Introduction to integrated methods in the vegetable garden
Chapter : Crop soil
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⇒ Agilo-humic complex and cation exchange capacity.
Some elements of humus, notably grey humic acids, humins produced by microbial flora and insolubilising humins, have the ability to bind to clay to form Clay-Humus Complexes (CHC) (CAH in french) which are known to be very stable. Clay micelles and electronegative humus molecules are linked together mostly by calcium cations. This type of bonding slows down the mineralisation of humus. The stabilisation of the whole is even greater when the humus compounds are polycondensed, favoured by soil conditions that are neither too acidic nor too basic (1). These aggregates play an important role in the structuring of soils by creating gaps between them filled with air and water.
CACs are also called "absorbing complexes", a term that includes other substances that have the ability to retain ions on their surface from the soil solution. The finer the clay micelles, the greater their retention capacity, which promotes the stability of the humus bonds. The negatively charged surfaces of CACs attract positively charged ions (cations). Among the cations bound by CACs, Mg₂⁺ (magnesium), K⁺ (potassium), Ca₂⁺ (calcium), Na⁺ (sodium), NH₄⁺ (ammonium) cations, which are plant nutrients, are also retained. For this reason, CACs are considered as nutrient reservoirs.
CACs are declared saturated when all H⁺ cations are replaced by other cations. In the soils usually studied, the proportion of cations fixed in clay-humus complexes is often this one: Ca₂⁺ = 75 to 90%, Mg₂⁺ = 10 to 30%, K⁺ = 5 to 10%, Na⁺ = 2 to 5% (2).
The bonds between CACs and cations are sufficiently weak to allow the cations to be retained in an exchangeable form. Specifically, a cation can only be released into solution if there is another cation capable of replacing it and if the binding strength of the latter to the absorbing complex is stronger than that of the cation already bound. CACs absorb cations in an order of decreasing affinity which is this: Ca₂⁺> Mg₂⁺> K⁺> NH₄⁺> Na⁺> (3). Exchanges take place until the electrical equilibrium of the medium is reached. Thus, uptake by plant roots of cations bound to CACs is only possible if other cations replace them. These exchanges take place in the rootlets.
In an acidic environment, H⁺ cations tend to replace other cations including Ca₂⁺ cations that bind humus and clay resulting in destructuring of the absorptive complex. To avoid this destructuring of CAC in calcium-poor soils, rapid calcium amendments should be applied as soon as possible. Lime can be used to quickly neutralise soil that is too acidic, but it is prohibited in organic farming.
Nitrates and chlorides are not retained by the absorbent complexes. On the other hand, ammonium ions are absorbed, which has the advantage of forming a nitrogen reserve that can be assimilated by plants. It is useful to remember that the assimilation of nitrogen by plants takes place in two ways:
Absorption of nitrates soluble in soil water produced by the decomposition of organic matter and industrial nitrogen fertilisers (urea, ammonium nitrate, etc.). Nitrates are the most important source of nitrogen used by plants.
Ammonia is absorbed by ion exchanges between the rootlets and the clay-humus complexes located at their contact. This is the only form of ammonia absorption by plants. This ammonia fixation by CAC has the advantage of limiting the losses of this form of nitrogen by volatilisation.
The fact that highly soluble nitrates cannot be fixed on CACs is a major problem in the control of nitrogen fertilisation, especially in organic farming, which prohibits the regulation of plant needs by a precise supply of nitrogenous mineral salts (see the article: the problem of nitrogen assimilation in organic farming). On the other hand, ammonium cations compete with other elements (notably calcium and magnesium) reducing the stock of ammonium cations fixed by CAC. Thus the stock of ammonium is often insufficient to meet the needs of the plants in the most critical periods.
If the soil solution is modified by the addition of fertilizers containing water-soluble elements, some cations (Ca₂⁺, H⁺...) will leave the clay-humus complex to be replaced by other cations from these fertilizers (except nitrates). It is obvious that these properties are very important in fertilisation. The way CACs fix these nutrients prevents them from being lost through leaching.
