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Cation exchange, between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful.
The negative charges on a colloid particle make it able to hold cations to its surface. The charges result from four sources.
- Isomorphous substitution occurs in clay when lower-valence cations substitute for higher-valence cations in the crystal structure. Substitutions in the outermost layers are more effective than for the innermost layers, as the charge strength drops off as the square of the distance. The net result is a negative charge.
- Edge-of-clay oxygen atoms are not in balance ionically as the tetrahedral and octahedral structures are incomplete at the edges of clay.
- Hydrogens of the clay hydroxyls may be ionised into solution, leaving an oxygen with a negative charge.
- Hydrogens of humus hydroxyl groups may be ionised into solution, leaving an oxygen with a negative charge.
Cations held to the negatively charged colloids resist being washed downward by water and out of reach of plants’ roots, thereby preserving the fertility of soils in areas of moderate rainfall and low temperatures.
There is a hierarchy in the process of cation exchange on colloids, as they differ in the strength of adsorption and their ability to replace one another. If present in equal amounts:
Al3+ replaces H+ replaces Ca2+ replaces Mg2+ replaces K+ same as NH4+ replaces Na+
If one cation is added in large amounts, it may replace the others by the sheer force of its numbers (mass action). This is largely what occurs with the addition of fertiliser.
As the soil solution becomes more acidic (an abundance of H+), the other cations bound to colloids are pushed into solution. This is caused by the ionisation of hydroxyl groups on the surface of soil colloids in what is described as pH-dependent charges. Unlike permanent charges developed by isomorphous substitution, pH-dependent charges are variable and increase with increasing pH.
As a result, those cations can be made available to plants but are also able to be leached from the soil, possibly making the soil less fertile. Plants will excrete H+ into the soil and by that means, push cations off the colloids, thus making those cations absorbable by the plant.