Evaluation of microbial inoculants as biofertilizers for the improvement of growth and yield of soybean and maize crops in savanna soils

Abstract

Certain rhizobacteria exert considerable influence on plant growth and development, particularly under limiting conditions. The effects of some indigenous soil microbial isolates and commercially produced microbial inoculants, referred to as bio-inoculants, on the growth and dry matter yield (DMY) of maize (Zea mays) and soybean (Glycine max) crops were assessed under greenhouse conditions. In two sets of experiments, one set comprised of free-living nitrogen-fixing microorganisms (Azospirillum spp.), three soils from Ibadan, Mokwa and Shanono located in different agro-ecological zones, and maize as the test crop. The other set consisted of microbial inoculants that can act as biocontrol agents applied to sterilized and non-sterilized soils; soybean was the test crop. The bio-inoculants were applied separately and also in combination. The treatments included a reference termed ‘mineral N’ where macro- and micro-nutrients were supplied at optimal rates, a control where bio-inoculants were not applied, and four replicates. All treatments, excluding the reference, received only macro-nutrients at suboptimal rates. The crops were grown for eight weeks and growth parameters were measured. The shoot DMY of maize was relatively large (42 to 63 g plant-1) and differed significantly among the soils but the bio-inoculants did not improve the shoot DMY significantly (P > 0.05) in any of the soils when compared with the control. However, sole inoculation of Mazospiriflo-2 enhanced nitrogen uptake significantly in maize grown in Shanono soil. For soybean, the shoot DMY was also not improved by the inoculation or the addition of the microbial products compared with the control.