ROOT-INDUCED DISSOLUTION OF PHOSPHATE ROCK IN THE RHIZOSPHERE OF LUPINS GROWN IN ALKALINE SOIL

Dissolution of North Carolina phosphate rock (PR) in the rhizosphere of white lupin (Lupinus albus) and narrow leaf lupin (L. angustifolius) was measured in a growth chamber experiment. Plants were grown for 8-13 days in an artificial soil (pure alumina sand) at alkaline pH to eliminate dissolution of PR due to reaction with the soil. Phosphate rock was supplied as the sole source of P and Ca for the plants at two rates of application (0.1 and 1 mg P g-1 soil). Both species dissolved considerable amounts of PR (up to 70% of PR present within 3 mm from the roots). Phosphorus extracted from the soil with 0.5 M NaOH showed that up to 69% of dissolved P accumulated in the rhizosphere of both species due to sorption by the soil, particularly at the high rate of application. Only white lupin utilized significant amounts of Ca. Thus P and Ca uptake were not driving forces for the root-induced dissolution of PR which was probably due to proton excretion that occurred concurrently, as evidenced by a decrease of rhizosphere pH of about 2 pH units. White lupin dissolved up to twice as much PR than narrow leaf lupin. This may be related to either the larger root biomass of white lupin or the particular excretion activity of its proteoid roots.