A long-term field experiment was conducted near Toulouse (south-west France) to verify to what extent increasing fertilizer dressings would increase yields on calcareous soils in a cereals-oilseeds-legumes rotation. Over a 2%year period, withholding K significantly depressed yield once; K inputs affected the total amount of K absorbed at flowering, without subsequent effects on yields. Exchangeable K content on non-fertilized plots declined slightly over the experimental period, but without reaching levels expected from nutrient balance estimates. Annual applications of K equal to average offtake in grain were adequate to maintain available soil K content at its initial value. Potassium release kinetics for the exchangeable fraction, evaluated from Ca-resin extraction, clearly discriminated between fertilized and non-fertilized treatments. Both soils exhibited similar patterns for diffusion-controlled K release. These results highlight the role of K reserves readily available in illite-rich calcareous soils and demonstrate the importance of cultural residues in K fertility management.