Response by wheat to phosphorus and nitrogen, with particular reference to 'haying-off'

Abstract

Heron wheat was grown with four levels of phosphorus (as superphosphate) and five levels of nitrogen (as ammonium sulphate) on two pairs of sites in the Riverina of New South Wales, to examine the 'haying-off' phenomenon. Within each pair of sites there was a large difference in soil fertility, as a result of contrasting pasture and cropping histories. Various attributes of crop growth and yield were measured. The crops responded more to phosphorus than to nitrogen. Phosphorus increased grain yield on two sites, and 1000-grain weights on all sites ; nitrogen increased grain yield on one very nitrogen-deficient site, and decreased 1000-grain weights on all sites. In each pair of sites, vegetative growth was considerably greater on the higher fertility one ; however, the difference in grain yield was much less. The failure of the crops on the higher fertility sites to achieve the grain yield potential indicated by their vegetative growth, is possibly an expression of the phenomenon commonly called haying-off. In the absence of adequate soil and plant moisture measurements, it cannot be stated unequivocally that the observed possible expression of haying-off was caused by a soil moisture deficiency induced by increased vegetative growth responding to higher fertility. The nature of the responses to phosphorus and nitrogen on the individual sites suggests that other factors besides induced moisture deficiency may be involved.