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

Zero till is commonly advocated as a preferred cropping system to conventional, multicultivation practices. Zero till is particularly attractive on clay soils, to minimise compaction and induce natural structure formation through shrink±swell cycles. Increases in soil water storage and increased numbers of (bene®cial) soil fauna with zero till have been reported, relative to traditional tillage. This work identi®es 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 traditional till treatment (TT) had received 34 tillage operations while zero tillage (ZT) had received none. Data collected were sorptivity and hydraulic conductivity of the soil surface at four tensions as measured by disc permeameters, water in®ltration with a rainfall simulator, and image analysis of soil structure from Araldite-impregnated intact soil blocks. Chloride concentrations to a depth of 4.5 m were also measured. The sorptivity and hydraulic conductivity data demonstrated that ZT had the greater volume of the largest pore size measured (1.5±3 mm), whereas TT had the greater volume of pores <1.5 mm diameter. Rainfall simulation at 100 mm hÿ1 showed that the time to ponding, ®nal in®ltration rate and the total in®ltration were all signi®cantly greater in ZT than TT. From the image analysis it was evident that ZT had an abundance of apparently continuous soil pores from the soil surface to depth that appeared round in cross section. In contrast, TT had a high-density surface crust and signi®cantly larger soil structure units than ZT from 0.15 m. The image analysis results corresponded well with the high counts of earthworms and termites previously recorded in ZT. Apparently earthworm channels and termite galleries are major contributors to measured increases in hydraulic conductivity and in®ltration in ZT. The chloride concentration pro®les showed great variability within each treatment. Although the averaged pro®les for each treatment showed that ZT had half the peak chloride value of TT and the peak occurred lower in the pro®le, these differences were not statistically signi®cant. Nevertheless, the chloride data indicate that more deep drainage occurred under ZT probably through preferred pathways. # 2000 Elsevier Science B.V. All rights reserved.