Effect of nitrogen and cobalt application on herbage and seed yields of serradella and subterranean clover

The response of serradella and subterranean clover to soil applications of cobalt in the absence and presence of regular applications of nitrogen fertilizer was measured in a field experiment on a nitrogen-deficient soil near Esperance, Western Australia. In an ancillary experiment, also sited near Esperance on a nitrogen-deficient soil, the responses of three serradella species and two cultivars of subterranean clover to regular applications of nitrogen were measured. Yields of herbage and seed were used to measure the response to the fertilizer treatments. Neither serradella, nor subterranean clover responded to cobalt application. However, in winter, herbage yields of serradella were markedly improved by regular applications of fertilizer nitrogen. Yield increases were three- to eight-fold, depending on the species. However, by springtime, there was no herbage response of serradella to fertilizer nitrogen. Seed yields of serradella and yields of subterranean clover at any harvest were unaffected by regular applications of nitrogen. It is concluded that low winter temperatures limit the rate of symbiotic nitrogen fixation for serradella which in turn limits dry matter production.