Changes in the Potassium Regime of Soddy-Podzolic Heavy Loamy Soil during Long-Term Extensive Cultivation and Fertilization of Agricultural Crops

Changes in the contents of the total, easily available (extraction with 0.01 M CaCl₂), available (extraction with 0.2 N HCl, Kirsanov’s method), and nonexchangeable (extraction with 10% HCl with boiling according to Gedroits) potassium in one-meter-thick layer of soddy-podzolic heavy loamy soil (Albiс Retisol (Abruptic, Aric, Loamic)) during long-term crop cultivation with and without application of fertilizers were studied on the fields of a stationary experiment established in 1968 in Perm region. Organic (cattle manure), mineral, and organomineral fertilizer systems leveled by nutrients (control (without fertilizers); manure 10 and 20 t/ha per year; NPK in equivalent rates; and manure 5, 10, and 20 t/ha per year + NPK in equivalent rates) were applied to the eight-field fallow–grain–row crop rotation. An assessment of changes in the potassium regime of arable soil was carried out in comparison with a virgin analogue (herbaceous meadow). It was found that the long-term (almost 50 years) crop cultivation without application of fertilizers resulted in a decrease in the reserves of total potassium in the topsoil (0–20 cm) by 13% and its available forms by 43%; the reserves of easily available potassium in the one-meter soil layer decreased by 40%. The application of fertilizers influenced the direction of changes in the potassium regime of soil. Maximum accumulation of easily available, available, and nonexchangeable potassium compounds was observed in the upper 40 cm; a tendency towards a decrease in the reserves of these compounds (relative to the trials without fertilizers) was seen in a deeper (40–100 cm) layer. The maintenance of the total stock of potassium in the layer of 0–20 cm at the level of the virgin analogue (46 t/ha) was observed when using an organomineral fertilizer system with the rate of manure application 10 t/ha per year + NPK in amounts equivalent to manure; an increase in the stock of potassium took place upon manure application rate of 20 t/ha per year + NPK equivalent to manure.