Abstract <p>The results of a study devoted to the study of the effect of the content of mobile potassium in various soils on the yield of major crops and the effectiveness of the use of nitrogen fertilizers for them are presented. According to the obtained data, the availability of mobile potassium in soils plays a very significant role in increasing the yield of cereals, potatoes, and flax. To the greatest extent, this affected sod-podzolic soils, on which the yield increase was higher due to an increase in their potassium level and a higher return on nitrogen fertilizers. Among the studied crops, winter wheat stood out, which reacts better than others not only to an increase in the K₂O content in the soil but also in general to their agrochemical cultivation. Its yield increased three times on sod-podzolic and gray forest soils and 2.5 times on leached chernozems, whereas the yield of spring wheat on the same soils increased 1.2–1.3 times, potatoes 1.5 times, and flax, depending on the forecrop, 1.7–1.9 times. The dependence of the efficiency of nitrogen fertilizers on the content of mobile potassium in soils has been established.</p>