The World Reference Base for Soil Resources and the FAO Guidelines for Soil Description are tools to identify, describe and classify a soil by diagnostic horizons, diagnostic properties, diagnostic materials, and other soil features. While bedrock and residual fragments can be defined by their lithology and abundance, the parent material of soil formation is not clearly classified. Therefore a substrate classification was implicated in descriptions of layered soils, because these consist often of allochthonous, polygenetic and heterogeneous substrates. Results of soil investigations in Yemen developed into a substrate classification key for parent materials that focuses on parameters of substrate variety. Primarily, the present approach was developed to describe layered soils on the Island of Socotra, but it is assumed to be valid for any semi-arid region in the Tropics and Subtropics. To assess the operational reliability of the system, it was applied to classify parent materials of soils from Yemen mainland and Jordan. The classification key is partly based on the FAO Guidelines, and partly on the German Soil Mapping Guidelines. The substrate variety includes lithology, weathering intensity, stratigraphical unit and substrate form. Different substrate varieties within one profile are called substrate orders and closely relate to sedimentary bedding. The proposed classification key is based additionally on parameters such as geogenesis and relief position. It can be used separately from soil description. However, soil identification and description become more precise by using this classification system. This contribution should be understood as a first approach to an international substrate classification.
Semiarid tropical regions are very suitable to study the polygenesis of layered substrates and soils. To determine relic and recent soil features of tropical soils, pedons in the northern part of Socotra Island, Yemen, were investigated. The aim of the study was to create developmental stages of widely distributed soils, which in particular are layered Cambisols and Calcisols. In most layers of those investigated soils, denudative components occur, which clearly indicate the relocation of coverloams and coverclays with an incorporation of extraneous materials, such as small pieces of relic clayey loams with different shapes. These fragments indicate strong chemical weathering, preceding erosion, and redistribution and/or embedding into younger sediments. Calcium carbonate found at a depth of up to 30 cm in 52% of the layered soils is very typical of an increasing aridity in parts of the semiarid tropics, particularly in areas with high groundwater levels and interflow. In addition, oxidation features and clay illuviation in noncalcareous substrates were detected. At the end, two major soil developmental stages of layered Cambisols and Calcisols were defined and demonstrated at two selected profiles. We assume that patterns of soil polygenesis depend not only on the composition of the respective substrates but also on changing climate conditions. Geochemical and soil physical methods as well as micromorphological analysis were used to describe and classify the soils.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.