EFFECTS OF LUPIN ON SOIL PROPERTIES AND WHEAT PRODUCTION

Effect of lupin on wheat production and soil properties was evaluated on a red earth, at Wagga Wagga, N.S.W. Soil physical and chemical properties as well as soil surface aggregate stability, soil water distribution and extraction by wheat crops from a 10-year-old wheat/lupin (WL) rotation were compared with those of continuous wheat (WW), with (WW+N) and without (WW-N) nitrogen fertilizer application. Averaged wheat yield over the 1989-1990 period was 4.17, 2.95 and 3.06 t ha-1 respectively for WL, WW-N and WW+N. Despite the higher yield, important changes in soil properties have been detected in the soil under wheat/lupin rotation when compared with that under continuous wheat. The major effect was surface soil acidifcation and an associated loss of cations. Ten years of WL, compared with WW-N resulted in 0.2 unit reduction in pH (4.35 v. 4.55) in 0.10-0.15 m with corresponding increases in extractable Al and losses in exchangeable Ca2+ (17% as present in WW-N) and Mg2+ (12%). In the continuous wheat, annual application of 100 kg N ha-1 as urea resulted in much greater acidification (by 0.48 pH unit from 4.63 to 4.15 at 0.05-0.10 m) and larger losses in Ca2+ (up to 40%) and Mg2+ (up to 52%) in the top 0.2 m. Ten years of WL rotation reduced K+ by 10% in the top 0.2 m layer compared with both of the continuous wheat rotations, presumably due to higher export of K in lupin grains. Inclusion of lupin in the rotation also resulted in differences in the quality of soil organic matter. Despite similar total soil organic carbon content to WW-N, in the top 0.1 m, soil organic matter under WL had lower C/N ratio and higher polysaccharide content. Lower macroaggregate stability was found under WL compared to WW-N, but this did not result in lower soil water storage over the summer fallow during the two seasons of measurement. However, the wheat crop under WW utilized less stored subsoil water than that under WL, even under conditions of moisture stress.