Abstract :
Nitrogen is widely used in agricultural practice in different organic and inorganic forms to enhance crop productivity. After the growing season, a part of the nitrogen remains in forms sensitive to changes of the conditions, such as nitrate. In years with above-average precipitation a significant amount of nitrate can leave the rooting zone of various crops even when land is cropped annually. Integration of knowledge related to environmental conditions of a certain area with the soil, water and crop management practices helps to prevent the simultaneity of the unfavourable processes leading to nitrate leaching, thus water resources may be protected from nitrate pollution of agricultural origin. It is of increasing importance that such an approach be applied in the Hungarian crop production. Since the great spatial variability of soil-forming factors is clearly reflected by the heterogeneous (sometimes mosaic-like) soil cover in Hungary, differentiation of categories within the soil types is strongly needed for agricultural practices. Basic features of a computerized fertilizer recommendation system developed in RISSAC to characterize the soil fertility levels in Hungary are: classification of the soils into a new land-site category system characterized by 4-digit codes, and also into aggregated land-site groups with regard to the major plant nutrients; and characterization of the nutrient status of soil by establishing nutrient supply categories which consider also the fertilizer requirements of the main crops. Since during the last few decades Hungarian farmers have applied more N---P---K fertilizers than the crops required, and at the same time there was a large increase in other potential pollution sources, harmful side-effects such as unfavourable changes in the quality of surface and subsurface water resources were detected. Hence, it was imperative to develop methods to calculate N balances which take into account the amount of N originating from other sources than the applied mineral fertilizers and also N output via nitrate leaching down the soil profile. Comparison of the results of several long-term field trials conducted simultaneously at several experimental sites which differed in environmental characteristics such as soil type and climatic conditions was shown to provide a good basis for a more generalized quantification of the overall turnover of nitrogen and for calculating improved N balances. The results indicated that when the rational use of organic manures and N-containing fertilizers is based on the plantʹs N demand, the nitrogen balances are in equilibrium. The amount of N needed to obtain economically viable yields, while at the same time being in balance with the requirements of the crops and keeping the environment uncontaminated, varied in long-term trials from 0 to 50 kg ha−1 yr−1 on fertile soils, 50 to 100 kg ha−1 yr−1 on coarse textured soils, while 100 to 150 kg ha−1 yr−1 in farm field analysis. Residual effects of nitrate in long-term experiments proved that after proper application of N fertilizer the amount of the residual nitrogen was low and no nitrate-N accumulation was detected under the root zone even for coarse textured soils.
