Assessment of potassium release threshold levels of Vertisols (shrink-swell soils) in different agro-ecological regions of India

Studies to evaluate the release threshold level of potassium in Vertisols (shrink-swell soils) in different agro-ecological regions of India and how that may help in K fertility interpretations are rare. Thus the objectives of this study were (i) to evaluate the K status of Vertisols through release threshold levels and (ii) to relate the findings with soil K release and fertilizer management. Three established soil series samples were collected from Vertisols of different agro-ecological regions of India. Release threshold level (RTL) was assessed after equilibrating soil with 0.01 M CaCl2 solution having series of soil: solution ratio, after which the remaining amount of exchangeable K was extracted with 1 N NH4OAc (pH 7). Total amount of K extracted (K-T = K desorbed in CaCl2 solution plus K extracted with 1 N NH4OAc) remained more or less constant with decreasing K-level up to a certain value (say threshold value) below which K-T increased sharply indicating K release from non-labile form. The threshold value in terms of K concentrations (Release Threshold Concentration) of Panjari, Nagpur, Maharashtra (Typic Haplusterts, Sub-humid dry), Teligi, Bellary, Karnataka (Sodic Haplusterts, Semi-arid dry) and Kheri, Jabalpur, Madhya Pradesh (Typic Haplusterts, Sub-humid moist) soils were 0.044-0.049, 0.034-0.062 and 0.043-0.11 mm, respectively. The high release threshold K levels in terms of concentration and activity ratio (AR) were observed in soils of Teligi and Kheri series as compared to soils of Panjari series due to high content of trioctahedral mica in the former two soils. The relationships between clay minerals, release threshold level, exchangeable potassium and non-exchangeable potassium determined by 1 N HNO3 and sodium tetra-phenyl borate (NaBPh4) have been discussed.