On the definition and identification of aquic soil moisture regimes

Abstract

The definition of an aquic soil moisture regime given in Soil Taxonomy is difficult to understand and, in places, ambiguous. The essence of the definition as written, however, is the requirement for reducing conditions. A soil which is saturated with groundwater or with capillary fringe water may, or may not, have an aquic moisture regime. Reducing conditions are defined in Soil Taxonomy in relation to the virtual absence of dissolved oxygen, though morphological criteria for the differentiation of soil classes are based on the reduction and segregation of iron. The absence of requirements relating to duration of reducing conditions, or to the thickness of reduced soil, suggests that the definition was intended to apply to soil material rather than to a pedon. The aquic moisture regime has a particular depth connotation only when it is applied to the differentiation of soil classes. A simple field test, based on the complexing agent �,�-dipyridyl, is useful for the identification of ferrous ions in soil solutions and hence is potentially useful in the recognition of aquic moisture regimes. The sensitivity of this test limits its usefulness to those soils which contain more than about 0.5% free iron (i.e. iron which can be solubilised as iron(II) under reducing conditions as measured by extraction with dithionite reagent).