Advisory systems for nitrogen fertilizer recommendations

During the last decades several methods have been proposed to optimize N fertilization, some of them being implemented in advisory systems. We focus on nitrogen fertilization of some important arable and vegetable crops. The recommendation systems can be divided into three groups: systems mainly based on soil analysis, systems based on plant analysis and systems based on simulation models. For these three systems we will discuss the possibilities, drawbacks and, if possible, ways to overcome the difficulties. Most systems based on soil analysis lean on the determination of mineral nitrogen content at the end of the winter period, sometimes also taking into account nitrogen mineralization during the crop growth period. Different approaches are possible. Plant analysis determines the actual nitrogen status of crops. As a single basis for nitrogen recommendation it may never satisfy, but it may be helpful in optimizing split applications of nitrogen fertilizer. Simulation models are interesting tools to assess nitrogen dynamics in the soil. Mechanistic models try to incorporate the best possible description of the known processes. As a consequence these mechanistic models are in general rather complex. Functional models, on the other hand, aim to give a reasonably good general description of the nitrogen dynamics in the soils without going into great detail. Modelling in combination with soil analysis can possibly result in more accurate nitrogen recommendations on field scale and predictions of the risk of nitrogen residues in the soil after harvest.