Phosphate availability and phosphate needs of soils under Siratro pastures as assessed by soil chemical tests

Abstract

In field trials covering a range of soils and environmental conditions, the variation in soil equilibrium phosphate potentialSP accounted for 86 per cent of the variation in the percentage yield response of Siratro (Phaseolus atropurpureus) to phosphorus, compared to 72 per cent accounted for by the Olsen extraction, 51 per cent by Truog's and 38 per cent by Morgan's. The Truog and Morgan extractants gave improved correlations with phosphate uptake, but all four methods were poorly correlated with phosphate concentration in the plant. For the prediction of the soil's phosphate requirement for 90 or 95 per cent of the maximum yield, the Morgan method was superior to the equilibrium potentialSP, Olsen and Truog methods, and comparable to the quantity measurement P sorbed, derived from the soil's quantity-intensity (Q/I) relation at the minimum, non-limiting solution activity of 2.2 ¦M P : each accounted for 67 per cent of the variation in P required for 90 per cent of the maximum yield. The precision of the extractions in estimating P required decreased as the extractant/soil ratio and the extraction period increased. Of the conventional extractants, a method could be chosen that was well correlated with either P required (Morgan's) or plant relative yield (Olsen's), but none that was well correlated with both criteria. On the other hand, the measurement: of the one Q/I relation provided intensity (equilibrium potentialSP) and quantity (P sorbed) terms for the prediction, respectively, of a soil's phosphate availability and its phosphate needs for optimum plant growth.