Factors affecting the structural stability of three contrasting soils of China

The structural stability of eight samples representing three soil profiles from tropical and subtropical regions of China (Latosol, Red Earth and Yell ow Brown Earth) was studied by dispersion treatments. The samples were trea ted with the following solutions in order without previous mechanical disru ption: (I) H2O, (II) 0.1 N NaCl, (III) 0.002% Na2CO3, (IV) 0.1 N NaOH, (V) acid oxalate, (VI) 0.1N NaOH. These procedures were designed to disperse so il samples by removal of potentially aggregating substances and by anion ad sorption. After each treatment the clay dispersed was separated by sediment ation and its mineralogical composition was studied by XRD, Mossbauer spect roscopy and magnetic susceptibility measurements to assess the role of mine ralogy in the maintenance of soil structure. The amounts of iron extracted by Na-dithionite-citrate-bicarbonate were 9.6-10.8% in the Latosol, 3.1-3.4 % in the Red Earth and 0.9-2.1% in the Yellow Brown Earth. It was concentra ted in clay fractions and existed mainly as superdispersed particles. The h ematite/goethite ratio varied from 1.7 for the Latosol to 0.2 for the Yello w Brown Earth. The clay fraction of the Yellow Brown Earth is vermiculitic and has a permanent negative charge. This soil's structural stability is ma inly influenced by exchangeable Ca2+. In the Latosol and the Red Earth, sur faces with variable (pH-dependent) charges prevail because of the large fre e Fe-oxides contents. Alkaline treatment (TV) promoted most dispersion of t hese soils, indicating the mainly electrostatic nature of interactions betw een mineral particles. Thus, the role of the Fe oxide minerals in these soi ls is one of aggregation rather than cementation. The aggregation propertie s of the Latosol and Red Earth are relict paleosol features inherited from an earlier period of cooler and wetter climate than the present. (C) 1999 E lsevier Science B.V. All rights reserved.