Response of commonly cultivated legumes in Zambia to non-indigenous legume inoculants

The use of inoculants is a critical strategy in legume production. Several factors determine the efficiency of an inoculant, including the competitiveness of the inoculated against indigenous strains. Although Rhizobia seems to be widely distributed, some soils may not support sufficient numbers of highly effective strains, necessitating the application of inoculants. In Zambia, inoculants are particularly used for the production of non-promiscuous genotypes of soya beans (Glycine max), but rarely for cowpeas (Vigna unguiculata) and groundnuts (Arachis hypogea). There are currently no locally isolated Rhizobial strains used as inoculants for soya beans, cowpea and groundnut in Zambia. This study evaluated the response of soya beans, cowpeas and groundnuts to BIOFIX legume inoculants produced in Kenya. Seeds were inoculated at the recommended or double the recommended rate at planting. Plants were grown under greenhouse conditions in a Completely Randomized Design for 7 weeks. Control, non-inoculated seeds were also planted and plants grown under the same conditions. At 7 weeks, nodule number and fresh weight per plant, nodule effectiveness (pinkness / redness), and above ground biomass were determined. Biologically fixed nitrogen was determined using the Nitrogen Difference Method and pearl millet (Eleusine coracana) as the reference crop. The results showed that nodule number per plant was higher by 31 and 36% with BIOFIX application in soya beans (recommend rate) and cowpeas (double rate), respectively. At the two rates, nodule number per plant in groundnuts increased by 178 and 207%, respectively. In soya beans and groundnuts, nodule fresh weight responded positively to inoculation, with increases of 178 (recommended rate) and 207% (double rate), respectively. All representative nodules assessed were pink or deep red, suggesting effectiveness. There was no significant benefit in inoculating seeds of the three legumes with BIOFIX on above ground biomass and biological nitrogen fixation. These results could suggest that possibly, the introduced strains could not outcompete the indigenous strains in the soils in which soya beans, cowpeas and groundnuts have been repeatedly grown before. This could be an indication that sufficient and appropriate effective strains are already present in this soil. In general, the results suggest that to obtain the full benefits of biological nitrogen fixation, legume growers need to be provided with the correct inoculant, where required. Further work under field conditions is recommended to confirm these findings.