The paper discusses the contents and chemical forms of potassium in post-mining soils after 35-year-long experimental cultivation. In the experiment the post-mining dump deposits were fertilized with N, P, K, NK, NP, PK or NPK. Wheat and rape were sowed on the fields. The control field was left unfertilized. In the years 2014-2016 surface (0-30 cm) and subsurface (30-60 cm) layers of fertilized and unfertilized soils were sampled for chemical analyzes. In <2 mm fraction of these soils we determined pH, C and N contents as well as the following K forms: H2O-soluble K (K-H2O), active K (K-CaCl2), bioavailable K (K-Dl), exchangeable K (K-CH3COONH4), fixed K (K-HCl) and non-exchangeable K (K-HNO3). We demonstrated that effect of fertilizing on the K fractionation in the soils was statistically significant. The most favorable chemical changes in the cultivated soils occurred due to NPK treatment. The NPK fertilizing resulted in the highest increase in H2Osoluble K and exchangeable K contents.