Research was conducted on the effects of mineral fertilizers and manure on urease activity and total nitrogen and ammonium nitrogen contents, and their correlation in a chernozem soil during four developmental stages of wheat. The study involved the following treatments: unfertilized control; N2 (100 kg/ha N); P2 (100 kg/ha P); K2 (100 kg/ha K); N2P2; N2K2; P2K2; N1P1K1 (50:50:50 kg/ha); N2P2K2; N3P3K3 (150:150:150 kg/ha), and N1P1K1 + solid manure (20 t/ha). Urease activity was determined by

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... e method of Hofmann, and total nitrogen and ammonium nitrogen by the Kjeldahl method. The combined application of low NPK fertilizer rates and manure resulted in increased urease activity. All treatments with three-component mineral fertilizers, two-component P2K2 and phosphorus P2 applied during the first two phenophases of wheat inhibited the soil urease activity, whereas at other developmental stages their effect and the effect of other mineral fertilizers on the activity of the enzyme were non-significant. The results indicate differing effects of mineral fertilizers on total N in the soil regardless of developmental stages. Of the single-component fertilizers, the highest total N level was recorded under K treatment. The use of two-component fertilizers produced a higher total N content compared to the single-component ones. Of the NPK treatments, total N content was highest under N2P2K2 treatment throughout the growing season. The N1P1K1 + manure treatment gave the highest amount of total N. Significant differences were found in ammonium N levels between the control and most of the other treatments during the growing season, but no definite pattern could be established. There were no correlations either between the mineral fertilizers used and urease activity or between urease activity and total nitrogen and ammonium nitrogen contents.