, and biological substrate. It is a heterogeneous material containing solid, liquid, and gaseous phases (see Chapter 4). All of these phases interact with mineral elements. The inorganic particles of the solid phase provide a reservoir of potassium, calcium, magnesium, and iron. Also associated with this solid phase are organic compounds containing nitrogen, phosphorus, and sulfur, among other elements. The liquid phase of the soil constitutes the soil solution, which contains dissolved mineral ions and serves as the medium for ion movement to the root surface. Gases such as oxygen, carbon dioxide, and nitrogen are dissolved in the soil solution, but in roots gases are exchanged predominantly through the air gaps between soil particles. From a biological perspective, soil constitutes a diverse ecosystem in which plant roots and microorganisms compete strongly for mineral nutrients. In spite of this competition, roots and microorganisms can form alliances for their mutual benefit (symbioses, singular symbiosis). In this section we will discuss the importance of soil properties, root structure, and mycorrhizal symbiotic relationships to plant mineral nutrition. Chapter 12 addresses symbiotic relationships with nitrogen-fixing bacteria.

Negatively Charged Soil Particles Affect the Adsorption of Mineral Nutrients

 Soil particles, both inorganic and organic, have predominantly negative charges on their surfaces. Many inorganic soil particles are crystal lattices that are tetrahedral arrangements of the cationic forms of aluminum and silicon (Al3+ and Si4+) bound to oxygen atoms, thus forming aluminates and silicates. When cations of lesser charge replace Al3+ and Si4+, inorganic soil particles become negatively charged. Organic soil particles originate from the products of the microbial decomposition of dead plants, animals, and microorganisms. The negative surface charges of organic particles result from the dissociation of hydrogen ions from the carboxylic acid and phenolic groups present in this component of the soil. Most of the world’s soil particles, however, are inorganic.  Inorganic soils are categorized by particle size:

Gravel has particles larger than 2 mm.

• Coarse sand has particles between 0.2 and 2 mm.

Fine sand has particles between 0.02 and 0.2 mm.

 • Silt has particles between 0.002 and 0.02 mm.

 • Clay has particles smaller than 0.002 mm (see Table 4.1).

The silicate-containing clay materials are further divided into three major groups—kaolinite, illite, and montmorillonite— based on differences in their structure and physical properties (Table 5.5). The kaolinite group is generally found in well-weathered soils; the montmorillonite and illite groups are found in less weathered soils. Mineral cations such as ammonium (NH4 +) and potassium (K+) adsorb to the negative surface charges of inorganic and organic soil particles. This cation adsorption is an important factor in soil fertility. Mineral cations adsorbed on the surface of soil particles are not easily lost when the soil is leached by water, and they provide a nutrient reserve available to plant roots. Mineral nutrients adsorbed in this way can be replaced by other cations in a process known as cation exchange (Figure 5.5). The degree to which a soil can adsorb and exchange ions is termed its cation exchange capacity (CEC) and is highly dependent on the soil type. A soil with higher cation exchange capacity generally has a larger reserve of mineral nutrients. Mineral anions such as nitrate (NO3 –) and chloride (Cl–) tend to be repelled by the negative charge on the surface of soil particles and remain dissolved in the soil solution. Thus the anion exchange capacity of most agricultural soils is small compared to the cation exchange capacity. Among anions, nitrate remains mobile in the soil solution, where it is susceptible to leaching by water moving through the soil. Phosphate ions (H2PO2 –) may bind to soil particles containing aluminum or iron because the positively charged iron and aluminum ions (Fe2+, Fe3+, and Al3+) have hydroxyl (OH–) groups that exchange with phosphate. As a result, phosphate can be tightly bound, and its mobility and availability in soil can limit plant growth. Sulfate (SO4 2–) in the presence of calcium (Ca2+) forms gypsum (CaSO4). Gypsum is only slightly soluble, but it releases sufficient sulfate to support plant growth.

