A SIMPLE MODEL FOR PREDICTING THE EFFECTS OF WINTER LEACHING OF RESIDUAL NITRATE ON THE NITROGEN FERTILIZER NEED OF SPRING CROPS

A model is described for predicting the amounts of nitrate residues remaining in the soil in the autumn and the extent to which these are lost by leaching during the winter. It is used to predict the effects of winter rainfall on the growth of spring crops. The model is based on the assumption that any nitrate which remains in the effective rooting zone of the spring crop after the winter will be equally available and any nitrate leached below it will be totally unavailable. The losses of nitrate were estimated from the excess rainfall and the water holding capacity of the soil. The model was tested against the results of published N response experiments in the Netherlands and the UK. It predicted the differential effects of winter rainfall with reasonable accuracy, but tended to overestimate the spring nitrate contents observed in the Dutch experiments and underestimate those for the English data. These deviations appeared to be associated with errors in the estimation of the amounts of nitrate remaining in the soil in the autumn. With certain reservations, the model would appear to provide helpful advice for adjusting spring fertilizer dressings for the effects of differences in winter rainfall.