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

The content of mineralizable N was studied in two sites where long-term stationary treatment trials were conducted in the years 1991 to 1993. Red clover, winter wheat and potatoes were grown on both sites. Fertilization treatments are in Tab. I. Soil samples were taken from a 0 to 30 cm depth in early spring and after the harvest of crops. The soil was dried after sampling at 40 degrees C. Aerobic incubation was used to determine mineralizable N. The soil diluted with sand (1:1) was incubated for seven days at 30 degrees C. Amount of mineral N (N min.) was determined prior to and after incubation by ion selective electrodes. The results were converted into soil dry matter and are expressed in ppm. Tabs II to VIII and Fig. 1 present them in the following way: A = increment of N min. during incubation = mineralizable N; B = N min. content before incubation; A + B = total N min. after incubation. High values of mineralizable N were found out after incorporation of clover in the autumn of 1991, and in the next year as well, especially in the site at Svitavy. Manuring in the autumn of 1991 did not affect more conspicuously content of N min. and mineralizable N in the spring of 1993. High values were found out as late as in autumn of 1993, particularly on the site at Lipa in combinations where fertilizers were applied (treatments 3, 4, 5, and 6). The clover stand raised more the values of mineralizable N and manure in combination with fertilizers raised more the values of N min. In autumn significant differences between sites were found. Higher values of N min. were set up at Lipa and lower ones of mineralizable N, and at Svitavy rather the opposite. It follows from statistical evaluation that mineralizable N together with N min. exhibits closer correlation with the yield of crops and N withdrawal by crops. Negative correlation was mostly found between content of N min. and mineralizable N.