The adsorption of cations and their exchange, together with that of anions on the surface of colloids, is probably second in importance only to photosynthesis. Understanding cation exchange properties of soil requires a knowledge of the source of negative charges responsible for adsorption and desorption of cations.
Within the soil profile we expect the A horizon to have a higher CEC as compared to the remainder of the profile because of high organic matter together with any clay that may be present. The B horizon with its higher clay content would also exhibit high CEC, but not as great as the A horizon; organic matter has a much higher CEC. A well-developed E horizon has low CEC since it is depleted in organic matter and clay. Soils with high CEC have a higher capacity to provide plants with nutrients, especially the cations, if present in the soil. Nutrient loss following fertilization on high CEC soils is lower of greater adsorption on exchange sites.
CEC is always expressed as centimoles of positive charge per kilogram of soil (cmolc/kg).
In a solution, colloids are dispersed about the liquid. Each particle moves independently of the others. The small size of the particles contributes to this independence. Dispersion is primarily due to the negative charge and the degree of hydration of the particles themselves. Refer to Figure 9.2 in the lab manual.
Like-charged particles repel each other, and unlike-charged particles attract (Fig. 9.2 a). Highly hydrated ions (i.e. Na) with a valence of 1 and a large hydrated radisu have a low charge density. When those cations are adsorbed on colloid surfaces, they do not effectively counter net negative charges. Consequently, the negatively charged colloid particles repel each other (Fig. 9.1b) and remain in suspension. The opposite occurs when higher valence cations which have a smaller hydrated radius (i.e. Al+3). The large charge density allows those cations to completly counter the net negative charge of the suspended colloids (Figure 9.1c). The net negative charge is neutralized and the particles which no longer repel each other are able to come into contact with each other. Van der Waals forces act to keep those particles together and the floccules fall out of solution. It is important to remember that the net negative charges are increased or decreased; the permanent negative charges remain unchanged. This has a practical application; when the electronegativity of particles is high, the particles are dispersed. We took advantage of this phenomenon in the texture exercise. If you recall, you added a dispersing agent, Na-hexametaphosphate, to the suspension before mixing. The purpose was to disperse the clay from the surfaces of sand and silt particles and from clay particles themselves so as to hold the clay in suspension while the sand and silt settled to the bottom of the cylinder.
The practical significance of dispersion from soil is two-fold. First, dispersion needs to occur before clays are translocated from the upper horizons to the subsoil as a pedogenic process. Secondly, clays are dispersed in soils that have high sodium or potassium concentrations, as in soils of semi-arid or arid regions. Dispersion prevents the formation of structure and so these soils are easily puddled. The addition of salty irrigation water in dry areas such as Arizona, California, or Colorado, has the same effect. To reduce the effects of salt water irrigation, farmers must use valuable underground freshwater to flush the salts out of the soil. This is a costly use of potable water.
Divalent or trivalent cations are not as highly hydrated as the monovalent cations. So, if we introduce Ca++ or Al+++ to exchange with the monovalent Na, not only are we replacing 2 or 3 Na ions and satisfying 2 or 3 negative charges on the particle, the attraction is stronger because the distance between the polyvalent cations and the charge source is smaller for the mulitvalen cation than it was for Na. The net negativity of the particle is reduced sufficiently so that the clay particles tend to be attracted to each other through the action of Van derWaal's forces.
Particles flocculate into large aggregates, the basis of structure. In a laboratory cylinder, the floccules settle to the bottom of the cylinder after coagulating. Flocculation of clay in the soil forms clay coatings or skins on ped surfaces. Calcium is a strong flocculating agent. The clay coats promote coherence of sand and silt particles which have no surface activity, and enhances the formation of clay aggregates as cementing agents, all together contributing to structure formation. Where this process predominates in the presence of organic matter, granular structure results. The binding effects of roots, freezing and thawing cycles and wetting and drying cycles also contribute to structure formation by causing shrinking and swelling of the soil. Microbial gums and slimes at as cementing agents for structural aggregates as well.
Finally, the soil cation exchange capacity is a for plant-available nutrients. The action is at the interface where soil colloid meets root colloid in a bath of soil solution! Our challenge is to focus on the action by some type of instant replay with the aim of developing a better understanding of the game being played.