Ten surface soil samples representing Red Mediterannean soils collected from different parts of Northern Greece,were investigated for their K availability and fixation characteristics. The samples were used for growing ryegrass, for a period of 335 days. Potassium was extracted using different extractants and the K-fixation characteristics of the soils were determined by varying the amount of potassium applications. The soils contained 0.9% to 2.0% total K of which 23.6% was HCl soluble, and 4.3% was N HNO₃ soluble (non-excangeable)and 1.6% exchangeable K. The non-exchangeable K utilized by the plants ranged from 54.8 to 91.5% of the total K under exhaustive cropping, and the rate of non-exchangeable K release in soils varied from 64 to 757 kg/ha/yr, with a mean value of 306 kg/ha/yr. The amounts of K fixed by the soils ranged from 13 to 56%(39.8 to 68.1 kg/100g soil)at the 300 mg/100 soil rate of K application. The K-fixation correlated significantly with the clay content(r=0.88***) and the K-buffering capacity of soils (r=0.90***). The available K reserves of soils ranged from 603 to 3086 kg/ha, and correlated significantly with the total K uptake by the crop (r=0.90***). The best correlations were obtained between total K uptake by ryegrass and resin-K (r=0.95***), NH₄NO₃ (r=0.93***), and NH₄OAc-K (r=.88***). Based on the size of the slope of electroultrafiltration-K (EUF-K) which displayed marked differences in the magnitude of K release by the soils employed, the soils could be grouped into three categories i.e.: soils with gentle, steep, and straight slopes of EUF-K curves, respectively.