In very calcareous soils, there is often a high content of water-soluble calcium, which is further accelerated by the CO₂ produced by high biological activity following the addition of organic matter. This calcium ends up saturating the clay-humus complexes to the detriment of other cations such as potassium and phosphorus, which may then be lacking. This is why soils with too much calcium have difficulty retaining mineral fertilisers. To reduce the influence of limestone, clay amendments must be applied, the details of which are described here.
CACs contribute strongly to the storage of phosphorus and potassium, which are important elements of soil fertilisation. A soil analysis every 3 to 5 years allows monitoring the evolution of this reservoir.
Phosphorus is present in the soil in organic or mineral form. The organic form comes from the degradation of organic matter, whereas the mineral form results either from the alteration of the parent rock (such as apatite, in which phosphorus is associated with calcium), or from the recombination of phosphate ions present in the soil solution with iron, calcium or aluminium to form phosphates that are insoluble in water.
This retrogradation phenomenon is more important in very calcareous or very acidic soils and is accentuated by high summer temperatures or when phosphorus is concentrated on the surface. Furthermore, in relation to a pH value < 7 or > 7, insoluble phosphorus salts will have different properties.
In acidic soils, phosphate ions react easily with aluminium and iron to form more stable compounds than the solid phosphate compounds that form in alkaline soils. This leads to a real blockage of phosphorus in acidic media, which can be reversed by liming to increase the pH.
Since solid phosphates cannot be assimilated by plants, they end up accumulating in the soil and represent 95% of the total phosphorus reserve in the soil.
Plants have access to phosphorus from the following 4 sources:
By assimilation of phosphate ions present in the soil solution, these ions coming from the parent rock or from organic fertilizers. Phosphorus can only be assimilated by the roots when it is in the form of ions present in the soil solution. This soluble phosphate is in dynamic equilibrium with orthophosphate fixed in solid compounds. This equilibrium is characterised by a very small amount of soluble phosphorus of the order of 0.1 to 0.4% of the total phosphorus present in the soil. This form of phosphorus uptake is therefore not very profitable. The concentration of orthophosphate ions in the soil solution depends on the pH, which affects the solubility of complexes formed with other elements. As soon as phosphorus disappears from the solution (absorbed by plant roots), it is replaced by drawing on the reserves of phosphorus fixed in the soil until the dynamic equilibrium between free phosphates and phosphates blocked in the soil is restored.
By chemical processes to make a phosphorus salt soluble in water. The dicalcium phosphate found in industrial fertilisers under the name of "phosphate soluble in ammonia citrate" is directly assimilated by plants.
By taking up phosphate ions stored in exchangeable form in the absorbing complexes. For this, the soil must be rich in CAC. The phosphate ions are then returned to the plants when they need them. Phosphate ions that are not absorbed by the plants are stored via calcium ions attached to the CAC. The positively charged calcium ions form a bridge with the negatively charged phosphate ions. In soils rich in absorbing complexes and especially CAC, this form of phosphate represents about 5% of the total mass of phosphate present in the soil and this reserve is available to the plants when it is close to the rootlets.
Phosphorus is released by fungi and bacteria such as Bacillus Amyloliquefaciens, which is a strict aerobic bacterium living in the soil. This microorganism, which colonises plant roots, generates a phytase enzyme that enables it to release organic phosphates from the soil. Mycorrhizal fungi, which thrive in symbiosis in plant roots, contribute up to 90% of the tricalcium phosphate absorption of mineral fertilisers when the soil contains sufficient calcium ions (4).
It is obvious that a good supply of CAC is necessary to allow a progressive assimilation of phosphates without forcing with fast-acting chemical fertilizers or nervous organic fertilizers. This is especially true since a massive supply of soluble phosphate is not necessarily the right solution when the soil is deficient in humus, as phosphates are rapidly fixed with iron and calcium to form new insoluble compounds. Very often, the reserves of phosphates blocked in the soil are sufficient to meet the needs of the plants. It is sufficient to release them by adding humus to a soil with a good clay content.
