TILLAGE AND CROP RESIDUE MANAGEMENT AFFECT VERTISOL PROPERTIES AND GRAIN-SORGHUM GROWTH OVER 7 YEARS IN THE SEMIARID SUBTROPICS .1. CROP RESIDUE AND SOIL-WATER DURING FALLOW PERIODS

The effects of fallow surface management treatments on stubble (crop residue) levels and soil water storage were studied during seven fallow periods between grain sorghum crops on a grey Vertisol near Biloela in central Queensland, Australia. Treatments were disc (D), blade (B) and zero (Z) tillage, each with stubble or residue from preceding crops either retained (+) or removed (-) at the start of the fallow periods, which were of 7-8 months duration. Where stubble was retained, stubble dry matter levels on the soil surface at the start of the fallow period were mainly influenced by stubble produced by the previous crop, but also by residual stubble on the soil surface before the previous crop. The general order was D +< B +< Z+. Stubble dry matter and stubble cover on the soil surface declined during the fallow period in all stubble-retained treatments, with the greatest reductions occurring after the initial disc or blade tillage. From the start to end of the fallow, mean reductions in stubble dry matter and stubble cover were, respectively, 60 and 74% in D+, 31 and 57% in B+, and 17 and 24% in Z+. Mean stubble dry matter levels on the soil surface at the end of the fallow period in December-January were 1030, 2030 and 2910 kg ha-1 in D+, B+ and Z+, respectively; corresponding stubble cover levels were 8, 16 and 35%. Mean plant available water capacity to 1.8 m was 201 mm. Mean fallow soil water accumulation varied between fallow periods from 11 to 102 mm. The corresponding variation in mean fallow water storage efficiency (percentage of rainfall over the fallow stored in the soil) was from 3 to 37%. Fallow soil water accumulation was significantly (P<0.05) higher in Z+ (116 mm) than in Z- (86 mm), D+ (96 mm) and D- (84 mm) in one fallow period. During the fallow period, B+ and Z+ generally resulted in higher plant available water than other treatments at mean values of 50-100 mm. However, these effects were not present at higher plant available water levels (mean of 128-164 mm), as occurred at the end of six fallow periods. The main treatment effect at the end of the fallow was for significantly (P<0.05) lower plant available water in Z-. © 1990.