POTENTIAL FOR INCREASING NITROGEN-FIXATION IN GRAIN LEGUMES

This review considers the main factors determining the level of nitrogen fixation in the major grain-legume crops (chickpea, common bean, cowpea, faba bean, lentil, pea, peanut, pigeon pea and soybean) and discusses the possibilities and means for improving symbiotic nitrogen fixation. Methods of determining whether nitrogen is limiting growth and yield are discussed, and responses of the various crops to both the addition of nitrogen fertilizer and inoculation with effective strains of rhizobia are described. The present situation and future prospects for increasing nitrogen fixation through plant breeding, development of superior strains, superior host-strain combinations, improved inoculation techniques, bioengineering and other non-traditional methods are discussed. The general conclusions are that increases in nitrogen-fixation potential should be possible in all grain legumes through both plant breeding and strain improvement. Major increases in the quantity of nitrogen fixed could be achieved simply by improving general cultural practices; however, in developing countries. where the gap between actual and potential crop yield is so large, such improvements are likely to occur very slowly.