Most nonacid soils contain substantial amounts of calcium; consequently, sulfate mobility in these soils is low, so sulfate is not highly susceptible to leaching.

clay mineral structure

Soil pH Affects Nutrient Availability, Soil Microbes, and Root Growth

Hydrogen ion concentration (pH) is an important property of soils because it affects the growth of plant roots and soil microorganisms. Root growth is generally favored in slightly acidic soils, at pH values between 5.5 and 6.5. Fungi generally predominate in acidic soils; bacteria become more prevalent in alkaline soils. Soil pH determines the availability of soil nutrients (see Figure 5.4). Acidity promotes the weathering of rocks that releases K+, Mg2+, Ca2+, and Mn2+ and increases the solubility of carbonates, sulfates, and phosphates. Increasing the solubility of nutrients facilitates their availability to roots. Major factors that lower the soil pH are the decomposition of organic matter and the amount of rainfall. Carbon dioxide is produced as a result of the decomposition of organic material and equilibrates with soil water in the following reaction: CO2 + H2O ~ H+ + HCO3 – This reaction releases hydrogen ions (H+), lowering the pH of the soil. Microbial decomposition of organic material also produces ammonia and hydrogen sulfide that can be oxidized in the soil to form the strong acids nitric acid (HNO3) and sulfuric acid (H2SO4), respectively. Hydrogen ions also displace K+, Mg2+, Ca2+, and Mn2+ from the cation exchange complex in a soil. Leaching then may remove these ions from the upper soil layers, leaving a more acid soil. By contrast, the weathering of rock in arid regions releases K+, Mg2+, Ca2+, and Mn2+ to the soil, but because of the low rainfall, these ions do not leach from the upper soil layers, and the soil remains alkaline.

5-8 rootapical
5-9 nutrient depletion
root fungi
t fungi 2

Excess Minerals in the Soil Limit Plant Growth

 When excess minerals are present in the soil, the soil is said to be saline, and plant growth may be restricted if these mineral ions reach levels that limit water availability or exceed the adequate zone for a particular nutrient (see Chapter 25). Sodium chloride and sodium sulfate are the most common salts in saline soils. Excess minerals in soils can be a major problem in arid and semiarid regions because rainfall is insufficient to leach the mineral ions from the soil layers near the surface. Irrigated agriculture fosters soil salinization if insufficient water is applied to leach the salt below the rooting zone. Irrigation water can contain 100 to 1000 g of minerals per cubic meter. An average crop requires about 4000 m3 of water per acre. Consequently, 400 to 4000 kg of minerals may be added to the soil per crop (Marschner 1995). In saline soil, plants encounter salt stress. Whereas many plants are affected adversely by the presence of relatively low levels of salt, other plants can survive high levels (salt-tolerant plants) or even thrive (halophytes) under such conditions. The mechanisms by which plants tolerate salinity are complex (see Chapter 25), involving molecular synthesis, enzyme induction, and membrane transport. In some species, excess minerals are not taken up; in others, minerals are taken up but excreted from the plant by salt glands associated with the leaves. To prevent toxic buildup of mineral ions in the cytosol, many plants may sequester them in the vacuole (Stewart and Ahmad 1983). Efforts are under way to bestow salt tolerance on salt-sensitive crop species using both classic plant breeding and molecular biology (Hasegawa et al. 2000). Another important problem with excess minerals is the accumulation of heavy metals in the soil, which can cause severe toxicity in plants as well as humans . Heavy metals include zinc, copper, cobalt, nickel, mercury, lead, cadmium, silver, and chromium (Berry and Wallace 1981).

