Effects of different moisture regimes on soil exchangeable potassium were investigated using 48 soil samples collected from different agricultural areas of Iran. The soil exchangeable potassium was determined after the application of 0, 10 and 20 cycles of wetting and drying, and following 10 and 20 days of incubation at field capacity moisture regime. In addition, variations in exchangeable K, following soil incubation at constant field capacity moisture was studied in five soils using 1M ammonium acetate as the K extractant, and in three out of these five soils the exchangeable K was also extracted by 1M sodium acetate. Results indicated highly significant (p<0.1) effects of wetting and drying and also constant moisture regimes on the exchangeable K. The mean increase in exchangeable K after the application of 10, and 20 cycles of wetting and drying, were 17.5 and 19.2 mgkg-1 and the mean decrease in the same parameter due to the incubation of the soils for 10 and 20 days at field capacity were 9.5 and 11.1 mgkg-1, respectively. The reaction of the dry soils to wetting were different; in some soils having low levels of exchangeable K, there was an increase in exchangeable potassium immediately after wetting, while in the others, it either decreased or remained unchanged. When 1M sodium acetate was employed instead of 1M ammonium acetate for the extraction of K, the exchangeable potassium remained unchanged by wetting, indicating that the effect of wetting is on the fraction of K which is not exchangeable by the large hydrated ions like Na+, but it is replaceable by NH4+ ions. This fraction of K is located mainly in wedge zones of 2:1 layer silicates. In all soils, the exchangeable K decreased exponentially with time of incubation at constant moisture. In this study, it was indicated that the results of Olk et al. (1995) method for determination of soil exchangeable K were highly correlated (r2=0.999, 22 soils) with those of Knudsen et al (1982) method
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