Exchangeable Cations

Exchangeable cations are those that can be exchanged by a cation of an added salt solution. This chapter discusses exchange of the basic cations Ca2+, Mg2+, K+, and Na+ and exchangeable acidity, primarily arising from the cations AI3+and H+. In the absence of excess CaCO3, the cations Ca2+, Mg2+, and Na+ appear to be exchangeable by NH4OAc except in certain soils containing trioctahedral vermiculites. Exchangeable acidity is an arbitrary quantity composed of four types of acidity. The first is the H ions obtained from the hydrolysis of exchangeable, trivalent AI. The second is from hydrolysis of partially hydrolyzed and nonexchangeable AI. The third type is from weakly acidic groups on organic matter, and the fourth is exchangeable H. A number of workers have used the pH 7 methods of Schollenberger and Simon wherein the exchange capacity is determined by the NH4+ retained from NH4OAc, the total exchangeable bases are determined in the NH4OAc leachate.

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