Potassium is present exclusively in 3 mineral forms in the soil:
It is part of the minerals of the parent rock (micas, feldspars), or it is trapped in clay sheets. Depending on the nature of these clays, potassium fixation is more or less high. It is the most abundant form, but its release is very slow.
It is present in the exchangeable form on the surface of clays and CAC.
It is in solution in the soil water.
Potassium is a more or less mobile element depending on soil characteristics. Leaching is limited when soils are well maintained and rich in CAC. It is more important in sandy soils poor in humus or with low clay content. Losses can be as low as 1 kg potassium ha/year if the soil contains 24% clay and as high as 46 kg ha/year when the soil contains only 5% clay (5). The reserves of exchangeable potassium fixed by the absorbing complexes can be multiplied by up to 23 depending on the type of soil (6).
Apart from CAC, there are other elements in the soil, in particular free clays (kaolites, montmorillonites, Illites...) or absorbent elements brought by the gardener (zeolites, blond peat...) which are able to retain cations. The Cation Exchange Capacity (CEC) is an indicator of the potential for fixing and storing cations that all absorbent complexes are capable of storing and releasing. The CEC depends on the clay and organic matter content and the composition of the clays. Other materials have a negligible influence on CEC (a).
As far as clays are concerned and to give some examples, humus has an exchange capacity 30 times higher than kaolites, 10 times higher than illites, 3 times higher than montmorillonites.
Exchangeable bases, an expression often encountered in agronomy studies or laboratory analysis reports, are cations (notably K⁺ Ca₂⁺ Mg₂⁺ Na⁺) fixed on clay-humus complexes and other colloidal substances that can be exchanged with those present in the soil solution. For these nutrient reserves, a laboratory analysis should at least specify the values found in relation to an optimum as well as the saturable absorption rate (SA/T) (T/S in french) of the absorbing complexes. The latter can retain strongly-bound substances such as strong acids that prevent cations useful to plants from binding to the uptake sites. The SA/T ratio gives the saturation level of weak acids (corresponding to the useful cations that can be absorbed) that an absorber complex is capable of binding. For example, a SA/T of 60 indicates that 60% of the sites in the complex are used to bind weak acids, with the remaining 40% taken up by strong-binding elements.
Source : Agr'eau -2b - Objectifs - Analyser son sol pour mieux le connaître - Chambre d'agriculture de la Drome - nov 2013
CEC is expressed in laboratory analyses most often in meq/100g. This value specifies the number of ammonium cations (NH₄⁺) expressed in number of charges per 100 g (milliequivalents per 100g) that all the exchange sites of the CEC are able to absorb measured by automatic spectrophotocolorimetry.
The higher the CEC, the more cations can be absorbed and released by the soil and made available to the roots. The CEC value is modulated by various factors such as the amount of calcium and the soil pH.
Most laboratories approved by the Ministry of Agriculture are able to carry out analyses to determine the CEC using a standardised method.
a) All minerals with extremely small particle sizes have a small CEC that forms at bond breaks. The CEC increases as the particle size decreases. But their exchange capacity due to broken bonds is insignificant
1) Le sol vivant Base de pédologie – biologie des sols – J.M. Gobat, M. Aragno, W. Matthey
2) Les facteurs chimiques de la fertilité des sols (bases échangeables ; sels ; utilisation des échelles de fertilité) - B. DABIN
3) Écologie – approche scientifique et pratique – 6e édition - Claude Faurie & all
4) The mycorrhizal contribution to plant productivity, plant nutrition and soil structure in experimental grassland - Marcel G. A. Van Der Heijden & all – New phytologist - 15 8 2006,
5) Askegaard et al. (2004)
6) Havlin et al., 2004