Contrasting soil physical properties after zero and traditional tillage ofan alluvial soil in the semi-arid subtropics

Zero till is commonly advocated as a preferred cropping system to conventio nal, multicultivation practices. Zero till is particularly attractive on cl ay soils, to minimise compaction and induce natural structure formation thr ough shrink-swell cycles. Increases in soil water storage and increased num bers of (beneficial) soil fauna with zero till have been reported, relative to traditional tillage. This work identifies reasons for improved soil and crop responses under zero till on an alluvial Vertisol from Biloela, Qld, All measurements were taken after 8 years of the trial. By then, the tradit ional till treatment (TT) had received 34 tillage operations while zero til lage (ZT) had received none. Data collected were sorptivity and hydraulic c onductivity of the soil surface at four tensions as measured by disc permea meters, water infiltration with a rainfall simulator, and image analysis of soil structure from Araldite-impregnated intact soil blocks. Chloride conc entrations to a depth of 4.5 m were also measured. The sorptivity and hydra ulic conductivity data demonstrated that ZT had the greater volume of the l argest pore size measured (1.5-3 mm), whereas TT had the greater volume of pores <1.5 mm diameter. Rainfall simulation at 100 mm h(-1) showed that the time to ponding, final infiltration rate and the total infiltration were a ll significantly greater in ZT than TT. From the image analysis it was evid ent that ZT had an abundance of apparently continuous soil pores from the s oil surface to depth that appeared round in cross section. In contrast, TT had a high-density surface crust and significantly larger soil structure un its than ZT from 0.15 m. The image analysis results corresponded well with the high counts of earthworms and termites previously recorded in ZT, Appar ently earthworm channels and termite galleries are major contributors to me asured increases in hydraulic conductivity and infiltration in ZT The chlor ide concentration profiles showed great variability within each treatment. Although the averaged profiles for each treatment showed that ZT had half t he peak chloride value of TT and the peak occurred lower in the profile, th ese differences were not statistically significant. Nevertheless, the chlor ide data indicate that more deep drainage occurred under ZT probably throug h preferred pathways. (C) 2000 Elsevier Science B.V. All rights reserved.