Nitrogen fertiliser replacement by single and multi-strain rhizobial inoculants for black gram, green gram and soybean cultivation in Sri Lanka

Various environmental, economic and health problems have arisen in the world due to the continuous application of N-fertilisers for crop production, especially in the third world countries. The current study was undertaken to develop effective rhizobial inoculants for three major legume crops in Sri Lanka namely, Vigna mungo, Vigna radiata and Glycine max to replace the application of nitrogen fertilisers. Rhizobial isolates were obtained from the root nodules of different cultivars of edible legumes, non-edible wild legumes and wild relatives of Vigna spp. Authentication and screening for effectiveness of the isolates were carried out, and five strains were selected as effective isolates and cross inoculated with the three legumes along with a stress tolerant strain which was previously screened. A pot experiment was followed by a field trial in the dry zone of Sri Lanka for single and multi-strain inoculations. The results of the pot experiment indicated that the addition of rhizobial inoculants increased the growth performance in all treatments. In the field trial, both single and multi-strain inoculants gave significant increases in yield, compared to N-fertiliser application in all three crops, viz; an increase of yield from 3 % to 39 % in V. mungo, 5 % to 14 % in V. radiata, and 4 % to 13 % in G. max. In conclusion, the current study has shown that single and multi-strain rhizobial inoculants are capable of completely replacing urea application to V. mungo, V. radiata and G. max, in Sri Lanka without any yield reduction.