Kinetics of non-exchangeable potassium release and availability in some calcareous soils of western Iran M. Jalali Department of Soil Science, College of Agriculture, Bu-Ali Sina University, Hamadan, Iran Received 21 October 2004; received in revised form 3 September 2005; accepted 3 November 2005 Available online 5 January 2006 Abstract 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 4 OAc (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 4 OAc (r = 0.689), which is used to extract K + in the soils of the studied area. D 2005 Elsevier B.V. All rights reserved. Keywords: Non-exchangeable K + ; Calcareous soil; Kinetics; Release; Iran 1. Introduction Potassium (K + ) is an element essential for plant growth and its importance in agriculture is well recog- nized (Sparks and Huang, 1985). Although the distri- bution of K + forms differs from soil to soil as a function of the dominant soil minerals present, total soil K + reserves are generally large. Soil K + is typically divided into four forms: soil solution K + , exchangeable K + , non-exchangeable K + , and K + in soil minerals (Sparks, 1987). There are dynamic, equilibrium reactions be- tween different forms of K + . Non-exchangeable K + can be an important reservoir of K + in soils. Several studies demonstrate that non- exchangeable K + from reserves makes an important contribution to plant K + supply (Mengel and Busch, 1982; Mengel and Uhlenbecker, 1993). For optimal nutrition of crop, the replenishment of a K + -depleted soil solution is affected predominately by the release of non-exchangeable K + from clay minerals and organic matter. Therefore, for maximal crop growth, soil solution and exchangeable K + need to be replenished continually with K + through the release of non-exchangeable K + 0016-7061/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2005.11.006 E-mail address: Jalali@basu.ac.ir . Geoderma 135 (2006) 63 – 71 www.elsevier.com/locate/geoderma