Kinetics of non-exchangeable potassium release and availability in some calcareous soils of western Iran

The rate of non-exchangeable K+ release from soils can significantly influence K+ fertility of soils. There are few studies about the relationship between the kinetics of K+ release and plant K+ uptake for calcareous soils. The objectives of this study were (i) to determine the kinetics of non-exchangeable K+ release from some calcareous soils and (ii) to compare the effectiveness of different extraction methods for the prediction of K-supplying capacities. The kinetics of non-exchangeable K+ release by successive extractions with 0.01 M CaCl2, extractable K+ using different soil extractants, and total K+ uptake by wheat was studied in surface samples of 10 calcareous soils in western Iran. Total K+ uptake by wheat grown in the greenhouse was used to measure plant-available soil K+. The kinetics of non-exchangeable K+ release from soils consisted of two phases and was best described by Elovich, power and parabolic models. Elovich b values were correlated with K+ uptake by wheat (r=0.785). This finding shows that the process of K+ release is initiated by a low K+ concentration in the soil solution and not by cation exchange. The following methods extracted increasingly higher average amounts of soil K+: 0.1 M HNO3 (194 mg K+ kg−1), 2 M NaCl (251 mg K+ kg−1), 1 M NaOAc (295 mg K+ kg−1), 1 M NH4OAc (312 mg K+ kg−1) and 1 M HNO3 (737 mg K+ kg−1). Potassium extracted by 0.1 M HNO3, 2 M NaCl and NaOAc showed higher correlation with K+ uptake by the crop (r=0.876, 0.790 and 0.758, respectively) than did NH4OAc (r=0.689), which is used to extract K+ in the soils of the studied area.