AbstractThe equilibrium activity ratio (ARo) of soil potassium and the K extracted by electroultrafiltration (EUF) within a desorption time of 10 min (EUF10) were compared as measures of K availability indicated by K uptake and dry matter yield of barley plants grown in four soils receiving different rates of K fertilization in a glasshouse experiment. As could be expected, both methods indicated that K intensity in soils increased with higher amounts of K dressings and decreased with progressive crop growth. With EUF10, 56–77% of the variation in plant response in dry matter yield and K uptake could be explained, where as with ARo, it was only 36–65%. There was good linear correlation between ARo, and EUF10for a particular soil; but there was differential variation in the two parameters for the different soils. When compared on the basis of plant response, EUF10was found to better describe K availability to plants and differentiate better between soils of different K selectiv