Content of mobile potassium in black topsoil during sugar beet crops rotation chain with peas changed as depending on fertilization system. In variant without fertilizers its content decreased to the initial in topsoil per 2,8, subsurface – 1,8 and becomes reciprocally 72,8 and 61,6 mg/kg soil.

Using for fertilizers green mass of mustard white, its combination with fertilizers and fertilizers itself  in optimal and increased rate, changed content of mobile potassium in soil by the end of rotation chain not significant.

Increasing mobile potassium content in black soil was observed under use organic-mineral fertilization systems. In this alternative fertilization system which included use by-products in combination with vegetative mass of white mustard for fertilizers was as affective as traditional system (on the base of manure) concerning its influence on exchangeable potassium fond in soil. Content of mobile potassium in topsoil under traditional organic-mineral fertilization system increased to initial per 7,7%, subsurface – 6,8%, alternative – reciprocally per 6,6% and 2,1%.

Key words:  potassium regime, black soil, rotation chain, fertilization system.