Potassium occurs in soils in four different forms, namely water-soluble, exchangeable, nonexchangeable and mineral (Reitemeir, 1951; Munson & Nelson, 1963). Water-soluble and exchangeable forms are considered to be directly available to plants. Under continuous cropping one may therefore expect depletion of these two forms, but in fact not much discernible difference in the amounts of available K was observed even after a few crops had been grown. Hence several people have stated that there exists an equilibrium among the following forms of K

This equilibrium implies that a change in magnitude of a form is compensated by the movement from, to and among other forms (Reitemeir, 1951; Garman, 1957; Munson & Nelson, 1963; Chevalier, 1971). If water-soluble and exchangeable forms of K are removed by crops more potassium would become exchangeable from non-exchangeable forms (Maurya & Ghosh, 1972). Whether soils when cropped can maintain the level of exchangeable K depends upon the type of soil, nature and intensity of cropping, the relative rate of removal and release from non-exchangeable form. Continuous cropping therefore alters the existing equilibrium of soil potassium in the profile as crop roots penetrate as deep as 2 m. Since all the potassium fertilizer for a crop is applied at sowing, the temporary overloading of the system may cause leaching losses of K in light soils. This may result in the enrichment of deeper layers.