SOLUBILIZATION OF HARDLY-SOLUBLE ALPO4 WITH P-SOLUBILIZING MICROORGANISMS

Four species Aspergillus niger, Penicillium simplicissimum, Pseudomonas sp. (P118/89) and Penicillium aurantiogriseum were found to be very effective in solubilizing hardly-soluble A1PO(4). A. niger produced citrate, oxalate and gluconate, whereas the other species did not produce any organic acid in detectable amounts. This indicates that the production of organic acids is an important solubilization mechanism of A1PO(4) but not the only effective one-as supposed in the great majority of publications. Nevertheless, organic acids alone are able to solubilize A1PO(4) to a certain extent, although they are less effective compared to biotic leaching. Proton-excretion accompanying NH4+-assimilation is thought to be the most probable explanation for microbial solubilization without acid production. In hydroponic studies with P-solubilizing microorganisms, the plant Lepidium sativum and A1PO(4) as the sole P source led to an increase in the dry weight and A1-content of plants. Plant P-content increased only when P. simplicissimum was inoculated.