Distribution and genesis of red and yellow soils in the central subtropics of southeast China

Abstract

In the subtropical climatic region of southeast China, which is influenced by the monsoon climate, red soils are widespread. According to the Chinese soil classification system the main soil groups are subdivided on the base of the degree of allitic weathering. This is mainly characterized by the SiO 2:Al 2O 3 molar ratio of the clay fraction (according to the Chinese classification the fraction <1μm). The weathering stage and soil properties are determined by the horizontal and vertical climatic zonation. In basin regions and lower altitudes of the mountains up to about 600 m above sea level Red Earths (central subtropics) and Lateritic Red Earths (south subtropics) are developed on silicate rocks. In higher altitudes these soils change to Yellow Earths with goethitic B-horizons. In order to obtain information of the main soil forming processes of typical and widely distributed soils in the central subtropical region, a Red Earth from Middle Pleistocene red sediments of a basin region (Quaternary Red Clay) and two profiles of a Red Earth - Yellow Earth toposequence from granite of the neighbouring mountainous region were investigated by morphological, mineralogical and pedochemical methods. During the youngest phase of morphodynamic stability the recent soils developed on pre-weathered parent materials (basin sediments and saprolite) containing kaolinite and gibbsite formed in ancient weathering periods. In the course of soil formation weathering proceeded with the transformation of halloysite to kaolinite and mica minrals through the stages illite and vermiculite to Al-chlorite. Pedochemical characteristics are dominated by an almost complete leaching of bases and a high Al-saturation of the sorption complex. The type and intensity of soil characteristics show a marked relation to the altitude of the sites, depending on differences in humidity and intensity of leaching processes. With increasing altitude the accumulation of organic substances and the neoformation of goethite and Al-chlorite increase, while with decreasing altitude the neoformation of hematite and intensity of lessivation increases. Bt horizons, which occur only in the Red Earths show that lessivation was an important process during soil genesis, although these soils developed from pre-weathered, acid parent materials. In Yellow Earths from different parent materials no indications to clay migration were found. According to the FAO legend the Red Earths in the area investigated belong to the soil unit of Ferric Acrisols and the Yellow Earths to the soil unit of Dystric Cambisols. It appears that the soil colour and the SiO 2:Al 2O 3 molar ratio of the clay fraction used as criteria for soil systematics cannot sufficiently indicate the degree of weathering, soil genesis and site properties.