Exchange characteristics of some acid organic rich forest soils

Abstract

The cation exchange characteristics were determined for several acid organic-rich soils formed under sub-alpine eucalypt forests growing in the Australian Capital Territory. Most of the exchange sites in the red and yellow earths studied are occupied by Al and are associated with soil organic matter. Unbuffered salt solutions extract more Al than could be associated with exchange sites, so that the sum of cations in the extracts is an overestimate of the effective cation exchange capacity (CECe). The CECe is not a unique property but depends upon pH and ionic strength (I) of the equilibrating solution. The exchange sites mainly carry negative charges of both permanent and variable nature. Higher carbon contents of the soils are correlated with increases in CECe due to increasing I, and with greater negative charge due to increasing pH. Some modifications to the compulsive exchange method of Gillman for determining CECe are proposed for its use with acid organic-rich soils.