EFFECTS OF PREVIOUS CROP, SOWING DATE, AND WINTER AND SPRING APPLICATIONS OF NITROGEN ON THE GROWTH, NITROGEN UPTAKE AND YIELD OF WINTER-WHEAT

Multifactorial experiments at Rothamsted Experimental Station in two c ontrasting seasons. 1985/86 and 1986/87, tested the effects of treatme nt combinations that varied the supply of nitrogen at important stages of crop development in autumn and spring on the grain yield and nitro gen content of September- and October-sown winter wheat. Treatments th at altered the nitrogen supply in autumn were an application of winter fertilizer N and sowing the wheat after rape or oats, which left diff erent amounts of residual N. These were combined with treatments which tested the effects of 200 kg N/ha in spring applied as early or late dressings and as single or divided dressings. The effect of applying a n additional 50 kg N/ha in summer was also tested in 1985/86. In both experiments, larger yields were obtained from sowing in September than in October. The September-sown wheat grew better over winter in 1986/ 87 than in 1985/86 but the early advantage in size and N uptake result ed in enhanced production of straw rather than grain. Residues of N fr om previous crops were smaller after oats than rape in both years. Thi s difference in soil N did not affect the over-winter growth and N upt ake of the October-sown wheats. Neither this difference in residual N nor an application of fertilizer N in winter affected the yield of the following September-sown wheat in 1985/86 because autumn growth and N uptake were restricted by adverse weather. In 1986/87, however, wheat that followed oats yielded 0.42 t/ha less grain than wheat that follo wed rape, and the deficit in yield was removed by an application of fe rtilizer N equivalent to the deficit in soil N. Yields were decreased when the spring N was applied as a delayed, single dressing in April e specially if the wheat was sown in September after oats, or was not gi ven winter N. Yields were not affected by any of the other combination s of single v. divided dressings or early v. late applications of spri ng N, despite these being given at very different stages of apical dev elopment. The percentage of N in the harvested grain was greatly incre ased by winter applications of fertilizer N, especially to wheat grown after oats, by applying the spring N as a late, single dressing and, in 1986, by applying N in summer.