Accurate estimation of the available potassium (K+) supplied by calcareous soils in arid and semi-arid regions is becoming more important. Exchangeable K+, determined by ammonium acetate (NH4OAc), might not be the best predictor of the soil K+ available to crops in soils containing micaceous minerals. The effectiveness of different extraction methods for the prediction of K-supplying capacities and quantity–intensity relationships was studied in 10 calcareous soils in western Iran. Total K+ uptake by wheat grown in the greenhouse was used to measure plant-available soil K+. The following methods extracted increasingly higher average amounts of soil K+: 0.025M H2SO4 (45 mg K+ kg21), 1 M NaCl (92 mg K+ kg21), 0.01 M CaCl2 (104mg K+ kg21), 0.1M BaCl2 (126 mg K+ kg21), and 1M NH4OAc (312mg K+ kg21). Potassium extracted by 0.01M CaCl2, 1M NaCl, 0.1M BaCl2, and 0.025M H2SO4 showed higher correlation with K+ uptake by the crop (P , 0.01) than did NH4OAc (P , 0.05), which is used to extract K+ in the soils of the studied area. There were significant correlations among exchangeable K+ adsorbed on the planar surfaces of soils (labile K+) and K+ plant uptake and K+ extracted by all extractants. It would appear that both 0.01M CaCl2 and 1M NaCl extractants and labile K+ may provide the most useful prediction of K+ uptake by plants in these calcareous soils containing micaceous minerals.
