THE EFFECT OF LONG-TERM FERTILIZATION ON THE CONTENT OF MINERAL NITROGEN IN SOIL

The content of soil mineral N (N min.) in early spring and after the h arvest of crops (beginning of September) was studied in two sites at L ipa and Svitavy in the years 1991 to 1993 in long-term stationary trea tment trials. In both sites, at Lipa since 1974 and Svitavy since 1980 , 12 treatments of fertilization have been included in trials. Trials were arranged in eight-plot crop rotation with 50% cereals, 25% forage and 25% root crops. The rotation of crops was identical in both sites and the following crops were cultivated in the years under observatio ns: 1991 - red clover, 1992 - winter wheat, 1993 potatoes followed by winter wheat. Nitrate and ammonium N were determined from freshly take n soil samples in extract of 1% K2SO4 by ion selective electrodes. Hig h content of mineral N was found in 1998 in the spring in the clover s tand (20.7 to 40.1 ppm N), but also in autumn after incorporation of c lover (20.7 to 30.1 ppm). Most of determined mineral N was in ammonium form. Marked increase in mineral N was found in autumn in the year 19 93 in treatments fertilized by gradated doses of NPK in combination wi th manure (applied in autumn of 1992). Gradated fertilization of P and K did not affect more significantly the content of mineral N. Positiv e correlation between mineral N content and wheat yield was found. Thi s relationship was not confirmed in potatoes. Positive correlation bet ween mineral N content was determined, in the spring and uptake of N b y wheat and in potatoes in the autumn.