Severe K nutritional problems in maize (Zea maysL.) and soyabean (Glycine max(L) Merr.) are common on the strongly K-fixing gleysols of east Croatia, Yugoslavia. There is, however, little information currently available regarding the K fertilizer requirements of these soils. The work reported here was conducted to obtain this information. Maize and soyabean were grown for three seasons in experiments incorporating seven initial K application rates (125, 275, 460, 650, 835, 1 580 and 2 220 kg ha−1). In the second and third seasons equal rates of K (125 kg ha−1) were applied to all plots. Both maize and soyabean responded dramatically. In the first season yield responses were curvilinear in nature, but by the third season responses were linear and K nutrition was clearly inadequate, even at the highest rate of K application. Soil and plant analytical data generally mirrored these effects and it is apparent that severe fixation of applied K resulted in a massive K requirement. Further work is needed to establish whether band placement of applied K will help to alleviate K-fixation problems more economically.