Effect of potassium fertilization on different forms of potassium in vertisol under soybean

A field investigation was carried out during Kharif 2015–16 to study the influence of graded levels of potassium on different potassium fractions in soil under soybean cultivated on farmers’ fields in Vertisols at Kanehri village, Tahsil Barshitakli, District Akola. The experiment consisted of four potassium treatments with six replications, where each farmer represented one replication, and was laid out in a Randomized Block Design. The treatments included 30:75:00 (T₁), 30:75:30 (T₂), 30:75:60 (T₃), and 30:75:90 kg NPK ha⁻¹ (T₄). Results indicated that progressive application of potassium significantly enhanced all soil potassium fractions. The highest concentrations of water-soluble K (34 mg kg⁻¹), exchangeable K (152 mg kg⁻¹), non-exchangeable K (586 mg kg⁻¹), lattice K (10535 mg kg⁻¹), and total K (11305 mg kg⁻¹) were recorded with the application of 30:75:90 kg NPK ha⁻¹. Among the different fractions, lattice K dominated, followed by non-exchangeable K, exchangeable K, and water-soluble K. The proportional contribution of potassium fractions to total K followed the order: lattice K (93.29%) > non-exchangeable K (5.14%) > exchangeable K (1.32%) > water-soluble K (0.26%). Significant and positive correlations among different potassium fractions indicated a dynamic equilibrium in soil. The contribution of non-exchangeable potassium to total potassium uptake increased with decreasing potassium application rates, with the maximum contribution (22.39 kg ha⁻¹ and 84.87%) observed under no potassium application. Potassium use efficiency was higher at lower potassium levels, recording a maximum value of 5.24% at 30 kg K₂O ha⁻¹.