Shelf-life of legume inoculants in different carrier materials available in East Africa

Abstract

Adoption of legume inoculation with rhizobia by small-scale farmers in East Africa, and the resultant increase in biological nitrogen fixation requires that quality inoculants meet minimum standards. BIOFIX is one of the commercially available rhizobia/legume inoculants in East Africa, whose standard is at least 10⁹ rhizobia g^-1. We examined the effect of carrier material and storage conditions on the populations of two industry standard rhizobia, Bradyrhizobium japonicum USDA 110 for soybean (Glycine max) and Rhizobium tropici CIAT 899 for common bean (Phaseolus vulgaris) over 165 days, using the drop plate method on Congo Red Yeast Extract Mannitol Agar. Viable populations of rhizobia differed significantly between carriers and rhizobia strains (P<0.05). Rhizobium tropici CIAT899, prepared with filter mud carrier, achieved a shelf-life of 135 days and B. japonicum USDA110 contained over 10⁹ cells g^-1 for 105 days. Both of these results fall below the stated six months expiry period of BIOFIX. Replacing filter mud carrier with vermiculite, resulted in an inferior product; although, both more thorough sterilisation and refrigerated storage, after a 14 day curing stage, improved the shelf-life thizobia in the inoculant packet.

Keywords: BIOFIX, CIAT 899, Kenya, USDA 110