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

nutrient table

N P K, C M S, I M Z, C B M.



Uptake form

Mobility in Plant
Mobility in Soil
Acid Deficiency
Alkaline Deficiency
gas and water C, H, O
Carbon, Hydrogen, Oxygen
Carbon © 50% dry weight  present in all macromolecules CO2, H2CO3
 Carbonic acid
Hydrogen (H) h2o is >80% plant weight part of many organic compounds and also forms water Hydron H+, OHHydroxide , H2O        
Oxygen (O)   necessary for cellular respiration; plants use oxygen to store energy in the form of ATP. O2 oxygen gas        


C M S , (Ca Mg S)

N, P, K    Ca Mg S
Nitrogen, Phosporous, Potassium ;
Calcium, Magnesium, Sulfur
Nitrogen (N) 4.00% A component of chlorophyll, nucleic acids, proteins, and
NO3Nitrate,  NH4+ ammonium ion,
also see Urea CH4N2O , and Ammonium nitrate (NH4NO3)
  Mobile as NO3, immobile as NH4+ y Volatilization Urea
Phosphorus 0.50% Required to store and transport energy HPO42- Hydrogen phosphate,
H2PO4 Dihydrogen phosphate
  Immobile y y
Potassium 4.00% Acts as a osmotic regulator in water absorption and loss
by the plant.
K+     y  
Calcium 1.00% Cell structure, secondary plant hormone Ca2+, see also Lime
calcium carbonate (CaCO3).
Magnesium 0.50% Central ion in the chlorophyll molecule Mg2+   Immobile y  
Sulfur 0.50% A component of nucleic acids and proteins SO4 sulfate ion     y  

I M Z , ( Fe Mn Z)

C B M ,  (Cu B Mo)

Fe, Mn, Zn ; Cu, B, Mo
Iron, Manganese, Zinc, Copper, Boron, Molybdenum
Iron 200 ppm Required for chlorophyll synthesis and
energy transferring pathways
Fe2+, Fe3+ Low Immobile   y
Manganese 200 ppm Required for chlorophyll production
and energy transferring pathways
Mn2+ Low     y
Zinc 30 ppm Activates enzymes Zn2+ Low Immobile   y
Copper 10 ppm Involved in respiration and oxidation/reduction reactions Cu2+ Low Immobile   y
Boron 60 ppm cell division and differentiation of young tissue H3BO3Boric Acid,
Low     y
Molybdenum 1 ppm Involved in nitrogen metabolism MoO4Molybdate Low   y  



Sodium 500 ppm Osmotic regulator Na+     10−3 g mg milligram
Chlorine 0.10% Required for photosynthesis Cl     103 g kg kilogram
Silicon 0.05-0.15% Pathogen defense, drought and heat tolerance H4SiO4     10−6 g µg microgram (mcg)
Cobalt     Co2+ Low   1 ppm =  1/1,000,000 =0.000001 = 0.0001%
= 1 mg/kg
Nickel     Ni2+    

Refrences –

Nutrient Uptake in Plants, Smart Fertilizer

Soil analysis: key to nutrient management planning, PDA

The most commonly found nutrient deficiency and toxicity symptoms are presented in the table below:

NutrientDeficiency SymptomsToxicity Symptoms
Nitrogen (N)Stunted growth and restricted growth of lateral shoots. Plants express general chlorosis of the entire plant to light green and yellowing of older leaves which proceeds to younger leaves. Older leaves become necrotic and defoliate earlyPlants are stunted, deep green in color, and secondary shoot development is poor. High N causes vegetative bud formation instead of reproductive bud formation. Ammonium toxicity can cause roots to turn brown, with necrotic root tips; reduce plant growth; necrotic lesions occur on stem and leaves; vascular browning occurs in stems and roots.
Phosphorus (P)Stunted growth. Purplish coloration of older leaves in some plants. Dark green coloration with tips of leaves dying. Delayed maturity, Poor fruit and seed development.Excess P in the plant can cause iron and zinc deficiencies.     
Potassium (K)Leaf margins turn chlorotic and then necrotic.  Tip and marginal burn starting on mature leaves.  Lower leaves turn yellow.  Weak stalks and plant lodge easily.  Slow growth. High amounts of K can cause calcium (Ca), magnesium (Mg) and N deficiencies.  
Magnesium (Mg)Interveinal chlorosis on older leaves which proceeds to the younger leaves as the deficiency becomes more severe.  The chlorotic interveinal yellow patches usually occur toward the center of the leaf with the margins being the last to turn yellow.   Curling of leaves upward along margins.  High Mg can cause Ca deficiency. 
Calcium (Ca)Light green color on uneven chlorosis of young leaves.  Brown or black scorching of new leaf tips and die-back of growing points.  Growing points of stems and roots cease to develop.  Poor root growth and roots short and thickened. High Ca can cause Mg or Boron (B) deficiencies. 
Sulfur (S)Uniform chlorosis first appearing on new leaves.  
Iron (Fe)Interveinal chlorosis of new leaves followed by complete chlorosis and or bleaching of new leaves.  Stunted growth. 
Zinc (Zn)Interveinal chlorosis of new leaves with some green next to veins.  Short internodes and small leaves.  Rosetting or whirling of leaves.  
Manganese (Mn)Interveinal chlorosis of new leaves with some green next to veins and later with grey or tan necrotic spots in chlorotic areas.  
Copper (Cu)Interveinal chlorosis of new leaves with tips and edges green, followed by veinal chlorosis.  Leaves at the top of the plant wilt easily followed by chlorotic and necrotic areas in the leaves.  Dieback of terminal shoots in trees.  
Boron (B)Death of terminal buds, causing lateral buds to develop and producing a ‘witches broom’ effect. Symptoms develop as a yellow-tinted band around the leaf margins.  The chlorotic zone becomes necrotic and gray, while the major portion of the leaf remains green. 
Molybdenum (Mo) Older leaves show interveinal chlorotic blotches, become cupped and thickened.  Chlorosis continues upward to younger leaves as deficiency progresses.  

