Studies of soil and soil water (50 cm) properties were performed six years after the final fertilization in an old field experiment. During a 20‐year period plots had been fertilized three times with different doses of ammonium nitrate. The highest dose was 600 kg N ha‐1, meaning that 1800 kg N ha‐1was given during the period. A substantial accumulation of organic matter had occurred in the forest floor as a result of the fertilization. At the dose giving the maximal response the humus layer had almost doubled its content of carbon and nitrogen, when comparing with the control. The pH was unaffected by the dose in the humus layer and in the eluvial layer. In the upper part of the spodic horizon the pH had decreased 0.2–0.3 units at intermediate doses. The amounts of exchangeable base cations were not changed as a result of the fertilization. The pH of the soil water was similar over all doses. The concentration of nitrate‐N was below the detection limit for all doses except for the highest.