The aim of our experimental work for the development of soybean fertilisation consultancy was to investigate the effect of N-, P- and K-supply on the protein content and amino acid composition of soybeans in well-separated soil nutrient supply levels in a long-term fertilisation experiment. The long-term fertilisation experiment was set up in 1989 on deeply calcareous chernozem meadow soil at 4–4 N-, P-, and K-supply levels, respectively, in a complete treatment combination with 64 treatments. In the present dissertation, the results of five years of quality testing from the 11-year long-term experiments with soy are presented, summarised as follows:

1. For the majority of the years, N fertilisation above 80 kg N/ha did not result in a further significant increase in protein content. In the chernozem meadow soil with a humus content of 2.8–3.2% and good N-supplying ability, in a year with better-than-average water supply, N-fertilisation did not significantly affect the protein content and amino acid composition of soybean seeds.

2. On chernozem meadow soil, the P supply did not cause any significant change in the protein content of soybean in the 120–360 mg kg-1 AL-P2O5 supply range of the cultivated soil layer.

3. Significant increase in amino acid content, expressed as a percentage of dry matter, was mainly caused by excessive N fertilisation (160, 240 kg N ha-1). The protein amino acid composition of soybean (g per 100 g protein) was only slightly affected by the N supply, and no consistent N effect was observed, i.e., there was no change in the ratio of a specific amino acid. N-supply did not substantially alter the ratio of essential and non-essential amino acids.

4. Compared to the supply level of 120–135 mg kg-1 AL-P2O5 of the soil, better P supply (150–360 mg kg-1 AL-P2O5) can potentially modify the amino acid composition of the soybean, depending on the given crop year. However, these effects are not clear in relation to the specific amino acids.

5. In the majority of years, the proportion changes in the amino acid composition of the soybean in the 150–250 mg kg-1 AL-P2O5 supply range of the soil are not consistent and crop year also affects the effects of P. For the majority of amino acids the effect of P resulted in a negative change at the supply level of 280–360 mg kg-1 AL-P2O5.

6. Crop year had a greater effect on the amino acid composition of soybean seed than the N and P supply. Yearly differences in amino acid composition cannot be consistently correlated with water supply, protein content, and yield levels.

Keywords: N and P supply, soybean, protein content, amino acid composition, long-term experiment