TURNOVER AND BALANCE OF NUTRIENTS OF ARAB LE FARMING ON SANDY SOILS IN DEPENDENCE ON SUPPLY OF NUTRIENTS AND CROP-ROTATION - RESULTS OF LONG-TERM FIELD EXPERIMENTS AT MUNCHEBERG

From 1976 to 1989/90 four long term experiments were implemented on lo amy sand with different content of N-t (N-t as a mark for humus), appl ication of rising amounts of organic manure and mineral fertilizer and crop rotations with different percentages of cereals and forages. In the experiments the input of NPK with fertilizing, deposition of nitro gen with precipitation (35 kg.ha(-1).a(-1)), NPK uptake in harvested y ield and the contents of P and K (Ca lactate extraction) were analysed . Effect of crop rotation and organic manure on content of N-t in soil was plainly recognizable. The effect of increasing amounts of mineral fertilizer was not significant. The changing of N-t in soil was modif ied by the degree of saturation with humus, described as quotient N-t : clay. Equal fertilizing treatments may cause an increase or decrease of the N-t-content in relation to humus saturation. There are signifi cant differences between simple or horizontal N-balances N-supply with fertilizer and seeds/N-uptake with yield) and ecological N-balance (c onsideration of the N-turnover in soil). In our experiments the differ ences between ecological and simple N-balance reach from -33 to +77 kg .ha(-1).a(-1) N in dependence on fertilization and crop rotation. The refore it has to be concluded that the simple N-balance is not useful to characterize the risk of nitrate-leaching. The P-eutrophication in the soil is a function of P-supply and uptake. The cumulative surplus of P-balance and the change in soil-P-content of top- and subsoil are highly correlated. The interpretation of the results appears concernin g the K content in soil problematically. Alterations in K content are correlated with K-balance only on plots without change of N content in soil. We observed a continuous decrease of K content in soil also in case of high surplus of K and increasing organic matter. Possible expl anations were discussed.