Rock phosphate solubilization by Aspergilli species grown on olive cake waste and its application in plant growth improvement

Abstract

Organic acids producing strains of Aspergillus niger and Aspergillus fumigatus grown on olive cake-based media were studied for ability to solubilize rock phosphate. The final fermented mixture was, acidified and contained mineralized organic matter, solubilized rock phosphate and fungal mycelium. Solubilization increased during the fermentation process, reaching a maximum of 400,330 µg/ml at pH 3.6, 4.0 and titrable acidity 25, 20 m mol/ml after9 and 12 days of incubation time with Aspergillus niger and Aspergillus fumigatus respectively. Various combinations of olive cake and rock phosphate, previously treated or untreated by the fungi were introduced into a phosphorus (P) deficient soil (0.016 mg/g soil) to improve the growth of broad beans in a pot experiment. Compared to other treatments, synergistic action of both the filamentous and arbuscular fungi caused considerable improvement of growth and P uptake. The greater growth rate and P concentration of mycorrhizal and non- mycorrhizal plant were achieved when microbes- treated olive cake and rock phosphate were applied to soil compared with all other treatments. Also, inoculation with the mycorrhizal fungus significantly increased plant growth than in the equivalent non- mycorrhizal treatment.