Residual effects of cotton-based crop rotations on soil properties of irrigated Vertosols in central-western and north-western New South Wales

Abstract

The residual effects of cotton (Gossypium hirsutum L.) based crop rotations on soil physical and chemical properties were evaluated in 2 irrigated on-farm experiments located at Warren (1999–2001) in the central-west and Merah North (2000–05) in the north-west of New South Wales. The soils in both sites were grey, self-mulching Vertosols. The rotations sown at Warren from 1993 to 1998 were: (1) continuous cotton (cotton sown every year); (2) long-fallow cotton (cotton alternating with a bare fallow); (3) cotton–high input wheat (Tricticum aestivum L.), in which wheat was sown at a rate of 100 kg/ha and fertilised with 180 kg/ha of urea; (4) cotton–low input wheat, in which wheat was sown at a rate of 40 kg/ha and did not receive any N fertiliser; and (5) cotton–green manured field pea (Pisum sativum L.). At Merah North the rotations sown from 1993 to 2000 were: (1) continuous cotton; (2) long-fallow cotton; (3) cotton–green manured faba bean (Vicia faba L.) until 1999 when sorghum was sown during the 1999–2000 growing season; (4) cotton–dolichos (Lablab purpureus L.)–green manured faba bean from 1993 to 1994 followed by cotton–unfertilised wheat in which wheat was sown at a rate of 50–70 kg/ha thereafter; (5) cotton–dolichos; and (6) cotton–fertilised dolichos with P and K removed by cotton replaced as fertiliser. Soil was sampled to a depth of 0.6 m at 0.15-m increments and analysed for pH (in 0.01 m CaCl2), EC1 : 5, ESP, specific volume, nitrate-N, organic C (SOC), plastic limit, and dispersion. Residual effects of rotation history were reflected in subsoil specific volume at both sites, and nitrate-N in the surface 0.3 m and SOC in the 0–0.6 m depth at Warren. In general, higher values of specific volume occurred where cotton–wheat rotations, and in particular, fertilised wheat, had been sown. At Merah North, subsoil specific volume in ex-long-fallow cotton was similar to that in the cotton–wheat rotation. At Warren, ex-continuous cotton had lowest subsoil specific volume, the ex-cotton–high input wheat rotation and ex-long fallow cotton had greater SOC sequestration, and the ex-cotton–high input wheat rotation had higher nitrate-N. These differences mirrored those present when the rotation treatments were in place. Residual effects of crop rotations are more likely to occur where the residues of the rotation crops are relatively recalcitrant or where cropping intensity is lower.