Cotton-based rotation systems on a sodic Vertosol under irrigation: effects on soil quality and profitability

Abstract

An experiment was established in 1993 on a sodic Vertosol (Vertisol, Typic Haplustert) at Merah North, north–western New South Wales, to evaluate the sustainability of selected irrigated cotton (Gossypium hirsutum L.)–rotation crop sequences. Crop sequences were selected following discussions with local cotton growers. The indices used to evaluate sustainability included soil quality, microbiology, yield and profitability. This paper presents data on soil properties [soil organic C, structure as air–filled porosity of oven–dried soil, exchangeable Ca, Mg, K and Na, pH, electrical conductivity (EC_1:5) and EC_1:5/exchangeable Na in the 0–0.6 m depth], lint yield and profitability (as gross margins/ha and gross margins/ML of irrigation water). The 6 cropping systems sown after minimum tillage were: continuous cotton (R1), long–fallow cotton (R2), cotton–green manured faba bean (Vicia faba L.) (R3), cotton–dolichos (Lablab purpureus L.)–green manured faba bean in the first year followed by cotton–wheat (Triticum aestivum L.) (R4), cotton–dolichos (R5), cotton–fertilised dolichos (with P and K removed by cotton replaced as fertiliser) (R6).

In 1996, air–filled porosity of oven–dried soil was highest with R4 at the surface but lowest with R1 in the 0.15–0.30 m depth. In subsequent years, air–filled porosity of oven–dried soil was higher with R2 and R4 in the deeper depths, although differences between cropping sequences were small. Air–filled porosity of oven–dried soil increased between 1996 and 1998 in all treatments, and was probably caused by the change from intensive to minimum tillage in 1993, irrigation with moderately saline water and application of gypsum resulting in an increase in EC_1:5/exchangeable Na. In general, differences in soil properties such as soil organic C, exchangeable Ca, Mg, K and Na, pH, electrical conductivity (EC_1:5) and EC_1:5/exchangeable Na between cropping sequences were far less than those which occurred with time. The key changes were decreases in pH, exchangeable sodium percentage, exchangeable cations and organic C between 1994 and 1996, and increases in air–filled porosity of oven–dried soil, EC_1:5 and EC_1:5/exchangeable Na between 1996 and 1998. A decrease in air–filled porosity of oven–dried soil occurred between 1998 and 1999 as a consequence of preparing land and sowing cotton under very wet conditions. R1 had the highest cumulative gross margin/ha and R3 had the lowest. R2 had the highest cumulative gross margin/ML of irrigation water and R3 again the lowest. Among crop sequences, R2 and R4 gave the best returns with respect to both land and water resources.