| Water is a major driving force for several processes in soil formation. To gain insight on the interactions between soil water dynamics and soil formation, ten pedons of a toposequence in São Paulo, Brazil, bearing different soil moistures regimes were evaluated for parent material uniformity and morphological, chemical and mineralogical properties reflecting the action of soil water. The Oxisols and Ultisols of the toposequence, formed on siliceous, reworked parent material have desilication, plinthitization and gleization as the main pedogenic processes. Two different base levels control the hydrology. In the lower toposequence segment, the hydrology is controlled by landscape position and flood plain groundwater. Due to longer periods of water saturation it produces less Fe segregation and plinthite, and more depletion of Fe from free oxides than the upper segment of the toposequence, where hydrology is controlled by landscape shape and position, and layers of lower permeability. In the upper toposequence segment, more frequent oscillation of the water table and, possibly, higher levels of dissolved O2 and Fe 2+ in the water produce Fe segregation in different stages, culminating in formation of plinthite in one pedon. Free Fe and free Mn contents enabled an estimation of redox potential differences among landscape positions. Less crystallinity of the Fe oxides seems to indicate aquic conditions. Incorporation of Fe into the structure of aluminosilicates, probably as Fe-hydroxy interlayers in HIM, was also a result of aquic conditions. Proportions of gibbsite to hydroxy-interlayered minerals (HIM) are also sensitive to variations in soil moisture regimes, with HIM tending to predominate in the wetter soils. The Fe oxide mineralogy was not useful for inferring the pedoenvironment because goethite occurred in low amounts and was the only Fe oxide mineral identified in the studied soils. |
