Abstract
In the published literature of Soil sciences a number of .mo cations of the I]apon equation have been proposed from time to time. These modifications have been examined in detail and put in the right perspective. The assumptions in this equation and their limitations are disc.ussecl.
INTRODUCTION
In considering the physical properties of soils and the mineral nutrition of plants growing on soils, the saturation of exchange sites by various ions and the relations between exchangeable ions and their concentrations in soil solution are of great importance. The equations for relating the distribution of adsorbed ions to solution ions may be classified into several categories. 'They include the kinetic and statistical approaches, mass action equations, Donnan distribution, double-layer theory and the application of thermodynamics. The soil tilth depends on the nature of exchangeable cations also, In an ideal soil, 65 of the exchange sites should be occupied by calcium, 10% by magnesium, 5 by potassium and 20% by, hydrogen (Toth, 1964.) Calcium saturated soils are flocculated and permeable to water, while sodium saturated soils are generally defloccuIated, dispersed and impermeable to good quality water. Some other physical properties of soil like swelling and plasticity and the chemical properties like availability of K and other nutrients are affected adversely by the excess of exchangeable sodium, The excess of sodium in irriga-tion waters may adversely affect soil properties and thus decrease soil productivity. It is for this reason that an arerupt is being made here to review arid consoli-date different modifications of the Capon equation since 19)3 to-date so that it can be used with more confidence and accuracy for the classification of salt•cffected
