Resultaten van stikstofopzoekingen

A survey is given of the most important experimental results from investigations on nitrogen carried out for 20 years by the Bodemkundige Dienst van België (Pedological Service of Belgium) with the support of I.W.O.N.L. (Instituut tot aanmoediging van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw). Four topics are treated. By means of tables, the share of nitrogenous fertilizers, in the productivity of the Belgian soils is pointed out. More in detail, the influence of nitrogenous fertilizers on the main crops is discussed. In cereals, the production components: number of ears per m², number of grains per ear and 1000-grain weight, are heavily influenced by nitrogenous fertilizers. In potatoes the number of tubers is increasing along with the N-dose. The sugar content in sugar beets and the starch content in potatoes are falling with increase of the N-dose. An increase of N-fertilizers causes an increase of N-content in the plant. A high N-dose, a late N-application or growth restraining conditions often will lead to nitrate accumulations in turnip, cabbages, mangolds and rye-grass. In the main part of this communication the factors, which basically bring about the variability between parcels, for nitrogen requirement, are analysed. These factors are: soil, N-aftereffect, climate, crop and the use of nitrogenous fertilizers. For sandy-loam and loamy soils, the depth of the profile and the organic matter content are important. For sandy soils, the organic matter content together with the humidity level plays a striking role. The after effect is mainly influenced by the N-balance of the previous years, the type of N-fertilizers, the type and depth of the soil profile, the winter rainfall and a possible intercropping. The factor climate is more complex and its influence is function of the soil, the crop and the period of the year. The efficiency of utilization of zoil nitrogen is very different from crop to crop. Beets and forage maize are more efficient than cereals in their utilization of soil nitrogen. A very low nitrgen efficiency has been revealed with Italian rye-grass. The N-fertilizer type, the moment of application, the pH of the soil and the climate on the time of the application may sensitively influence the experimental results. The arosen side-effects are often important. In the last topic, attention has been invited for the draft of an advice for N-dressing in the practices. In this connection, a new kind of N-determination is introduced by which the soil is sampled till a depth of 100 cm in the beginning of the growth period. From field experiments on winter wheat in loamy soils a method has been developed to obtain a more adapted advice for N-application with the help of the determination of the organic matter content of the arable layer and the determination of the mineral nitrogen content till a depth of 100 cm.