Single and double inoculation with Azospirillum/Trichoderma: the effects on dry bean and wheat

A plant growth-promoting rhizobacterium ( Azospirillum brasilense Sp7) and a bio-control fungus, which can solubilize insoluble phosphorus ( Trichoderma harzianum Rifai 1295- 22), were evaluated for their single and combined effects on dry bean ( Phaseolus vulgaris) and wheat ( Triticum aestivum L.) grown in soil. A pot experiment with bean and a field experiment with both bean and wheat were established. In contrast to single inoculation of Trichoderma, the single inoculation of Azospirillum and the double inoculation did not significantly ( P > 0.05) increase nodule numbers and nodule mass at 45 days after planting in pot grown beans. However, the Azospirillum inoculation with supplementary phosphorus significantly ( P < 0.05) increased nodule mass. There were no significant ( P > 0.05) differences among the inoculation treatments for plant dry weight, total plant nitrogen, and total plant phosphorus at 45 days after planting in both pot and field experiments with bean. However, the combined inoculation and rock phosphate application at 1 Mg ha(-1) significantly ( P < 0.05) increased bean seed yield, total seed nitrogen and phosphorus in the bean field trial. This treatment more than doubled the mentioned properties compared to the control. The microbial inoculations, with the exception of the combined inoculation, significantly ( P < 0.05) increased total seed nitrogen, but never affected seed yield in the wheat field trial ( P > 0.05). The combined inoculation improves many plant and yield parameters and, therefore, has some advantages over single inoculation provided that rock phosphate was supplied at an amount not exceeding 1 Mg ha(-1). Higher rock phosphate application rates decreased many plant and yield parameters in our study.