2. Nature and properties of soil colloids.

The minute silicate-clay colloid particles, which are referred to as micelles, ordinarily carry a negative charge. As a consequence; thousands of positively charged ions or cations are attracted to each colloidal crystal. This gives a rise to what is known as ionic double layer:

Fig. 1. Double ionic layer on a colloidal crystal of a silicate clay.

The colloidal particle constitutes the inner ionic layer, being essentially a huge anion, the surface of which is highly negative in charge. Outer ionic layer is made up of a swarm of rather loosely held cations. They surround and in some cases penetrate particle. Thus a clay particle is accompanied by a tremendous number of adsorbed cations. Associated with the layer of cations is a large and indefinite amount of water. Part of this water is carried by adsorbed cation which are hydrated. In addition, most silicate clays hold numerous water molecules packed between the plates that make up a clay micelle.

Schematically the colloidal micelles of some soil colloids may be represented as follows:

(1) - micelle

(2) silicate – clay micelle:

(3) Al (OH)₃ micelle in acid solution (H<S,1):

(4) Al (OH)₃ – micelle in alkaline solution (pH>8,0):

(5) Organic colloidal micelle:

(pH<8,0)

(pH>8,0)

Colloids capable to change their charge depending on soil reaction are called ampholitoides. But the majority of soil colloids are acid ones: they carry a negative charge compensated by the cations, a part of which are more loosely held and can be exchanged for an equivalent number of cations from the surrounding solution. These are called exchangeable cations. Between them and the rest of a colloidal particle there arises the so-called electrokinetic potential (z-potential).

Exposed crystal edges and isomorphic substitutions may be an additional source of the negative charge. The charge of many soil colloids is pH-dependent and so is their CEC. For organic colloids, kaolinite, and non-silicate clays, the CEC rises with increasing soil pH. For smectites it also rises but much less significantly.

Among the other properties of soil colloids are plasticity, cohesion, flocculation and dispersion.

3. Pole in soil genesis and soil productivity development.

Soil colloids and colloidal states of substances are very important in soil formation. An accumulative type of soil profile is the result of soil organic matter and other colloid flocculation when united with calcium and other exchangeable cations. In chernozems this is a flocculated gel of calcium humate. On the other hand, the formation of differentiated profiles is connected with dispersion of flocculated humus and other colloids by the substitution of exchangeable Ca (sometimes Mg) by hydrogen and sodium cations in podzolic and solonetzic soils respectively. Soil structure and proper granulation are to a great extent the result of soil colloid flocculation. Flocculation is a term applied to a coagulation of the dispersed particles. A very good example of flocculation is afforded by treating a colloidal suspension with a small amount of calcium hydroxide. The tiny clay particles almost immediately coalesce into floccules. Because of their combined weight, these sink to the bottom of the containing vessel, leaving the supernatant liquid clear. The coagulating capacities of the several cations differ, but ranked more or less in the order of Al³⁺>Ca²⁺ and H⁺>Mg²⁺>K⁺>Na⁺. Flocculation is associsted with z-potential covering. Soil plasticity (the pliability and the capacity of being molded) is due to the plate like nature of the clay particles and the lubricating yet binding influence of the adsorbed water. Plasticity is extremely important because it encourages a ready change in soil structure, soil pliability to tillage, etc.

Cohesion is the tendency of the clay particles to stick together. Montmorillonites exhibits cohesion to stick to a much more noticeable degree then does kaolinite or hydrous oxides. Humus, by contrast, tends to reduce the attraction of individual clay particles to each other.

The adsorbing capacity of soil colloids favors the accumulation and economic expenditure of many nutrient cations (and anions) in plant nutrition. The sanitary (toxic substance binding) function of the soil is very much dependent on the amount and properties of soil colloids.