Characterization of the Grey Layer in the Gezira Soil - Sudan

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Elamin, E.A.
Saad, S.A.
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Faculty of Agriculture, University of Khartoum, Shambat, Sudan
The Gezira soil is an extensive unconsolidated clay plain (part of the central clay plain of Sudan) characterized by a dark coloured sub-surface layer called melanic or grey horizon. It is a genetic horizon in a soil profile continuum that belongs to the Order Vertisols. Disputes were raised about its formation and chemical and physical properties, hence the economic importance. This study aimed to characterize the grey layer (melanic horizon) in the Gezira soil. Therefore, five profiles were selected in El-Maseed, El-Laota, El-Turabi and Wad Medani; the selection criteria were the grey layer thickness and depth. The profiles were described in the field and three samples were collected from the grey layer and the layers above and below the grey layer. The samples were analyzed for physical, chemical and mineralogical properties. The textural class was clay; the bulk density increased with increasing depth, and the matrix was alkaline in reaction. The cation exchange capacity and organic matter were high in the grey layer compared to the other sampled layers. The nitrogen content was invariably low in all examined sites. Trace elements analysis revealed the prevalence of iron over zinc, copper and manganese. The mineralogical analysis, using X-ray diffraction, showed similar mineralogical assemblages, with smectite (montmorillonite and beidellite) being the dominant clay mineral. Different combinations of zircon, tourmaline, rautile, staurolite, kynite, epidote, hornblende and garnet formed the heavy minerals of the sand fraction. Highly stable heavy minerals of zircon and tourmaline constituted more than 50% of the combinations, suggesting different environments of deposition and/or perhaps negligible pedological processes and alterations. The uniform physiography and similar mineralogy in the studied pedons as well as in the sediments of the Blue Nile and its tributaries suggest common origin of parent material, being derived from the Ethiopian Highlands.
Page(s): 8 (2), 51-68,24 Ref