Improvement in bioavailability of tricalcium phosphate to Cymbopogon martinii var. motia by rhizobacteria, AMF and Azospirillum inoculation

The interactive effects of phosphate solubilizing bacteria, N-2 or fixing b acteria and arbuscular mycorrhizal fungi (AMF) were studied in a low phosph ate alkaline soil amended with tricalcium insoluble source of inorganic pho sphate on the growth of an aromatic grass palmarosa (Cymbopogon martinii). The microbial inocula consisted of the AM fungus Glomus aggregatum, phospha te solubilizing rhizobacteria Bacillus polymyxa and N-2 fixing bacteria Azo spirillum brasilense. These rhizobacteria behaved as "mycorrhiza helper" an d enhanced root colonization by G. aggregatum in presence of tricalcium pho sphate at the rate of 200 mg kg(-1) soil (P1 level). Dual inoculation of G. aggregatum and B. polymyxa yielded 21.5 g plant dry weight (biomass), whil e it was 21.7 g in B. polymyxa and A. brasilense inoculated plants as compa red to 14.9 g of control at the same level. Phosphate content was maximum (0.167%) in the combined treatment of G. aggr egatum, B. polymyxa and A. brasilense at P1 level, however acid phosphatase activity was recorded to be 4.75 mu mol mg(-1) min(-1) in G. aggregatum, B . polymyxa and A. brasilense treatment at P0 level. This study indicates th at all microbes inoculated together help in the uptake of tricalcium phosph ate which is otherwise not used by the plants and their addition at 200 mg kg-1 of soil gave higher productivity to palmarosa plants.