Comparative performance of six Stylosanthes species in three acid soils

The yield rcsponse of six Stylosanthes species to a factorial combination of four lime rates and two phosphorus levels applied to three acid soils (Coolum, Kogan, Rochedale) was determined in a pot experiment. The unlimed soils were of similar pH, but differed widely in exchangeable aluminium content. Aluminium toxicity appeared to be a major limitation to growth in the three soils. In the unlimed soils, the most severe yield restriction was observed in the Coolum soil, which had the highest soluble aluminium concentration (55 �M), and the least restriction in the Kogan soil, which had the lowest soluble aluminium concentration (37 �M). All six species achieved maximum yield in the three soils when the soluble aluminium concentration was reduced to values below 21 �M . The strong yield responses observed with little change in soluble aluminium at the higher lime rates in the Coolum and Rochedale soils may be due to either a further reduction in aluminium toxicity associated with increasing concentration of soluble polymeric species or the direct alleviation of hydrogen ion toxicity. The observed responses to lime do not appear to involve direct calcium effects, nor do they involve alleviation of manganese toxicity or molybdenum deficiency. Maximum yield was associated with reduction in aluminium saturation to less than 5% of the effective cation exchange capacity in all three soils. However, when examined across the three soils, aluminium saturation and also the exchangeable aluminium content were both unsatisfactory predictors of plant performance. The largest and smallest restrictions on growth were observed in soils with similar aluminium saturation (Coolum 14.4%, Kogan 17.6% respectively), while the Rochedale soil with its much higher aluminium saturation (42.0%) was intermediate in degree of growth restriction. Phosphorus and nitrogen deficiencies also limited plant growth, but the magnitude of their effects varied among soils and species.