Effects of sowing cowpea on properties of an irrigated Vertisol and growth and yield of succeeding cotton

Abstract

The effects of sowing a break crop of cowpea on soil properties, nutrient uptake, growth, and yield of furrow-irrigated cotton were evaluated in a trial conducted from 1993 to 1995 in a Typic Haplustert (Vertisol) with high subsoil compaction in north-western New South Wales, Australia. The experimental treatments were cowpea (Vigna unguiculata Walp.)-cotton (Gossypium hirsutum L.), continuous cotton, and long-fallow cotton (cotton alternating annually with a bare fallow). Cotton was sown in all plots in 1994 with minimum tillage. Soil was sampled to a depth of 0.6 m in August 1993, May–June 1994, and April 1995. Soil properties evaluated were organic matter fractions, resilience (a measure of the self-mulching ability of the soil), plastic limit, strength (as cone resistance), shrinkage indices derived from shrinkage curves, exchangeable Ca, Mg, K, and Na, pH, nitrate-N, and electrical conductivity. Profile water content and water extraction, nutrient uptake, crop vegetative growth, cotton lint yield, and fibre quality were also quantified. Compared with fallowing, sowing either cotton or cowpea in 1993 decreased pH and aggregate size formed after puddling and drying, and increased soil organic matter and plastic limit. Cropping also increased exchangeable cations in the surface 0.3 m of the soil. Nitrate-N in the 0–0.15 m depth was 23% higher after cowpea than after fallow. Amelioration of soil compaction was quickest in the short-term where continuous cotton was sown, but was also observed with time in plots where cowpea was sown in 1993. In comparison with cowpea-cotton and long-fallow cotton, continuous cotton had the highest nutrient uptake. Growth of cotton sown after either fallow or cotton was greater than that of cotton sown after cowpea, with lint yield being highest in the previously fallowed plots. Lint fibre quality did not differ significantly between treatments. Overall, the benefits of sowing cowpea in terms of improvements in soil properties and crop yields were either small or negligible in comparison with fallowing or sowing cotton.