Abstract

Sodic soils are characterised by their poor structural stability. This is thought to be caused mainly by high levels of exchangeable sodium and low electrolyte concentrations. Historically, soil sodicity has been reported as the exchangeable sodium percentage, ESP [(exchangeable Na/∑ exchangeable cations) × 100]. However, some authors believe that exchangeable sodium content alone is a better indicator of sodicity, whereas others suggest that an effective sodicity index is one which includes both the exchangeable sodium levels and electrolyte concentration (EC_1:5). Some examples are the electrochemical stability index (EC _1:5/ESP) and EC_1:5/exchangeable Na. The objective of this study was to evaluate which of 3 empirical sodicity indices (ESP, EC_1:5/ESP, EC_1:5/exchangeable Na) was best related to soil dispersion in Vertosols sown to cotton farming systems.

Soil was sampled between 1995 and 2001 from 4 irrigated and dryland sites in New South Wales and Queensland, where the cropping systems included continuous cotton (Gossypium hirsutum L.), cotton–rotation crop sequences, and 2- and 1-m beds. Tillage systems ranged from zero to minimum tillage. Soils from all sites were analysed for EC_1:5, exchangeable Ca, Mg, K, and Na, and dispersion index, and ESP, EC_1:5/ESP and EC_1:5/exchangeable Na derived. Long-term dispersion was best predicted by EC_1:5/exchangeable Na, except where zero tillage was practised when none of the sodicity indices were related to dispersion. Aggregate stabilisation under zero tillage was speculated to be determined largely by labile soil organic matter and microbial activity rather than sodicity.