In Volume 136, 407–426, you published a paper entitled ‘Responses of potato (Solanum tuberosum) to potassium fertilizers’ by M. F. Allison, J. H. Fowler and E. J. Allen. We consider that the authors' conclusions were not justified and they have mis-interpreted and criticized earlier published work. One of their criticisms is levelled at the use of exchangeable potassium (Kex – rapidly plant-available K) to categorize soils on the basis of the likely response of crops to an application of potassium (K) fertilizer. While Kex is not perfect, it is the best rapid method currently available (Johnston & Goulding 1990). To support their contention about Kex, the authors quote in Table 15 data from Rothamsted experiments done in the late 1950s and early 1960s. They state that despite the large amounts of K applied in fertilizers and manures the effect on Kex was relatively small. This comment fails to recognize that it is the K balance (K applied minus K removed) that must be related to Kex, not the K applied, and the inevitable transfer of part of the K balance from Kex to fixed K (less readily available K). Warren & Johnston (1962) showed that on these soil types as much as 60% of the K balance is rapidly transferred to fixed K.