Effects of fertilizer nitrogen on soil nitrogen availability after a grazed grass ley and on the response of the following cereal crops to fertilizer nitrogen

Abstract

SUMMARYNitrogen nutrition of two succeeding wheat crops was studied after ploughing of grassland in July 1987 on a clay soil at ADAS Drayton. The four plots of grassland had received 100, 250, 450 and 750 kg N/ha per year for 4 years from 1984 and were grazed by beef cattle at stocking densities which varied according to grass growth.Determinations of soil mineral N taken to 60 cm every 3 weeks from July to the following May were particularly variable. However, in the first 2 years after ploughing the means of the series of mineral N measurements were directly proportional to the amounts of fertilizer N applied to the grass.N offtake in winter wheat grain without fertilizer N was directly proportional to fertilizer N applied to grass but this had little effect on maximum grain yields. Large soil N supplies did not necessarily predispose the wheat crops to large grain N concentrations because fertilizer N caused grain N offtake to reach a similar maximum, irrespective of previous grass N.Optimum amounts of fertilizer N for the wheat were 188, 147, 87 and nil kg/ha in 1988 and 152,130, 89 and 25 kg/ha in 1989 after 100, 250, 450 and 750 kg N/ha per year applied to the grass. Soil N supply as indicated by both the amount of fertilizer applied to grass and means of mineral N measurements accounted for almost all of this variation. Mean soil mineral N over winter was no better as an indicator of soil N supply than the amount of N applied to the grass. However, before adopting N applied to grass as a more general index of N supply, it would need to be adjusted for variation in N removed and lost during grass growth; these were controlled in this experiment.