Residual soil nitrate after application of nitrogen fertilizers to crops

In Western Europe residual soil nitrate, i.e. nitrate present in the soil in the fall after harvest of the crop, is a major source of nitrate that may possibly leach during the subsequent winter period. At the currently recommended N fertilizer application rates high levels of residual soil nitrate are found after potatoes, most field vegetables, grazed grassland, and silage maize. It is to be expected that in these cases the maximum permissible nitrate concentration in the shallow groundwater layers is exceeded. Measures to reduce the levels of residual soil nitrate include refinement of the current recommendations, reduction of N application rates, and also growing winter crops after the cash crops. Current N fertilizer recommendations generally only take into account the amount of soil nitrate present shortly before fertilizer application. Soil N mineralization rates during the growing season and yield levels, which have a dramatic effect on the N fertilizer requirement of the crop, may vary considerably from field to field. Therefore, N mineralization and expected yield level should be explicitly included in N fertilizer recommendations. This can be done by using simulation models. If even after refinement of the recommendations still too high levels of residual soil nitrate occur, recommended rates should be reduced to prevent leaching of nitrate at an unacceptable level. Obviously, this causes a loss of yield. Future research should aim at developing N fertilizer strategies which are able to supply agricultural crops with sufficient N for optimum growth and development, while keeping losses to the environment at a minimum.