The B horizon of duplex soils in the wheatbelt of Western Australia is sometimes enriched with potassium (K). K supply from this source is, however, not taken into account in fertiliser recommendations based on soil tests on samples from the 0-10 cm layer. Grain yield responses of wheat to K were measured over 4 years on 10 duplex soils in the medium rainfall (400-600 mm) regions of the West Australian wheatbelt. The depth at which the texture changed markedly ranged from 25 to 75 cm. K was measured through the profile and the concentration was reduced to a single value with a weighting equation that takes account of the distribution of root length. The weights chosen by the fitting procedure discounted the K content of the subsoil severely and provided no evidence that subsoil K was important in determining yield. Furthermore, the improvement obtained by using the weighted K concentration in the 0-1 m layer compared with using the topsoil (0-10 cm) analysis was not statistically significant. K deficiency can be diagnosed in those soils using analysis of the 0-10 cm samples only.