Increasing shelf-life of rhizobium inoculants and efficacy of elite rhizobium strains of soybean

Abstract

Legumes integrated in farming systems contribute to improved soil fertility through biological nitrogen fixation. Therefore, rhizobia inoculant quality and strain effectiveness become central considerations. Elite rhizobia strain from various tropical climate localities with promising effectiveness but have not been tested in Uganda. Secondly, peat soil is non-renewable resource is getting depleted in Uganda. Thus an alternative carrier material ought to be selected for shelf-life and quality of rhizobia inoculants. This study determined (i) Municipal Solid Waste compost and peat carrier formulation ratio on shelf-life of rhizobia inoculants, (ii) room temperature storage on shelf-life of rhizobia inoculants, and (iii) effectiveness of elite rhizobia strains on soybean productivity. A laboratory experiment testing five carrier formulations (Municipal Solid Waste Compost: Peat w/w) at 100:0, 60:40, 40:60, 20:80 and 0:100 ratios. Another laboratory experiment tested the effect of room temperature on quality of inoculant for 270 days while elite strains NAK 128, NAK 84, USDA 532C, RANI 22, IRJ 2130A, RACA 6 were evaluated in the field against USDA 110 and urea tested on soybean. Results showed that compost carrier formulations maintained rhizobia populations greater than 108 rhizobia/g for the study duration of 270 days, with pure compost containing a significantly highest population. It was noted that room temperature storage maintained rhizobia population greater than 109 rhizobia/ g for 270 days. Inoculating with elite strains NAK 84 and USDA 532C had highest relative BNF effectiveness (> 90 %) and significantly increased grain yield. Therefore, MSW compost in Uganda is good alternative for peat soil and the inoculants can be stored in shade at room temperature for 270 days. Elite strains RANI 22, IRJ 2180A, NAK 128, NAK 84 and USDA 532C are potential candidate strains to USDA 110, which is currently used in Uganda for commercial inoculant production.