The temporal behaviour of soil microbial biomass C (SMBC) and the protease, urease, phosphatase and dehydrogenase activities were measured in the organic production of tomato and cucumber crops under greenhouse conditions. Soil was fertilized with either farmyard manure or poultry manure. Manures were applied either alone or with two biological preparations, E2001 and Allgrow Bioplasma. The effect of solarization on microbial activity was also studied. Soil samples were taken three times in each growing season. Activities of protease, urease and phosphatase decreased by 26% and dehydrogenase by 50% after solarization. In both crops, the highest amounts of organic C and SMBC were determined in soils with applied poultry manure. Protease and urease activities showed a strong increase after the first manure application. After the second manure application, enzyme activities stabilized at lower levels than in the first 6 months. Low temperature in the winter season may have limited phosphatase activity. Dehydrogenase activity was very low in all treatments, probably because the greenhouse soils were intensively fertilized with nitrogenous fertilizers before the experiment. Among the organic sources used, poultry manure had a long-term effect on enzyme activity.