After reading this article you will learn about the importance of cation exchange in plant nutrition.
Cation exchange is an important reaction in soil fertility, in correcting soil acidity and basicity, in altering soil physical properties, and as a mechanism in purifying or altering percolating waters. Plant roots, when they come in contact with colloidal particles, adsorb exchangeable cations directly by interchange or contact exchange between the root hairs and colloidal complex.
In fact, the usual “soil test” to predict a soil’s ability to furnish potassium to the plant is a measure of its exchangeable potassium content. The amounts of cation in the soil solution are intimately related to the exchangeable ions, and any change in concentration of a cation in the soil solution forces a change in proportions of all other exchangeable ions.
Thus, soil acts as a large cation exchanger. Any water moving through the exchanger will lose many of its soluble cations to the exchanger and pick up those cations that have been replaced from the exchange sites. Cation nutrients, such as potassium (K+), ammonium (NHJ) and calcium (Ca2+), will not move far in soil before they are adsorbed to the exchange sites.
This reduces their leaching loss but also reduces their mobility in soil. When potential water pollutants, such as lead (Pb2+) or cadmium (Cd2+), are disposed of a soil, instead of leaching easily they are adsorbed to the cation exchange sites.
They may finally form insoluble-hydroxides, carbonates, or other forms, but cation exchange helps initially to make them immobile and “cleanses” the percolating liquid and the environment.
The nature and content of exchangeable bases in soil have an important bearing on its physical, chemical and biological properties. In normal fertile soils the total exchangeable bases (Ca, Mg, K, and Na) constitute about 80-90 per cent of the total CEC. Out of all exchangeable bases, calcium is the dominant cation.
The pH value of such soil varies from 6.5-7.5 which is most conducive for the availability of plant nutrients. When the proportion of exchangeable hydrogen is high it gives rise to acid soil. In such soil, clay is saturated with hydrogen cations and forms H-clay and it is called base unsaturated soil. It is less fertile.
Out of total exchangeable cations in soil, when exchangeable sodium occupies more than 15 per cent it gives rise to soils of alkaline reactions. The pH value of such soil is usually greater than 8.5. The soil turns into a sodium saturated and forms Na-clay. This soil is less fertile.