SUMMARYThe recuperation of the soil during bush fallows after a period of cultivation is an important part of the success of shifting cultivation in the lowland humid tropics. However, the sources of the nutrients which enable the observed rapid growth on apparently exhausted soils has never been satisfactorily explained. Some soils from the Manaus region of Brazil have been examined in an attempt to identify reserves of potassium. X‐ray diffraction, physical and chemical analyses, and microcalorimetric measurements of K‐Ca exchange show that small reserves of K occur in 2:1 clay minerals which exist in trace amounts in the soils. The results suggest that the clay fraction of the soil may play a more important role in nutrient supply than previously realized. The reserves are considered in terms of the land use of the a