Selecting Indigenous P-Solubilizing Bacteria for Cowpea and Millet Improvement in Nutrient-Deficient Acidic Soils of Southern Cameroon

A trial of a screening and selection strategy for phosphate-solubilizing bacteria based on phosphate solubilization ability, and the subsequent effect of these bacteria on plant growth promotion under in situ conditions, was conducted. Of the 277 (187 from soils and 90 from roots) microorganisms tested, 10 bacteria (BOR8, LEJ14, DR5, DR9, EDJ4, EDJ6, EDJ8, SR7, EMJ5, LR7) were selected. All the bacteria were able to show P dissolution halo zone particularly on agar plates containing sparingly soluble iron phosphate as well as they were able to mobilize important amount of P in liquid media supplemented with either Ca3(PO4)2 or AlPO4·H2O or FePO4·2H2O. Calcium phosphate (Ca-P) solubilization resulted from combined effects of pH decrease and carboxylic acids synthesis. However, the synthesis of carboxylic acids was the main mechanism involved in the process of aluminium phosphate (Al-P) and iron phosphate (Fe-P) solubilization. Both nutrients were mobilized at pH 4 corresponding to their natural occurrence by citrate, malate, tartrate, on much lower level by gluconate and trans-aconitate. Subsequently, a greenhouse trial using cowpea and millet inoculated with selected bacteria showed a significant improvement of plant phosphorus uptake as well as root and shoot dry weight. However, the selection of phosphate-solubilizing bacteria as possible inoculation tools for phosphate-deficient soils should focus on the integral interpretation of laboratory assays, greenhouse experiments, and field trials.