EFFECTS OF FALLOW MANAGEMENT AND CROPPING HISTORY ON AGGREGATE BREAKDOWN UNDER RAINFALL WETTING FOR A RANGE OF QUEENSLAND SOILS

This paper reports changes in stability of soils to wetting under a range of fallow management/cropping treatments for eight experiment sites in Queensland. Because of its greater relevance to dryland soils, and its better correlation with field soil behaviour, wetting with high energy rain (HER) was used to wet samples and cause aggregate breakdown. The water-stable size fraction considered was that <0.125 mm, with the proportion of particles <0.125 mm referred to as P125. To study interactions between wetting method and tillage management, several other wetting methods were applied to some or all of the eight soils. These other methods were immersion (IMM), wetting by rainfall of low energy (LER) or under tension (TENS). Where fallow management treatments significantly reduced P125 under HER wetting, those reductions were generally small, and likely to increase steady infiltration rates of high energy rain into bare soil by only 2-6 mm h-1. In contrast, steady infiltration rates could be increased by 15-42 mm h-1 if those same soil surfaces were completely covered by stubble so that they received low energy rain. At least under Queensland conditions, retention of crop residues to protect soils from drop impact is therefore likely to be of much greater importance in improving water storage during fallows than any improvement in soil aggregation under 'improved' fallow management practices. Effects of fallow management and cropping history on P125 under HER varied across the sites. Three out of four virgin soils were more stable than corresponding cropped soils; stubble retention improved stability to HER wetting on three out of four sites where it was tested, and direct drill or reductions in tillage frequency improved stability at two out of eight sites. IMM showed responses to treatments that, for some soils, differed greatly from those obtained using HER. Neither LER nor TENS appeared useful, as LER gave occasional inconsistent results, and TENS caused little aggregate breakdown for most soils.