Soil Development: Numerical Indices

Abstract

Numerical indices of soil development provide highly useful tools for comparing soil profiles or soil horizons in terms of the degree of pedogenesis that has taken place in them. In this way, numerical indices may also help to establish chronologies of the age of different land surfaces. Common indices used in soil sciences, and the pedogenic processes associated with them, comprise the theme of this entry. Indices based on field data include the redness rating and the profile development index, or PDI. Clay formation can be evaluated by the clay accumulation index (CAI), and pedogenic iron oxides by ratios of iron fractions. Various ratios of mobile versus immobile elements are used as indices that reflect silicate weathering and leaching of released mobile elements. Indices of profile anisotropy are based on the concept that pedogenesis increases anisotropy in soils over time. Some indices reflecting specific processes (carbonate accumulation and podzolization) are also described. Appropriate use of any soil development index requires that the soil-forming processes which the index was designed to reflect match with the processes that take place in the soils under investigation. Hence, knowledge on both the concepts of the indices and the processes taking place in the soils is needed, including variations in soil-forming processes in both space and time (especially over time spans that include major climatic shifts).