SCREENING FOR INOCULANT-QUALITY STRAINS OF RHIZOBIA
Studies of the Rhizobia-legume association (RLA) historically have been motivated largely by practical considerations. 1 This unique plant-bacteria association commonly results in "fixation" or reduction of dinitrogen by Rhizobia within nodules, which they induce on the roots of compatible host legumes. This activity can provide nitrogen in quantity sufficient to supplement significantly the available soil nitrogen required for growth of the legume. Soil nitrogen is a consumptive element in plant nutrition and must be replenished periodically by chemically fixed nitrogen fertilizer or by biological nitrogen fixation (BNF), as in the Rhizobia-legume association. It is the mineral element most often found limiting for crop production. Nitrogen fertilizer is unavailable or prohibitively expensive, especially for subsistence farmers, in many parts of the world.2 The Rhizobia-legume BNF system is, potentially, universally available and economically feasible. It therefore represents a vital technology of inestimable value for increasing and sustaining crop productivity in agricultural soils where soil nitrogen is a real or potential limitation.~ A critical aspect of the RLA is the fact that it often can be manipulated under nitrogen-limiting field conditions in such a way that BNF, and hence crop production, can be enhanced easily and inexpensively. One management practice by which the RLA is exploited-manipulated is referred to as "inoculation". 1 This is the practice of placing Rhizobia on legume seed or in the soil at the time of planting, thus greatly increasing the probability of development of a successful BNF system in the legume crop.