AbstractInterest in potassium (K) manuring has decreased in recent years because applying K leads to no environmental problems, few soils are K deficient and K is cheaper than N. However, fresh K optimises yields, especiatly with high‐yielding crops and through its interaction with N, and K residues benefit crops in a way that often cannot be offset by fresh K. To achieve a rational basis for K manuring, the various sources of K and demands for K by the crop must be considered and used, together with soil and crop analyses, to predict K fertiliser requirements. K should be applied annually, perhaps as an NK compound, on light‐textured soils. On heavier soils it can be applied at any convenient point in a rotation, probably just before the most K‐sensitive crop is grown. Applications should match or slightly exceed crop requirements. There is little to gain from omitting K and every reason, both economic and agronomic, to include it. Such a rational policy will provide due reward in current crop yields and quality and in future soil productivity.