Plants Develop Extensive Root Systems

The ability of plants to obtain both water and mineral nutrients from the soil is related to their capacity to develop an extensive root system. In the late 1930s, H. J. Dittmer examined the root system of a single winter rye plant after 16 weeks of growth and estimated that the plant had 13 × 106 primary and lateral root axes, extending more than 500 km in length and providing 200 m2 of surface area (Dittmer 1937). This plant also had more than 1010 root hairs, providing another 300 m2 of surface area  the desert, the roots of mesquite (genus Prosopis) may extend down more than 50 m to reach groundwater. Annual crop plants have roots that usually grow between 0.1 and 2.0 m in depth and extend laterally to distances of 0.3 to 1.0 m. In orchards, the major root systems of trees planted 1 m apart reach a total length of 12 to 18 km per tree. The annual production of roots in natural ecosystems may easily surpass that of shoots, so in many respects, the aboveground portions of a plant represent only “the tip of an iceberg.” Plant roots may grow continuously throughout the year. Their proliferation, however, depends on the availability of water and minerals in the immediate microenvironment surrounding the root, the so-called rhizosphere. If the rhizosphere is poor in nutrients or too dry, root growth is slow. As rhizosphere conditions improve, root growth increases. If fertilization and irrigation provide abundant nutrients and water, root growth may not keep pace with shoot growth. Plant growth under such conditions becomes carbohydrate limited, and a relatively small root system meets the nutrient needs of the whole plant (Bloom et al. 1993). Roots growing below the soil surface are studied by special techniques

oots 1
roots by species differ

Root Systems Differ in Form but Are Based on Common Structures

The form of the root system differs greatly among plant species. In monocots, root development starts with the emergence of three to six primary (or seminal) root axes from the germinating seed. With further growth, the plant extends new adventitious roots, called nodal roots or brace roots. Over time, the primary and nodal root axes grow and branch extensively to form a complex fibrous root system (Figure 5.6). In fibrous root systems, all the roots generally have the same diameter (except where environmental conditions or pathogenic interactions modify the root structure), so it is difficult to distinguish a main root axis. In contrast to monocots, dicots develop root systems with a main single root axis, called a taproot, which may thicken as a result of secondary cambial activity. From this main root axis, lateral roots develop to form an extensively branched root system (Figure 5.7). The development of the root system in both monocots and dicots depends on the activity of the root apical meristem and the production of lateral root meristems. Figure 5.8 shows a generalized diagram of the apical region of a plant root and identifies the three zones of activity: meristematic, elongation, and maturation. In the meristematic zone, cells divide both in the direction of the root base to form cells that will differentiate into the tissues of the functional root and in the direction of the root apex to form the root cap. The root cap protects the delicate meristematic cells as the root moves through the soil. It also secretes a gelatinous material called mucigel, which commonly surrounds the root tip. The precise function of the mucigel is uncertain, but it has been suggested that it lubricates the penetration of the root through the soil, protects the root apex from desiccation, promotes the transfer of nutrients to the root, or affects the interaction between roots and soil microorganisms (Russell 1977). The root cap is central to the perception of gravity, the signal that directs the growth of roots downward. This process is termed the gravitropic response (see Chapter 19). Cell division at the root apex proper is relatively slow; thus this region is called the quiescent center. After a few generations of slow cell divisions, root cells displaced from the apex by about 0.1 mm begin to divide more rapidly. Cell division again tapers off at about 0.4 mm from the apex, and the cells expand equally in all directions. The elongation zone begins 0.7 to 1.5 mm from the apex (see Figure 5.8). In this zone, cells elongate rapidly and undergo a final round of divisions to produce a central ring of cells called the endodermis. The walls of this endodermal cell layer become thickened, and suberin (see Chapter 13) deposited on the radial walls forms the Casparian strip, a hydrophobic structure that prevents the apoplastic movement of water or solutes across the root (see Figure 4.3). The endodermis divides the root into two regions: the cortex toward the outside and the stele toward the inside. The stele contains the vascular elements of the root: the phloem, which transports metabolites from the shoot to the root, and the xylem, which transports water and solutes to the shoot.

