UPTAKE OF PHOSPHORUS FROM DIFFERENT SOURCES BY LOTUS-PEDUNCULATUS AND3 GENOTYPES OF TRIFOLIUM-REPENS .2. FORMS OF PHOSPHATE UTILIZED AND ACIDIFICATION OF THE RHIZOSPHERE

Improvements in the ability of legumes to utilise applied phosphate (P ) could lower fertiliser requirements. Earlier studies have found diff erences between legume species and genotypes of Trifolium repens in th eir ability to take up P from P sources that vary in solubility. Acidi fication of the rhizosphere may account for different P utilisation, e specially from reactive phosphate rocks.To investigate differences in the ability of legumes to acidify their rhizosphere and extract variou s forms of P, Lotus pedunculatus and 3 genotypes of white clover that varied in their ability to respond to added P were grown in root study containers with either unfertilised soil, or soil fertilised with eit her monocalcium phosphate (MCP) or North Carolina phosphate rock (NCPR ). After 6 weeks of plant growth, the containers were dismantled and 1 0 thin soil slices were taken 0-8 mm from the rhizoplane. These slices were fractionated for various P forms and pH was measured. All 4 legu mes, especially lotus, acidified their rhizosphere. This resulted in n egligible depletion of the small, H2SO4-P-i fraction (P-i, inorganic p hosphorus) of unfertilised and MCP-fertilised soil, but caused direct dissolution of NCPR. The predominant forms of P depleted in the MCP-fe rtilised soil were the resin-P-i and NaOH-P-i forms, whereas in the NC PR treatment the H2SO4-P-i fraction was depleted. No significant deple tion of any P fraction was observed in the unfertilised soils as plant s were very small and P change in the soil slices was below that which could be measured using the experimental technique. Suggestions are m ade on how the experimental technique could be improved.