Plasmic fabrics of two sudan vertisols

Abstract

From several vertisol profiles of the Sudan which were described in the field, plasma separations have been studied in thin sections. Two profiles were selected for the present study, one with an annual rainfall of 400 mm and one with 800 mm precipitation. The former soil has no gilgai and a churning zone of 40 cm thickness, the latter has a pronounced gilgai microrelief and a churning zone of 120 cm. Most other properties of these soils are the same, except the grain size distribution of the non-clay fractions. A quantitative micromorphological study on thin sections, using point counting techniques, was performed to investigate the plasma separations. Field observations showed that slickensided ped surfaces are increasing in size and distinctness with depth. This structural development reflects the intensity of the churning process, an observation which is supported micromorphologically. Most of the planar voids in both soils are surrounded by a vosepic plasmic fabric. In the present study it was not found that the frequency of the vosepic plasmic fabric increases with depth. The width of the brims of planar vosepic plasmic fabric was found to be larger in the higher rainfall profile than in the lower rainfall profile. The fraction of masepic plasmic fabric of the total plasma is larger in the higher- than in the lower-rainfall profile. Within the churning zone of both profiles it increases with depth. Most skeleton grains are surrounded by a brim of skelsepic plasmic fabric. The width of this fabric may be influenced by the intensity of the churning process or by the grain size, or by both. The conclusion is drawn that in the profiles studied the width of the planar vosepic plasmic fabric and the fraction masepic fabric of the total plasma can be used as micromorphological parameters of the stress processes active in vertisols.