MOBILIZATION OF DIFFERENT PHOSPHATE FRACTIONS IN THE RHIZOSPHERE

Availability of soil P fractions and mechanisms of acquisition by plan ts were studied. Plants mobilize soil P by desorption via depletion of P solution concentration around roots. In an oxisol, the process was enhanced by nitrate N nutrition of ryegrass, which increased soil pH, and by carboxylate release by white lupin. Ligand exchange and Fe/Al s olubilization are assumed to be the mechanisms. Ammonium N nutrition o f ryegrass decreased pH and allowed P mobilization in a luvisol but ha d no such effect in an oxisol, due to acid solubility of P in these so ils. Organic P dissolved in soil solution contributed one third to the P uptake of field-grown barley on a luvisol. Laboratory experiments s uggest that organic P is hydrolyzed by phosphatases at the root surfac e and replenished by micro-organisms.