The expert system nitrogen (N)‐index calculates N fertilizer recommendations for arable crops in temperate regions. The knowledge base of N‐index leans on different fundamentals, such as the measurement of the mineral N stock in soil in spring, the calculation of the evolution of the mineral N content in time and depth in the soil profile as well as the crop response to N fertilizer rates. The calculations with the empirical and theoretical founded knowledge base results in a N‐index value, which is subdivided into different values each representing a major point of vue to know the N availability for the growing crop. Since the introduction of the N‐index expert system in Belgium and the northern part of France in 1979, many crops were added to the knowledge base. The number of samples and correlated analysis data increased, making it possible to review accurately the mineral N stock in the soil after the winter period as a function of many factors,e.g.previous crop and organic fertilizer application.