Accessibility of subsoil potassium to wheat grown on duplex soils in the south-west of Western Australia

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.