Phloem develops more rapidly than xylem, attesting to the fact that phloem function is critical near the root apex. Large quantities of carbohydrates must flow through the phloem to the growing apical zones in order to support cell division and elongation. Carbohydrates provide rapidly growing cells with an energy source and with the carbon skeletons required to synthesize organic compounds. Sixcarbon sugars (hexoses) also function as osmotically active solutes in the root tissue. At the root apex, where the phloem is not yet developed, carbohydrate movement depends on symplastic diffusion and is relatively slow  (Bret-Harte and Silk 1994). The low rates of cell division in the quiescent center may result from the fact that insufficient carbohydrates reach this centrally located region or that this area is kept in an oxidized state   Root hairs, with their large surface area for absorption of water and solutes, first appear in the maturation zone (see Figure 5.8), and it is here that the xylem develops the capacity to translocate substantial quantities of water and solutes to the shoot. 

Different Areas of the Root Absorb Different Mineral Ions

The precise point of entry of minerals into the root system has been a topic of considerable interest. Some researchers have claimed that nutrients are absorbed only at the apical regions of the root axes or branches (Bar-Yosef et al. 1972); others claim that nutrients are absorbed over the entire root surface (Nye and Tinker 1977). Experimental evidence supports both possibilities, depending on the plant species and the nutrient being investigated: • Root absorption of calcium in barley appears to be restricted to the apical region. • Iron may be taken up either at the apical region, as in barley (Clarkson 1985), or over the entire root surface, as in corn (Kashirad et al. 1973). • Potassium, nitrate, ammonium, and phosphate can be absorbed freely at all locations of the root surface (Clarkson 1985), but in corn the elongation zone has the maximum rates of potassium accumulation (Sharp et al. 1990) and nitrate absorption (Taylor and Bloom 1998). • In corn and rice, the root apex absorbs ammonium more rapidly than the elongation zone does (Colmer and Bloom 1998). • In several species, root hairs are the most active in phosphate absorption (Fohse et al. 1991). The high rates of nutrient absorption in the apical root zones result from the strong demand for nutrients in these tissues and the relatively high nutrient availability in the soil surrounding them. For example, cell elongation depends on the accumulation of solutes such as potassium, chloride, and nitrate to increase the osmotic pressure within the cell (see Chapter 15). Ammonium is the preferred nitrogen source to support cell division in the meristem because meristematic tissues are often carbohydrate limited, and the assimilation of ammonium consumes less energy than that of nitrate (see Chapter 12). The root apex and root hairs grow into fresh soil, where nutrients have not yet been depleted. Within the soil, nutrients can move to the root surface both by bulk flow and by diffusion (see Chapter 3). In bulk flow, nutrients are carried by water moving through the soil toward the root. The amount of nutrient provided to the root by bulk flow depends on the rate of water flow through the soil toward the plant, which depends on transpiration rates and on nutrient levels in the soil solution. When both the rate of water flow and the concentrations of nutrients in the soil solution are high, bulk flow can play an important role in nutrient supply. In diffusion, mineral nutrients move from a region of higher concentration to a region of lower concentration. Nutrient uptake by the roots lowers the concentration of nutrients at the root surface, generating concentration gradients in the soil solution surrounding the root. Diffusion of nutrients down their concentration gradient and bulk flow resulting from transpiration can increase nutrient availability at the root surface. When absorption of nutrients by the roots is high and the nutrient concentration in the soil is low, bulk flow can supply only a small fraction of the total nutrient requirement (Mengel and Kirkby 1987). Under these conditions, diffusion rates limit the movement of nutrients to the root surface. When diffusion is too slow to maintain high nutrient concentrations near the root, a nutrient depletion zone forms adjacent to the root surface (Figure 5.9). This zone extends from about 0.2 to 2.0 mm from the root surface, depending on the mobility of the nutrient in the soil. The formation of a depletion zone tells us something important about mineral nutrition: Because roots deplete the mineral supply in the rhizosphere, their effectiveness in mining minerals from the soil is determined not only by the rate at which they can remove nutrients from the soil solution, but by their continuous growth. Without growth, roots would rapidly deplete the soil adjacent to their surface. Optimal nutrient acquisition therefore depends both on the capacity for nutrient uptake and on the ability of the root system to grow into fresh soil.

