THEORETICAL ASPECTS OF NITROGEN ECONOMY IN GRAZING EXPERIMENTS.

An hypothesis relating to the circulation of nitrogen between soil, plants and animals in fertilized grassland is presented. It embodies the results of small-plot experiments and grazing trials on seral grassland in South Africa. Where the value of land is low in comparison with the cost of fertilizer, it is essential that the maximum response be obtained from fertilizer as well as from excretal N. The assumption that 80% of herbage N is returned as excreta, of which 25% is recovered by the grasses in the first season and 25% in the second season, appears to be in accord with liveweight data. Using average values of soil N and fertilizer recovery under local conditions, it is argued that applying 30 lb N per acre (with an initial heavy dressing) should provide higher percentage recoveries of fertilizer N than fixed annual dressings of 60 lb N per acre. In grazing trials over five years, in which these systems are compared, the apparent recoveries of fertilizer N in animal liveweight were respectively 22.4% and 11. 6%. A distinction is drawn between the initial build-up of nitrogen in circulation and the long term maintenance of a nitrogen level providing maximum recovery of fertilizer N. [ABSTRACT FROM AUTHOR]