The aim of our experimental work supporting the development of the fertilisation consultancy service of soybean was to examine the effect of the N, P and K supply on the yield of soybean at well distinguished soil nutrient supply levels, as well as to determine the N, P and K supply limit values for chernozem meadow soil. The long-term fertilisation experiment was established in 1989 on chernozem soil with deep calcareous layer at 4 supply levels of N, P and K, respectively. The experiment had a comprehensive treatment combination with 64 treatments. This study focuses on the experimental findings of 11 years between year 4 and 20. The following main conclusions can be made:

1. The yield of soybean without N fertilisation was between 0.69–3.08 t ha-1 (average yield: 1.78 t ha-1) depending on the water supply of the growing season on the high N supply chernozem meadow soil with a humus content between 2.8–3.2%. Averaged over the experiment years, 87% of the yield could be achieved without N fertilisation. The 80 kg ha-1 N dose significantly increased seed yield in each year. Averaged over the 11 examined years, the yield surplus was 0.27 t ha-1. In the majority of years, N over fertilisation resulted in yield decrease.

2. The P reaction of soybean is weak. Compared to the 118–150 mg kg-1 AL-P2O5 supply level of the soil, better P supply rarely results in a slight and non-economical yield increase. The high P supply of the soil results in yield depression in certain years. The P fertilisation of soybean is justified primarily on soil with weak P supply levels.

3. The seed yield of soybean did not notably change in the 200–560 mg kg-1 AL-K2O supply range of the soil. In some cases, excessive K supply may be coincided with yield decrease. The K fertilisation of soybean is not recommended on chernozem meadow soil with a 200–250 mg kg-1 AL-K2O supply level and above.

4. According to the performed examinations of interactions, the yield increasing effect of N fertilisation is more pronounced at proper P supply levels. Under N fertilisation, better K supply than the 320 mg kg-1 AL-K2O supply of the soil may result in yield decrease.

Keywords: N, P and K supply, interaction, soybean, seed yield, long-term experiment