Mycorrhizal Fungi Facilitate Nutrient Uptake by Roots

Our discussion thus far has centered on the direct acquisition of mineral elements by the root, but this process may be modified by the association of mycorrhizal fungi with the root system. Mycorrhizae (singular mycorrhiza, from the Greek words for “fungus” and “root”) are not unusual; in fact, they are widespread under natural conditions. Much of the world’s vegetation appears to have roots associated  with mycorrhizal fungi: 83% of dicots, 79% of monocots, and all gymnosperms regularly form mycorrhizal associations (Wilcox 1991). On the other hand, plants from the families Cruciferae (cabbage), Chenopodiaceae (spinach), and Proteaceae (macadamia nuts), as well as aquatic plants, rarely if ever have mycorrhizae. Mycorrhizae are absent from roots in very dry, saline, or flooded soils, or where soil fertility is extreme, either high or low. In particular, plants grown under hydroponics and young, rapidly growing crop plants seldom have mycorrhizae. Mycorrhizal fungi are composed of fine, tubular filaments called hyphae (singular hypha). The mass of hyphae that forms the body of the fungus is called the mycelium (plural mycelia). There are two major classes of mycorrhizal fungi: ectotrophic mycorrhizae and vesicular-arbuscular mycorrhizae (Smith et al. 1997). Minor classes of mycorrhizal fungi include the ericaceous and orchidaceous mycorrhizae, which may have limited importance in terms of mineral nutrient uptake. Ectotrophic mycorrhizal fungi typically show a thick sheath, or “mantle,” of fungal mycelium around the roots, and some of the mycelium penetrates between the cortical cells (Figure 5.10). The cortical cells themselves are not penetrated by the fungal hyphae but instead are surrounded by a network of hyphae called the Hartig net. Often the amount of fungal mycelium is so extensive that its total  mass is comparable to that of the roots themselves. The fungal mycelium also extends into the soil, away from this compact mantle, where it forms individual hyphae or strands containing fruiting bodies. The capacity of the root system to absorb nutrients is improved by the presence of external fungal hyphae that are much finer than plant roots and can reach beyond the areas of nutrient-depleted soil near the roots (Clarkson 1985). Ectotrophic mycorrhizal fungi infect exclusively tree species, including gymnosperms and woody angiosperms. Unlike the ectotrophic mycorrhizal fungi, vesiculararbuscular mycorrhizal fungi do not produce a compact mantle of fungal mycelium around the root. Instead, the hyphae grow in a less dense arrangement, both within the root itself and extending outward from the root into the surrounding soil (Figure 5.11). After entering the root through either the epidermis or a root hair, the hyphae not only extend through the regions between cells but also penetrate individual cells of the cortex. Within the cells, the hyphae can form oval structures called vesicles and branched structures called arbuscules. The arbuscules appear to be sites of nutrient transfer between the fungus and the host plant.  Outside the root, the external mycelium can extend several centimeters away from the root and may contain spore-bearing structures. Unlike the ectotrophic mycorrhizae, vesicular-arbuscular mycorrhizae make up only a small mass of fungal material, which is unlikely to exceed 10% of the root weight. Vesicular-arbuscular mycorrhizae are found in association with the roots of most species of herbaceous angiosperms (Smith et al. 1997). The association of vesicular-arbuscular mycorrhizae with plant roots facilitates the uptake of phosphorus and trace metals such as zinc and copper. By extending beyond the depletion zone for phosphorus around the root, the external mycelium improves phosphorus absorption. Calculations show that a root associated with mycorrhizal fungi can transport phosphate at a rate more than four times higher than that of a root not associated with mycorrhizae (Nye and Tinker 1977). The external mycelium of the ectotrophic mycorrhizae can also absorb phosphate and make it available to the plant. In addition, it has been suggested that ectotrophic mycorrhizae proliferate in the organic litter of the soil and hydrolyze organic phosphorus for transfer to the root (Smith et al. 1997).

