Extension of symbiotic biological nitrogen fixation technology in developing countries

Abstract

There is critical involvement of microbial activities in the management of nitrogen in tropical soils. One of the most important of these activities is the provision of nitrogen to the soil via symbiotic biological nitrogen fixation, mediated primarily by the rhizobia-legume association. This symbiotic system commonly produces a significant amount of the nitrogen required by plants growing in a nitrogen-deficient soil. The exploitation of this system is especially important for subsistence farmers in developing countries, who typically are relegated to exist on a few hectares of land of marginal potential for agricultural use. Nitrogen, the nutrient most frequently found limiting to crop growth, is generally not available, economically or logistically, in the form of chemical fertilizers in sufficient amounts to produce a crop. Legumes are utilized throughout the world as sources of food, forage, fuel and cover crops. The efficiency of their use in farming practice depends heavily on the nitrogen-fixing properties of the native rhizobia with which they form symbiotic nitrogen-fixing associations. Native rhizobia are often less effective in N-fixation than are selected strains isolated and screened specifically for high N-fixation. These latter strains are mass produced to form a seed-soil inoculum to be applied at planting, thus assuring a high rate of nodulation and N-fixation in the legume host. This legume inoculation is simple, inexpensive and an invaluable component of the farming systems of agriculture-based cultures throughout the world. This invaluable technology is largely unavailable to subsistence farmers, world-wide, who need it most. The absence or ineffectiveness of appropriate extension programs is discussed as a major contributing factor in this unrealized potential.