Formation of an Oxisol‐Ultisol Transition in São Paulo, Brazil: I. Double‐Water Flow Model of Soil Development

This study of Oxisol‐Ultisol transitions on hill slopes in transported material with oxic properties allowed the development of a double‐water flow model explaining the formation of soil structure that occurs during the soil development. According to this view, Ultisols form from Oxic materials through the action of both lateral‐ and base‐water flow, integrated in the landscape. In the sites studied both argillic and oxic horizons are formed from the same parent material. Oxisols are formed in thick colluvium on nearly level surfaces where the lateral‐water flow is not developed and the base‐water flow is too deep to affect the solum. The argillic horizon of the Ultisols on the slopes below the Oxisols is hypothesized to form inside a layer having blocky structure initiated by saturation ⇆ desiccation‐induced compression. The formation of the compressed layer is controlled by the geomorphology, which promotes the formation of the lateral‐water flow as the relief becomes steeper, as well as by the degree of vertical textural anisotropy.