Calcareous Soils

What is Calcareous Clay?

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

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

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

Excerpts- papers on nutrition in calcareous soils

Food Availability in Alkaline-Calcareous Soils (pdf)

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


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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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


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

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

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

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

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

b.         Potassium absorption increases with decrease in pH value.

c.         Calcium absorption increases with decrease in pH value.

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

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

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

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

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

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


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

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

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

Phosphorus Availability with Alkaline/Calcareous Soil (pdf)

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


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

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


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

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


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

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

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


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

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

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


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

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

Nitrogen Application in Warm, Dry Weather

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

Nitrogen in the Environment: Denitrification

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

Two main factors influence denitrification:

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

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

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

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

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

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

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


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

 Specifically, we found positive linear correlations between

 soil test K and tissue N,

soil test B and tissue N, and

soil test K and tissue K

concentrations, and found a significant negative inverse relationship between

 soil test Fe and tissue Mn concentration.

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

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

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

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

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

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


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

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

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


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

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

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

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

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

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

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

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

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

2.2.         Soil iron availability

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

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

Other iron containing compounds

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

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

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

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

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

Citrus Fertilizer Management on Calcareous Soils (pdf)

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

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

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

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

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

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

The Effect of CaCO3 On Magnesium and Potassium

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

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

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

The Effect of CaCO3 On Phosphorus

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

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

The Effect of CaCO3 On Zinc and Manganese

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

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


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


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

The Real Dirt on Austin Area Soils

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

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

Edwards Plateau Description and Gardening Strategies

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

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

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

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

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

Slow Down Runoff

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

Add Organic Matter for Soil Health

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

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

Chemistry pH

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

Table 6.2

Names of the Monatomic Anions


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

Table 6.3

Common Polyatomic Ions

NH4+ammonium3- PO4phosphate2- SO4sulfate
2- CO3carbonate    

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

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

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

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

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

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

HCl Water
acid solution

Types of Arrhenius Acids

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

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

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

Acid Review

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

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

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

completion reaction

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

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

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

reversible reaction

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

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

ammonia water 8-1
ammonia water solution

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

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


nitric solution

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

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

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

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

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

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

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

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

H3O+(aq) + OH-(aq)      ®    2H2O(l )

nitric acid solution
niitric acid

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

H+(aq) + OH-(aq)   ®    H2O(l )


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

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

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

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

ph scale solutions chemistry

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

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

Active and exchangeable acidity

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

pH = – log [H+]

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

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

titrate acidity

Soil pH and Salt Concentration

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

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

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

soil pH scale


Single nutrient (“straight”) fertilizers

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

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

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

Multinutrient fertilizers

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

Binary (NP, NK, PK) fertilizers

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

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


Soil pH

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

Sources of Soil acidity

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

Sources of Soil alkalinity

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

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

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

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

Treatment of High pH Soil

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

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

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

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

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

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

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


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

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

Bishop chemical reactions

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

Interpreting a Chemical Equation

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

H2 gas burn
Gas burn 7-2

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

2H2(g) + O2(g) → 2H2O(l)

Chemical equations give the following information about chemical reactions.

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


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

cations and acidity
acid cations soil space

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

What is basic cation saturation?

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


 Chapter 5: Chemical Compounds

Chapter 6: More on Chemical Compounds

Chapter 7: An Introduction to Chemical Reactions

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

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

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

Soil and Roots

Clay and CEC


Molecular FormulaAl2H2O12Si4
silicon tetrahedron
aluminum octahedron
clay layers
Montmorillonite structure

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

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

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

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


Cation Exchange:

cation exchange CEC 1
CEC clay

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

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

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

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

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

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

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



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

Negatively Charged Soil Particles Affect the Adsorption of Mineral Nutrients

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

Gravel has particles larger than 2 mm.

• Coarse sand has particles between 0.2 and 2 mm.

Fine sand has particles between 0.02 and 0.2 mm.

 • Silt has particles between 0.002 and 0.02 mm.

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

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

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

clay mineral structure

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

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

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

Excess Minerals in the Soil Limit Plant Growth

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

Plants Develop Extensive Root Systems

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

oots 1
roots by species differ

Root Systems Differ in Form but Are Based on Common Structures

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

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

Different Areas of the Root Absorb Different Mineral Ions

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

Mycorrhizal Fungi Facilitate Nutrient Uptake by Roots

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

Nutrients Move from the Mycorrhizal Fungi to the Root Cells

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


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