The objective of the study was to determine modifications in the content of available nutrients, organic carbon, total nitrogen and mineral forms of nitrogen occurring under the effect of long-term mineral and mineral-organic fertilization. The experiments were carried out on two fields between 1998 and 2001. They covered the fourth rotation of a long-term field trial established in 1986. On grey-brown podzolic soil formed from loamy clay, the following crops were grown in rotation: sugar beet, spring barley, maize and spring wheat. The soil analyses carried out prior to the tests showed that the soil contained 7.9 g kg-1 organic carbon and 788.6 mg kg-1 total nitrogen. The content of available nutrients was as follows: 100.0 mg K, 53.2 mg Mg and 41.3 mg P kg-1. One of the fields was fertilized with minerals and manure. The other field received only mineral fertilizers. The rates of mineral fertilizers were similar on both fields. Soil samples for analyses were taken from the arable layer after harvest. The content of organic carbon and total nitrogen in soil depended mainly on manure fertilization. The differences in the content of org.-C and total-N in the soil from objects fertilized with minerals and manure versus those fertilized exclusively with minerals were 50 and 40%, respectively. No significant effect of different modes of fertilization on the soil properties mentioned above was noticed. The content of available forms of P, K and Mg in soil to a high extent depended on the fertilization system as well as on the rates of each fertilizer component. The concentration of these nutrients tended to decrease as the rates of nitrogen went higher. This was more evident in the rotation with manure fertilization. The concentration of ammonium nitrogen and nitrate nitrogen in the arable layer of soil was influenced by the organic and mineral fertilization.