THE P-FERTILIZER REQUIREMENTS OF SUBTERRANEAN CLOVER, AND THE SOIL-P STATUS, SORPTION AND BUFFERING CAPACITIES FROM 2 P-ANALYSES

The phosphorus fertilizer requirement of subterranean clover (Trifolium subterraneum) was determined on a number of soils in field and glasshouse experiments. The solution phosphorus concentration in a 0.01 M CaCl2 extract (SPC0) determined from a phosphorus sorption curve was found to be closely related (R2 = 0.80 to 0.84) to relative yield. The critical SPC required to achieve 90% of relative yield (SPC(C)) was 0.128-mu-g P mL-1 in the field and 0.106-mu-g P mL-1 in the glasshouse experiments. Phosphorus sorbed at an SPC of 0.10-mu-g P mL-1 (Ps0.1 at SPC0.1) was highly correlated with the phosphorus requirement of subterranean clover in both the field and glasshouse, accounting for 89% and 73% respectively of the variation in phosphorus requirement. A modified two-point sorption curve was found to give a good estimate of phosphorus sorbed at both SPC0.1 (Ps0.1) and SPC0.3 (PS0.3), and greatly increased the number of soil samples that can be processed, compared with a five-point sorption curve. The two-point test involved the addition of solutions containing 0 and 8-mu-g P mL-1 at a soil: solution ratio of 1 : 10. The sorption curve constructed using this method was suitable for assessing soils with Ps0.1 levels up to 90-mu-g P g-1. The soil phosphate buffering capacity at SPC0.1 (PBC0.1) was also accurately estimated using calculations from the two data points, but PBC0.3 values were poorly estimated. It was concluded that the two-point sorption curve is a sensitive and efficient method of predicting the present phosphorus status of clover pastures, their phosphorus fertilizer requirements, and of calculating PBC0.1, a useful soil characteristic.