Potassium Status of Soils on Unfertilized Cropland in Russia

The potassium status of different types of soils with various textures, and the content and distribution of forms of soil potassium, were studied in long-term experiments in various agrolandscapes. It was shown that the content of forms of soil potassium naturally increased from soils of the non-chernozem zone to soils of the forest-steppe and the steppe, and within a zone when the texture was fine. The profile distribution of forms of potassium in the explored soils was unequal. It was found that under agro-depleting conditions (long-term agricultural use of soils without fertilization) as a result of uncompensated removal of soil potassium by plants, a minimum level of exchangeable potassium (K2Omin) was reached. The part played by exchangeable and non-exchangeable soil potassium in crop formation under agro-depleting conditions was determined in various agricultural landscapes. It was shown that without the application of fertilizers, potassium removal with a harvest significantly exceeded the decrease in the amount of exchangeable potassium from soils, and a harvest formed due to the mobilization of the natural non-exchangeable potassium under the influence of plants. The annual mobilization of potassium from stocks of sod-podsolic soils and gray forest soils was 40–62 kg/ha of K2O, while that from chernozem and light chestnut soils was 57–130 kg/ha. To ensure that plants receive the optimal level of potassium and to prevent the soil from being depleted of potassium, the regular use of potash fertilizers is necessary.