Residual effects of tillage and crop rotation on soil properties, soil invertebrate numbers and nutrient uptake in an irrigated Vertisol sown to cotton

The residual effects of tillage and cropping sequence on soil physical and chemical properties, surface-active and soil invertebrate composition and abundance, nutrient uptake, growth and yield of cotton were evaluated from 1994 to 1996 in a compacted Typic Haplustert (Vertisol) of north-western New South Wales, Australia. The experimental treatments from 1985 to 1992 were intensive tillage (disc-ploughing to 200 mm, chisel ploughing to 300 mm followed by ridging every year) sown with continuous cotton (Gossypium hirsutum L.); minimum tillage (planting on ridges retained intact from previous years with soil disturbance being limited to deepening of the furrows with disc-hillers and shallow cultivation on ridge surfaces) sown with either continuous cotton or a cotton-winter wheat (Triticum aestivum L.)-fallow rotation where wheat was sown with no-tillage. The tillage treatments were repeated in May 1993, and the plots were either followed or cropped by sowing either cowpea (Vigna unguiculata Walp.) or cotton. Cotton was sown with minimum tillage in 1994 and 1995 in all plots. Soil was sampled from the 0–150 mm, 150–300 mm, 300–450 mm and 450–600 mm depths, and analyzed for organic carbon, dispersion index, soil resilience (a measure of the self-mulching ability of the soil), plastic limit, soil strength, pH, exchangeable Ca, Mg, K and Na, and nitrate-N. Profile water content, nutrient uptake, numbers of soil invertebrates, cotton growth and lint yield, and fibre quality were also quantified. Soil strength was lowest where intensively tilled continuous cotton had been sown, whereas in plots where minimum tillage and cotton-wheat-fallow rotation were combined soil fertility was best (indicated by lowest values of pH, exchangeable Na, exchangeable sodium percentage and dispersion, and highest values of organic C) and water extraction by cotton greatest during periods of reduced water availability. The latter was attributed to cotton utilizing stable pores with a high degree of pore-continuity created by the root systems of preceding crops or associated macrofauna as ‘by-pass channels’ to avoid the restrictions of the soil matrix, thereby facilitating rapid access to sub-soil water. Cotton growth reflected these differences such that vegetative and reproductive growth, nutrient uptake and lint yield were greater and fibre quality superior wherever minimum tillage had been imposed, and best in plots under minimum tilled cotton-wheat-fallow rotation. Composition and abundance of surface-active and soil invertebrates were determined primarily by soil microclimate and pesticide application regime rather than by tillage and cropping system. Ant numbers were lowest in intesively tilled plots whereas Collembola activity was limited to periods when the soil was moist.