The objective of this work was investigation of the growth dynamics of proteolytic bacteria and the enzymatic activity in soil for the cultivation of maize (Zea mays L.), as well as the maize yield under application of some selected organic fertilizers. Intensity and the direction of the developed changes in the soil depended on the type of applied organic fertilizer, the size of its dose introduced into the soil and on the developmental phase of the grown plant (maize). On the basis of obtained results it was found that all tested organic substances stimulated the activity of proteases. Yields ofmaize as raw material for silages obtained from soil fertilized with sewage sludge were similar to those obtained after the application of pig slurry and they exceeded maize yields harvested from other ferti I ization objects.

The laboratory experiment was set up on a podzolic soil in two variants. In one of them non-sterile sewage sludge was introduced into the soil, and in the second - the same sludge but subjected previously to the process of sterilisation. In both variants the same doses of the sludge were applied: 30 (1%), 75 (2.5%), 150 (5%), 300 (10%) and 600 Mg·ha-1 (20%). Then, after 0.5, 1, 2, 3, 4 and 5 months, the soil of both experimental variants was analysed for the numbers of bacteria and fungi decomposing proteins, the rate of the process of ammonification, the rate of the process of nitrification, and for proteolytic activity. The results obtained revealed a stimulating effect of the sludge, both sterile and non-sterile, on the numbers of the microbial groups under study and on the rate of nitrification and protease activity. Only the process of ammonification was subject to inhibition. The observed effects of the sludge were the most pronounced in the case of the higher sludge doses. Significantly greater numbers of protein-decomposing fungi and higher activity of almost all (except for ammonifcation) analysed biochemical parameters in the soil with non-sterile sludge compared to that with sterile sludge indicate an effect of microorganisms from the sludge on the microbiological transformations of nitrogen in soil amended with sewage sludge.