NITROGEN IN HUNGARIAN SOILS - NITROGEN MANAGEMENT RELATION TO GROUNDWATER PROTECTION

Nitrogen is widely used in agricultural practice in different organic and inorganic forms to enhance crop productivity. After the growing se ason, a part of the nitrogen remains in forms sensitive to changes of the conditions, such as nitrate. In years with above-average precipita tion a significant amount of nitrate can leave the rooting zone of var ious crops even when land is cropped annually. Integration of knowledg e 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 ori gin. It is of increasing importance that such an approach be applied i n the Hungarian crop production. Since the great spatial variability o f soil-forming factors is clearly reflected by the heterogeneous (some times mosaic-like) soil cover in Hungary, differentiation of categorie s within the soil types is strongly needed for agricultural practices. Basic features of a computerized fertilizer recommendation system dev eloped 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 gro ups 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. Si nce during the last few decades Hungarian farmers have applied more N- P-K fertilizers than the crops required, and at the same time there wa s a large increase in other potential pollution sources, harmful side- effects such as unfavourable changes in the quality of surface and sub surface water resources were detected. Hence, it was imperative to dev elop methods to calculate N balances which take into account the amoun t of N originating from other sources than the applied mineral fertili zers and also N output via nitrate leaching down the soil profile. Com parison of the results of several long-term field trials conducted sim ultaneously at several experimental sites which differed in environmen tal characteristics such as soil type and climatic conditions was show n 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 a nd N-containing fertilizers is based on the plant's N demand, the nitr ogen balances are in equilibrium. The amount of N needed to obtain eco nomically viable yields, while at the same time being in balance with the requirements of the crops and keeping the environment uncontaminat ed, varied in long-term trials from 0 to 50 kg ha(-1) yr(-1) on fertil e soils, 50 to 100 kg ha(-1) yr(-1) on coarse textured soils, while 10 0 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 nit rate-N accumulation was detected under the root zone even for coarse t extured soils.