The influence of urea phosphate on physicochemical properties of acid peat substrate (pHH2O = 5.23) was examined in an incubation experiment. The experiment included six objects which received, together with urea phosphate, 0; 94.68; 189.54; 284.22; 378.90; 473.76 mg N dm-3 of the substrate and 0; 100; 200; 300; 400 and 500 mg P·dm-3 of the bedding. The incubation was carried out for 96 days at room temperature (19-21ºC). The indicators of physicochemical features were determined by standard methods used in the analyses of substrates in horticulture (a universal method). After the application of the highest dose of this fertilizer, the peat substrate lowered its reaction the least. N-NO3 content was significantly responsible for the acidification of the bedding (r = 0.75). Increasing doses of urea phosphate resulted in a significant, almost two-fold, drop in available calcium for plants. A significant positive correlation (r = 0.73) was demonstrated between the amount of mineral nitrogen (N-NH4+N-NO3) and EC in the substrate. The smallest fluctuations related to the occurrence of nitrates content in the bedding over time were found after applying doses 2 and 3 of urea phosphate. As the dose of the fertilizer increased, the concentration of N-NH4 in the analysed environment of the tested bedding grew. Incubation of peat substrate of combinations 5 and 6, regardless of the time, resulted in considerably exceeded limits, even by a factor of 10.