Rock-Phosphate Solubilising Bacteria and their Effect on Soybean (Glycine max) Growth under Pot Grown Conditions

The modern agriculture is dependent on phosphorus (P) derived from phosphate rock. However, the direct application of low-grade rock phosphate as a P source in soils need an addition of inoculums of phosphate solubilising microorganisms to improve the rock phosphate efficiency as a phosphorus source. Phosphate solubilising bacteria (PSB) were screened for their phosphate solubilising ability on plates and in liquid cultures supplemented with Malian, Moroccan or Mexican rock phosphates. They were subsequently tested on soybean grown in pots filled with non sterile soil amended with Moroccan rock phosphate for their aptitude in promoting soybean growth. The activity of the different strains on plates indicates Panthoea sp. and Bacillus sp. as the most efficient strains able to show halo zone on plates supplemented all different rock phosphates, with a solubilisation index (SI) of 3.65, 4.10 and 5.42 (Panthoea sp.) and 2.93, 3.13 and 2.13 (Bacillus sp.) respectively for Malian, Moroccan and Mexican rock phosphates. Panthoea sp. remains the strains showing the highest concentration of the solved P with all rock phosphates: 1038.25, 996.67 and 1207.87 µgP/g for Malian, Moroccan and Mexican rock phosphates respectively. It is followed by Klebsiella sp. (862.57, 615.19 and 426.29 µgP/g respectively) and Bacillus sp. (810.86, 270.92 and 180.95 µgP/g). In general, Panthoea sp. and Bacillus sp. better contribute to the soybean growth with the effect of 35% and 34% respectively compare to non inoculated control supplied with non soluble Moroccan rock phosphate. The activity of Klebsiella sp. (13%) that is low in general seems to be stimulated when associated with the two other strains (33%). This suggests that the use of rock phosphate combined with the co-inoculation with those strains would ensure soybean production in economically profitable and environmentally friendly conditions.