Nutrients Move from the Mycorrhizal Fungi to the Root Cells

Little is known about the mechanism by which the mineral nutrients absorbed by mycorrhizal fungi are transferred to the cells of plant roots. With ectotrophic mycorrhizae, inorganic phosphate may simply diffuse from the hyphae in the Hartig net and be absorbed by the root cortical cells. With vesicular-arbuscular mycorrhizae, the situation may be more complex. Nutrients may diffuse from intact arbuscules to root cortical cells. Alternatively, because some root arbuscules are continually degenerating while new ones are forming, degenerating arbuscules may release their internal contents to the host root cells. A key factor in the extent of mycorrhizal association with the plant root is the nutritional status of the host plant. Moderate deficiency of a nutrient such as phosphorus tends to promote infection, whereas plants with abundant nutrients tend to suppress mycorrhizal infection. Mycorrhizal association in well-fertilized soils may shift from a symbiotic relationship to a parasitic one in that the fungus still obtains carbohydrates from the host plant, but the host plant no longer benefits from improved nutrient uptake efficiency. Under such conditions, the host plant may treat mycorrhizal fungi as it does other pathogens (Brundrett 1991; Marschner 1995).  


  The soil is a complex substrate—physically, chemically, and biologically. The size of soil particles and the cation exchange capacity of the soil determine the extent to which a soil provides a reservoir for water and nutrients. Soil pH also has a large influence on the availability of mineral elements to plants. If mineral elements, especially sodium or heavy metals, are present in excess in the soil, plant growth may be adversely affected. Certain plants are able to tolerate excess mineral elements, and a few species—for example, halophytes in the case of sodium—grow under these extreme conditions. To obtain nutrients from the soil, plants develop extensive root systems. Roots have a relatively simple structure with radial symmetry and few differentiated cell types. Roots continually deplete the nutrients from the immediate soil around them, and such a simple structure may permit rapid growth into fresh soil. Plant roots often form associations with mycorrhizal fungi. The fine hyphae of mycorrhizae extend the reach of roots into the surrounding soil and facilitate the acquisition of mineral elements, particularly those like phosphorus that are relatively immobile in the soil. In return, plants provide carbohydrates to the mycorrhizae. Plants tend to suppress mycorrhizal associations under conditions of high nutrient availability.  

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MIT Chemistry

Lecture 8: The Periodic Table and Periodic Trends
Lecture 9: Periodic Table; Ionic and Covalent Bonds
Lecture 10: Introduction to Lewis Structures
Lecture 11: Lewis Structures: Breakdown of the Octet Rule

MIT Biochemistry

Bishop-Introduction to Chemistry

Chapter 3: Chemical Compounds

Section 3.1: Classification of Matter

Section 3.2: Compounds and Chemical Bonds

Section 3.3: Molecular Compounds

Section 3.4: Naming Binary Covalent Compounds

Section 3.5: Ionic Compounds

Chapter 4: An Introduction to Chemical Equations

Section 4.1: Chemical Reactions and Chemical Equations

Section 4.2: Solubility of Ionic Compounds and Precipitation Reactions

Chapter 5: Acids, Bases, and Acid-Base Reactions

Chapter 5: Acids, Bases, and Acid-Base Reactions

Section 5.1: Acids

Sections 5.2 and 5.3: Acid Nomenclature and Summary of Chemical Nomenclature

Section 5.4: Strong and Weak Bases

Section 5.5: pH and Acidic and Basic Solutions

Section 5.6: Arrhenius Acid-Base Reactions

Section 5.7: Brønsted-Lowry Acids and Bases

soil reactions