Biological nitrogen fixation in upland and marginal areas of the U.K.

The limitation of pasture, and hence livestock production, in the uplands of the United Kingdom by lack of available soil N is described. It is shown that fixation of 100-150 kg N ha $^{-1}$ annually by white clover (Trifolium repens) is the key to sustained pasture improvement and to economically viable production systems so making agriculture less marginal in these areas. Some results of research on the nutrient, microbial and management requirements to establish and maintain upland pastures containing at least 25% clover dry matter (d.m.) in the total herbage d.m. production in spring are illustrated. Enhanced N $_2$ fixation by white clover in upland agriculture will depend on increases in efficiency due to selection of rhizobia and rigid matching of them with clover genotype and the environment, on extension of the season of fixation by choice of clover with low-temperature thresholds and by carefully controlled grazing of the swards, and on extension of the area of improved pasture. The potential for the latter alone is considerable since improvement of an additional 10% of the better upland soils by establishment of existing varieties of white clover and strains of Rhizobium would result in fixation of approximately 50 kt N annually, worth [pound]20 M at today's fertilizer prices.