PHOSPHORUS EFFICIENCY AND THE FORMS OF SOIL-PHOSPHORUS UTILIZED BY UPLAND RICE CULTIVARS

Experimental measurements of phosphorus (P) uptake and the forms of soil P depleted from an Ultisol by 6 upland rice cultivars are reported. In both P-fertilized and -unfertilized soil, the majority of P taken up was solubilized from a 0.1 M NaOH-soluble pool by root-induced changes. The soil pH within 4 mm of the roots was lowered by up to 0.5 units (from 4.6), but this by itself could not account for the P solubilized, and nor could increased phosphatase activity near the roots. The possible role of root-released low molecular weight organic acid anions in P solubilization is discussed. No significant differences in the extent of solubilization by a given root mass could be detected between cultivars. In P-unfertilized soil, but not in P-fertilized soil, there were significant differences between cultivars in 'internal' P efficiency as measured by shoot dry weight per unit total plant P. In unfertilized soil, root growth and P uptake were strongly correlated with the P content of the seeds from which the plants were grown.