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Fixation of K was studied in seven soil samples collected from different areas of Karaj . 5 grams of soil were transfered into centrifuge tube and 1 milliliter KCI ( 1 meq K+ / 100 g soil ) was added . After 48 hours the - extractable K were replaced by ammonium acetate and K+ was determined by flame photometer . The results of the experiment indicated that the amount of K+fixed were considerable and moreover it was noticed that increase o. the amount of fixed K+ was proportional to increased amount of KC1 added to the soil , but the relationship was not linear at all levels of K+ concentration . Maximum K+ was fixed with addition of little amount of KC1 . The results indicated that with the passage of time, there was a decrease in the amount of K+ fixed , that is , maximum amount of K+ was fixed during the early days of saturation. The results also indicated that as soil temperature increased , more K was fixed in soil . The effect of various anions on K+ fixation was also studied and it was concluded that anions had no effect on K+ fixation in soils