We examined the impact of the potassium supply of the soil on the grain yield and K supply level of soil in a long-term maize fertilisation experiment and we also determined the K supply level limit values which are satisfactory for plant analysis for diagnostic purposes based on the correlation between the K concentration of the maize leaf and grain yield.

The basic fertilisation experiment was established on chernozem meadow soil with deep calcareous layers in 1989. 4-4 P and K supply levels were used and 64 treatments were performed in a full treatment combination. Main soil characteristics: thickness of the humus layer: 85-100 cm; humus content: 2.8-3.2%; pH(KCl) of the cultivated soil layer: 5.0-5.2; plasticity (KA): 50; clay content: 32%; AL-K2O content depending on K fertilisation: 200-550 mg/kg.

This study deals with the K fertilisation results of the experiments carried out in years 7-19 (between 1996 and 2008) of the long-term experiment. Consequently, the following conclusions can be drawn:

1. In 1989, at the time of establishing the experiment, the AL-K2O content of the cultivated soil layer was 322 mg kg-1 which decreased to 200-235 mg kg-1 by the 16th-19th years of the experiment. Depending on the level of K fertilisation, the level of the K supply of the soil changed between 200 and 550 mg kg-1 over the 13-year-long experimental period.

2. There was not any occasion when significant yield surplus was detected as a result of better K supply due to K fertilisation over the 13-year-long experimental period. Without K fertilisation, the dry grain yield changed between 3.28 and 11.01 t ha-1, depending on the crop year. Abundant K supply (380-550 mg kg-1) decreased yield only slightly as a tendency.

3. Based on the maize leaf analysis results performed at the 5-6-leaf development stage and at the early tasseling stage, better K supply was accompanied by higher K concentration in the leaf. The satisfactory K supply limit value determined on the basis of the correlation analyses between the K concentration of the maize leaf and the grain yield is reached at 2.3-4.3% K concentration at 10–14 t ha-1 grain yield and 5–6-leaf development stage, and at 1.5–2.6% K-concentration at the early tasseling stage.

Keywords: K fertilisation, K supply, maize, long-term experiment