ABSTRACT
Soils possess electrostatic charge as a result of atomic substitution in the lattices of soil
minerals (permanent charge) and because of hydrolysis reactions on broken edges of the
lattices and the surfaces of oxides, hydroxides, hydrous oxides, and organic matter (pH-
dependent charge). These charges attract counterions (exchangeable ions) and form the
exchange complex. The principle of the methods used to measure exchangeable ions is to
saturate the exchange complex with some ion that forces the exchangeable ions already
present on the charged surfaces into solution (law of mass action). Exchange capacity can
then be calculated as the sum of the individual cations displaced from the soil (summation
method); or the ion used to saturate the exchange complex, termed the index ion, can be
displaced with a concentrated solution of a different salt and the exchange capacity calcu-
lated as the amount of the index ion displaced (displacement method).
