DISSOLUTION OF PHOSPHATE ROCK IN THE RHIZOSPHERE OF 5 PLANT-SPECIES GROWN IN AN ACID, P-FIXING MINERAL SUBSTRATE

The dissolution of phosphate rock (PR) was studied in the presence and absence of plants grown in an acid mineral substrate (alumina sand) t hat simulates an acidic, P-fixing soil. This substrate being devoid of P and Ca, PR was the only source of both P and Ca fbr the plants, In the absence of plants, the low pH of the substrate (pH about 4) result ed in dissolution of about 8 and 30% of the applied PR, at high and lo w levels of application (1.0 and 0.1 mg P as PR per g of substrate), r espectively. Additional dissolution of PR occurred in the presence of plant roots as indicated by a significant decrease in total Ca content in the PR-amended substrate and a concomitant accumulation of dissolv ed P in the rhizosphere and plant material of the five species studied . The largest root-induced dissolution was achieved by ryegrass (Loliu m rigidum) and rape (Brassica napus) and amounted to 19 to 32% of the PR present in the first two mm of the rhizosphere. The root-induced di ssolution of PR in this zone was thus of the same order of magnitude a s the substrate-mediated dissolution, and two to three times larger fo r the highest level of application. The depletion of dissolved Ca whic h occurred in the rhizosphere of all species and particularly rape sug gests that Ca uptake may have promoted PR dissolution. Rhizosphere pH increased by 3 units for rape, while little or no change in pH occurre d for the other species. Nevertheless, proton excretion was suspected to have occurred for all species but rape, with a subsequent neutraliz ation of these protons due to PB dissolution. When supplied with PR in an acid, P-fixing mineral substrate, plant roots may thus mobilize so me Pas a consequence of a combination of Ca uptake and proton excretio n which significantly enhances PR dissolution.