Clay-humus complexes and cation exchange capacity

Clay-humic complexes (CHCs)

Certain elements of humus, particularly grey humic acids and humins, have the ability to bind with clay to form clay-humic complexes (CHCs), which are known to be very stable. Clay micelles and electronegative humus molecules are most often linked together by calcium cations. This type of bond slows down the mineralisation of humus.

The stabilisation of the whole is all the greater when the humic compounds are polycondensed, favoured by soil conditions that are neither too acidic nor too alkaline (1). These aggregates play an important role in soil structure by creating gaps between them that are filled with air and water.

CAHs are also known as “adsorbent complexes”, a term that encompasses other substances capable of retaining ions present in the soil solution on their surface.

The finer the clay micelles, the greater their retention capacity, which promotes stable bonds with humus. The negatively charged surfaces of CAHs attract positively charged ions (cations). Among the cations fixed by CHCs, Mg₂⁺ (magnesium), K⁺ (potassium), Ca₂⁺ (calcium), Na⁺ (sodium) and NH₄⁺ (ammonium) cations, which are nutrients for plants, are also retained. For this reason, CAH are considered to be nutrient reservoirs.

CHCs are declared saturated when all H⁺ cations are replaced by other cations. In soils that are commonly studied, the proportion of cations fixed in clay-humic complexes is often as follows: Ca²⁺ = 75 to 90%, Mg²⁺ = 10 to 30%, K⁺ and NH₄⁺ = 5 to 10%, Na⁺ = 2 to 5% (3).

Weak bonds in CHCs that allow cation exchange with roots

The bonds between the CHCs and the cations are weak enough to allow the cations to be retained in an exchangeable form. More specifically, a cation can only be released into a solution if there is another cation capable of replacing it and if the bond strength of the latter with the absorbing complex is stronger than that of the cation already bound. CHCs absorb cations in descending order of affinity, as follows: Ca₂⁺> Mg₂⁺> K⁺> NH₄⁺> Na⁺> (3). Exchanges take place until the electrical equilibrium of the medium is reached. Thus, the absorption by plant roots of cations fixed on CAHs is only possible if other cations replace them. These exchanges take place at the level of the rootlets.

An acidic environment destroys the bonds in clay-humic complexes.

In an acidic environment, H⁺ cations tend to replace other cations, including Ca²⁺ cations, which bind humus and clay, leading to the breakdown of the absorbent complex. To prevent this breakdown of CHCs. in calcium-deficient soils, calcium amendments must be applied as soon as possible. Lime can be used to quickly neutralise overly acidic soil, but it is prohibited in organic farming.

Nitrogen fixation by clay-humic complexes

Nitrates and chlorides are not retained by the adsorbent complexes. However, ammonium ions (NH₄⁺) are absorbed, which has the advantage of forming a reserve of nitrogen that can be assimilated by plants. It is worth remembering that nitrogen assimilation by plants occurs in two ways:

• Absorption of water-soluble nitrates in soil produced by the decomposition of organic matter and industrial nitrogen fertilisers (urea, ammonium nitrate, etc.). Nitrates are the most widely used source of nitrogen for plants.
• Absorption of ammonia through ion exchange between rootlets and clay-humic complexes located in contact with them. This is the only form of ammonia absorption by plants. This fixation of ammonia by clay-humic complexes has the advantage of limiting losses of this form of nitrogen through volatilisation.

The fact that highly soluble nitrates cannot be fixed on CHCs is a major problem in controlling nitrogen fertilisation, especially in organic farming, which prohibits regulating plant needs through the precise addition of nitrogenous mineral salts (see article: the problem of nitrogen assimilation in organic farming). Furthermore, ammonium cations compete with other elements (particularly calcium and magnesium), reducing the stock of ammonium cations fixed by HACs. As a result, the ammonium stock is often insufficient to meet plant needs during the most critical periods.

Clay-humic complexes and reduction of soluble element losses from fertilisers

If the soil solution is modified by the addition of fertilisers containing water-soluble elements, certain cations (Ca₂⁺, H⁺, etc.) will leave the clay-humic complex and be replaced by other cations from these fertilisers (except nitrates). It is clear that these properties are very important in terms of fertilisation. The way in which these nutrients are fixed by CHCs prevents them from being lost through leaching or 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.