RELATING THE NITROGEN-FERTILIZER NEEDS OF WINTER-WHEAT CROPS TO THE SOILS MINERAL NITROGEN - INFLUENCE OF THE DOWNWARD MOVEMENT OF NITRATE DURING WINTER AND SPRING

Optimum applications of N fertilizer, N(opt) have been related successfully to the amount of mineral N in the soil, N(min) in some parts of Europe but not always in the UK. If there is a body of mineral N, Q(N), that ultimately lessens the need for N fertilizer, it will not remain constant in its amount or its position. Mineralization will add to Q(N), while the nitrate component of Q(N) will be leached downwards. Also, part of Q(N) will be taken up into the crop where it will continue to lessen the need for fertilizer N but will be safe from leaching. A computer model was used to simulate these processes for 23 experiments, covering five sites and five years, in which N(opt) had been estimated. From these simulations we derived trial values of Q(N) that took account of mineral N to a series of depths on a series of dates. For each date we used the trial values to find the depth for which N(opt) was best correlated with Q(N) and assumed that this was the depth, d(L), of the lower boundary of Q(N) on that date. Thus d(L) was a collective value for all 23 experiments. The value of d(L) increased throughout the winter and the spring and was very closely related to the cumulative average drainage through 0.5 m soil at Rothamsted. By 15 April, d(L) was 1.66 m, a depth that was compatible with observations by others that winter wheat can remove mineral N to a depth of at least 1.5 m. We inferred two likely reasons why N(min) may fail as a predictor of N(opt) in the UK: insufficient depth of sampling, and too wide a spread of sampling dates. The values of N(opt) were shown to be related satisfactorily to the values of Q(N) computed, without any measurements of mineral N, for appropriate depths on single dates.