Relating the nitrogen fertilizer needs of winter wheat crops to the soil's mineral nitrogen. Influence of the downward movement of nitrate during winter and spring

Abstract

SUMMARY: 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 andassumed 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_Lincreased 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.