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 CaCl 2, 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 HNO 3 (194 mg K + kg − 1 ), 2 M NaCl (251 mg K + kg − 1 ), 1 M NaOAc (295 mg K + kg − 1 ), 1 M NH 4OAc (312 mg K + kg − 1 ) and 1 M HNO 3 (737 mg K + kg − 1 ). Potassium extracted by 0.1 M HNO 3, 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 NH 4OAc ( r = 0.689), which is used to extract K + in the soils of the studied area.