Phosphate concentration in siratro as a guide to its phosphate status in the field

Abstract

Siratro (Phaseolus atropurpureus) was grown under a range of soil and climatic conditions in sub-tropical Queensland, over the years 1964 to 1967. Phosphate concentration in the tops, sampled immediately prior to flowering, was poorly correlated with either the relative yield response to applied phosphorus, or the amount of phosphorus required to lift yield to a maximum. Fifty per cent of the variation in P required could be accounted for by regression on great soil group, which crudely reflected differences in phosphate buffering capacity between soils. The critical value of phosphorus in the plant, corresponding to a yield of 90 per cent of the maximum, varied from 0.16 to 0.29 per cent. Twenty-five per cent of this variation was due to differences in the rain falling in the 28 days prior to sampling : the lower the rainfall, the lower the critical value. Rain falling 28 days prior to sampling and great soil group together accounted for 41 per cent of the variation in critical value. The effect of moisture stress on the critical value was more likely plant physiological, than one of a diminished phosphate supply from the soil.