Differences in the properties of a red earth after ten years of wheat-lupin and wheat-subterranean clover rotations

Differences in the properties of a red earth caused by 2 different rotations, wheat-lupin (WL) and wheat-subterranean clover (WC) were compared under 2 different tillage systems, direct-drilled (DD) and conventional tillage (CT) at a 10-year-old experimental site in Wagga Wagga, New South Wales. Significant differences in soil organic carbon concentration between the 2 rotations were found only under the DD systems; soil under WC rotation had a significantly higher organic carbon in the top 0.05 m than the WL soil. No such difference was detected under CT. Under both tillage systems, WC soil had lower pH (by 0.32 unit), with accompanying lower exchangeable magnesium but higher extractable aluminium than the WL soil in the top 0.05 m. Despite similar earthworm population and surface macroporosity, there was evidence suggesting that, under direct drilling, 10 years of WC rotation had reduced the subsoil water storage after summer fallow and reduced the effective rooting depth of wheat when compared with the WL rotation. These had not resulted in difference in wheat yield between the rotations even though a smaller average grain size was found under WC rotation in the 1990 season.