Effects of nitrogen and sulphur fertilisers on the yield, N and S content, and amino acid composition of the grain of spring wheat

Abstract

Spring wheat (cv. Kleiber) was grown in three pot experiments in a soil containing 10 mg kg⁻¹ KH₂PO₄‐soluble sulphur with different rates of nitrogen and sulphur fertilisers. Without added S the yields of grain dry matter and crude protein were either unchanged (glasshouse experiment) or significantly decreased by the N treatments (controlled environment experiments). With added S the N fertiliser increased the yields by 40–110%. The experiments produced grain with a wide range of N and S contents, the latter ranging from 0.06–0.21% on a dry matter basis. Grain from treatments giving the largest yields of dry matter and crude protein had a N:S ratio of about 15. The amino acid composition of the whole grain was changed significantly by the fertiliser treatments. When plants were grown without added S at the highest level of N the grain contained less than half the amount of cyst(e)ine and methionine (as % of total amino acid content) found in grain from plants given adequate amounts of S fertiliser. Grain from S‐deficient plants also had a much greater content of aspartic acid + asparagine but less threonine, leucine, isoleucine and lysine. The N:S ratio in grain from plants grown with insufficient S was well in excess of 15. The relation of these findings to the nutritive value of wheat as a feed and to the provision of enough disulphide bonding capacity for its use in bread‐making are discussed. Our results emphasise the necessity for having an adequate amount of S to balance the larger N applications now given to wheat to ensure that the grain is of the highest quality possible with any particular cultivar.