Competition for nutrients between grasses and white clover

In a grass-clover pasture, the clover content is of considerable importance because of the dependence on symbiotic fixation for supplying the vast nitrogen needs of a highly productive sward. It is well realised that the grass-clover balance is a dynamic factor of all pastures, and considerable work has been done to show the effects of pasture management and fertilizer treatment on the clover content of the sward with a view to encouraging the longevity of the clover constituent. However, the mechanisms by which these practices encourage or suppress the clover in the association are less well understood. Clements, Weaver and Hanson (27) explained that the botanical composition of plant associations is determined through the outcome of competition among the associated species for the available light, moisture, and nutrients, but since this time, 1929, little has been done to assess the relative importance of such competition on clover when associated with grass, or to discover the means whereby some grasses can often compete with clover with such aggression as to reduce the legume content of an association to a level well below that necessary to maintain high production. While light competition is fairly obvious when it occurs and can easily be controlled by frequent defoliation, nutrient competition is much more subtle and in order to maintain the clover content when such competition is operative, it is often necessary to apply fertilizer several times in excess of the requirements of both the grass and clover because of luxury consumption by the strongly competitive grasses. Of the grasses commonly grown in association with clover, it is well recognized that some species are considerably more aggressive competitors for a limiting nutrient than are others. The need of further knowledge on the precise mechanism of nutrient competition is of great importance in that through such knowledge, it may be possible to find ways of reducing competition and thus reduce fertilizer application, reduce the dominance of highly competitive grasses on poorer pasture country, and increase the longevity of good grass-clover associations. The aim of the present experiments was to test the competitive effects of three species of grasses on white clover and to endeavour to throw some light on the causal agents concerned with nutrient competition in pastures.