An objective assessment of the ability of the soil to supply plants with potassium is possible only when determining a set of indicators - the content of various forms of potassium in the soil and the degree of their mobility. In this regard, we conducted studies to study the effect of long-term fertilizer and ameliorant application on the content of various forms of potassium and potassium buffer capacity of the soil. The studies were carried out in a field experiment laid down in 1986. A six-field crop rotation was mastered in the experiment. We studied 7 options for experience. Soil samples were taken after the fifth rotation of the crop rotation. The results showed that long-term fertilizer application creates quite high reserves of various forms of potassium in the soil. The most intensive accumulation of forms available for plant nutrition (mobile and metabolic) was facilitated by the introduction of mineral fertilizers against the background of organic aftereffects. The use of ameliorant leads to the increase in non-exchange fixation of potassium. The study of the potential buffering capacity of leached chernozem with respect to potassium showed that even with a sufficiently high content of its mobile and metabolic forms, the soil can be characterized by a deterioration of the potash state.