DEVELOPMENT OF NITROGEN-FERTILIZER RECOMMENDATIONS FOR ARABLE CROPS IN THE NETHERLANDS IN RELATION TO NITRATE LEACHING

In the Netherlands, current nitrogen fertilizer recommendations for arable crops are based on the amount of soil mineral nitrogen in early spring. The larger the amount of soil mineral nitrogen, the lower the recommended application rate of fertilizer nitrogen. A more refined method is to draw up a balance sheet in which the nitrogen requirement of the crop is given on the one side and the contributions of fertilizer nitrogen, soil mineral nitrogen, and the amount of nitrogen mineralized during the growing period on the other. The most refined method of nitrogen fertilizer recommendation is the use of a simulation model that predicts the daily crop nitrogen requirement and nitrogen supply to the crop from various pools during the growing period. A simulation model thus adds the time element to nitrogen fertilizer recommendations. Moreover, in contrast with the other two methods, a simulation model allows identification of environmental side-effects of nitrogen fertilizer application. The current Dutch nitrogen fertilizer recommendations aim at predicting the economically optimum application rate of fertilizer nitrogen. From the environmental point of view it is interesting to know how much soil mineral nitrogen has accumulated in the soil at harvest, because this nitrogen is a potential loss to the environment through nitrate leaching during the subsequent winter period. If the economically optimum application rate of fertilizer nitrogen is applied to arable crops, it is unlikely that soil mineral nitrogen accumulates, except in the case of potatoes. Model calculations have shown that accumulation of soil mineral nitrogen after potatoes can be prevented when the recommended nitrogen application rate is reduced by 25%. In that case tuber yield is reduced by only 2%.