Diagnosing Nutrient Deficiencies

EC and pH

pH and ECsymptomscorrections
High salts ECSmall, thick, and dark colored leaves. Lower leaves may turn yellow and brown, especially on the leaf margins. Plants become stunted.   Roots are less vigorous, may have brown leaves, and are likely to become diseased.The solution to high salt levels is usually to “leach out” (apply excess water to wash through the container) the growing medium or potting mix (termed the “root substrate” throughout this book).
Low EC ( see specific nutrient deficiency)“chlorosis” (yellowing, resulting from lack of the green chlorophyll pigment that is essential for photosynthesis). These symptoms can be seen on this geraniu   Nitrogen, phosphorus, and other nutrients are being moved out of its lower leaves to provide N and P to the growing points. The plant is essentially cannibalizing itself, because it cannot get nutrients from the roots.   Growth and flowering can be greatly reduced    The solution to low fertilizer levels is simply to apply more fertilizer. However, it is important to diagnose that nutrient levels are indeed low using an onsite soil test method and checking EC is below the adequate range
Substrate pH is too high (alkaline, pH values above 7 are “basic”)  e.g. Iron Symptoms to look for with iron deficiency at high pH:    Chlorosis either in the entire leaf or between the darker leaf veins (“interveinal chlorosis”)     Symptoms show up first in young leaves, in contrast to an overall low nutrient level, which may be in all or older leaves.   With sensitive plants such as the calibrachoa above, growth is severely stunted and growing points turn white and even necrotic (the tissue dies).Reduce excess limestone, or alkaline water, nitrate fertilizer’s basic effects.   To correct a high pH problem, a combination of acid fertilizer (containing ammonium or urea nitrogen) and adding acid into the irrigation water to remove alkalinity can usually drop substrate-pH.   Drenches (irrigations applied to the root substrate) with additional iron in a highly soluble form are also very effective at corr
Substrate pH is too low (acidity)Mn and Fe toxicity (over availability)   Chlorotic (yellow) or necrotic (brown and dead) spots or leaf margins in older leaves,   In marigold, the symptoms appear as brown sandy-colored spots in older leaves  The most common causes of a drop in substratepH are insufficient limestone, a fertilizer high in ammonium or urea nitrogen, which has an acidic reaction. Some plant species  such as geranium also tend to drop substrate-pH. An application of a liming material such as flowable limestone or potassium bicarbonate, and a change to a basic nitrate fertilizer are needed to raise substrate-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 1. Soil-borne elements essential for plant growth, the form occurring in the soil and taken up by the plant, and their relative soil mobility.
Element (symbol)Form taken up by the plantMobility in the soil
Nitrogen (N)(NH4)+ Ammonium form
(NO3) Nitrate form
Somewhat immobile
Phosphorous (P)(H2PO4) Dihydrogen phosphate, (HPO4)-2,  Hydrogenphosphate PO-3  phosphate anionImmobile
Potassium (K)K+Somewhat mobile
Calcium (Ca)Ca+Somewhat mobile
Magnesium (Mg)Mg+2Somewhat mobile
Sulfur (S)(SO4)-2 sulfateMobile
Chlorine (Cl)ClMobile
Iron (Fe)Fe+2Immobile
Boron (B)(BO3)Mobile
Manganese (Mn)Mn+2Immobile
Zinc (Z)Zn+2Immobile
Molybdenum (Mo)(MoO4)Mobile
Mn, ZnLet’s talk about manganese (MN) and zinc (Zn)
Manganese Deficiency in Palms – Ask IFAS
Manganese (Mn) and Zinc (Zn) for Citrus Trees

Manganese Fertility in Soybean Production
Zinc Deficiencies and Fertilization in Corn Production

mosaic Resources Zn, and Mn
Troubleshooting Magnesium Deficiencies in Corn
Sulfur Fertility for Crop Production
Mosaic Resources S

Sulfur Amendments to Lower Soil pH – UF/IFAS Extension ..

Iron (Fe) and Copper (Cu) for Citrus Trees – Ask IFAS
Iron (Fe) Nutrition of Plants – University of Florida

Iron Chlorosis
Management of Soybeans on Soils Prone to Iron Deficiency Chlorosis
Soil Fertility – Base Saturation and Cation Exchange Capacity
Environmental Fates, Nutrient Demands, and Efficient Nitrogen Fertility Programs
mosaic Micronutrients

UF/IFAS Standardized Fertilization Recommendations for …
Basics of Soybean Fertility

Phosphorus and Potassium Fertility for Corn and Soybean
Molybdenum Fertility in Crop Production
Molybdenum (Mo) and Nickel (Ni) for Citrus Trees

Growing Pomegranates in Florida … – University of Florida
Guide to Olive Tree Nutrition in Florida –
Develop Your Own Florida Olive IPM … – University of Florida