This paper concerns the influence of a single application of various doses of organic-mineral fertilizer, combined with yearly application of ammonium nitrate, and a single application of manure plus NPK, on the content of nitrate and ammonium nitrogen in the soil, considering the soil moisture. It was found that application of organic-mineral fertilizer at the dose of 12 t ha^–1 plus N, caused a significant increase of the nitrates content in the soil. The content of ammonium nitrogen in the soil did not vary in the years, independently of the fertilizer doses. The nitrate nitrogen content in the soil was dependent on the moisture conditions, the lower the water content, the higher that of nitrates in the soil. The moisture content had a greater impact on the mineral nitrogen content in the soil than the applied fertilization. With the plants growth and development, the nitrate content in the soil decreased, which indicates a steady uptake of this nitrogen form by the plants during the whole growing season. In the case of ammonium nitrate no such connection was found, and the ammonium content in the soil was stable. The nitrates content in the soil was higher than that of the ammonium forms, in the course of the whole experiment.