The Principle of Hard and Soft Acids and Bases as Applied to Exchangeable Cation Selectivity in Soils

The principle of hard soft acids and bases, HSAB, which can predict stable complex formation, is reviewed. In the soil system, cation exchange selectivity can be compared to complex formation, which can then be described in terms of HSAB theory. It is then demonstrated by comparing “softness” valves to observed cation exchange selectivity in soils, that the majority of soil exchange materials behave as relatively soft bases.