SOIL PROPERTIES, NUTRIENT-UPTAKE AND CROP GROWTH IN AN IRRIGATED VERTISOL AFTER 9 YEARS OF MINIMUM TILLAGE

The effects of three contrasting tillage/cropping combinations on soil properties, nutrient uptake, growth, and yield of cotton and cowpea d uring 1993-1994 were evaluated in a trial running since 1985 in a Typi c Pellustert (Vertisol) of north-western New South Wales, Australia. T he treatments from 1985 to 1993 were: (a) maximum tillage (disc-plough ing to 0.2m depth, chisel ploughing to 0.3m depth followed by ridging every year) sown with continuous cotton (Gossypium hirsutum L.); (b) m inimum 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, if required) w ith continuous cotton; and (c) a cotton-winter wheat (Triticum aestivu m L.)-summer (bare) fallow-cotton sequence where cotton was sown with minimum tillage and wheat was sown with no-tillage. Soil was sampled t o a depth of 0.6 m in August 1993 and May-June 1994. Soil properties e valuated were organic matter fractions, dispersion index, soil resilie nce (a measure of the self-mulching ability of the soil), plastic limi t, soil strength (as cone index), soil shrinkage indices derived from shrinkage curves, exchangeable Ca, Mg, K and Na, pH, nitrate-N and ele ctrical conductivity. Profile water content and water extraction, nutr ient uptake, crop vegetative growth, cotton lint yield and fibre quali ty were also quantified. In comparison with maximum tillage, values of exchangeable Na, ESP and dispersion index were lower with minimum til lage in 1993 and 1994, and nitrate-N, particulate and total organic ma tter were greater and pH lower in 1994. Soil compaction (evaluated fro m shrinkage indices) in ridges was in the order minimum tillage/cotton -wheat-fallow-cotton < minimum tillage/continuous cotton < maximum til lage/continuous cotton, whereas subsoil compaction in maximum tilled p lots was less than that in minimum tilled plots. In comparison with pu blished data, however, subsoil compaction was large in all treatments. Nonetheless, water extraction during extended drying cycles was deepe r and more extensive with minimum tillage and suggests that ''by-pass channels'' may have played a significant role in determining water ext raction patterns at this site. Nutrient concentrations in plant tissue s were not significantly affected by tillage/crop combinations. In com parison with maximum tillage, vegetative growth of cotton and cowpea i n January and February (the peak of the summer crop growing season), a nd cotton lint yield were higher, and cotton lint fibre quality was be tter with minimum tillage. The highest lint yield and value, and the b est fibre quality occurred where minimum tillage/cotton-wheat-summer f allow-cotton had been sown in the past. (C) 1997 Elsevier Science B.V.