A Study on Properties and Genesis of Cracking Clay .Soils in Southern Kordofan

Abstract

This study was conducted in a region of South Kordofan state located between latitudes 11o56' and 12o09' N and longitude 29o35' and 32o40' E. The topography of this regions includes mountains traversed by clay plains extending and sloping from west to east, dominated by dark cracking clay soils classified in this and previous studies as Vertisols. The cracking clay soils of South Kordofan are part of the Central Clay Plain of the Sudan that traverses the country from east to west. Southwards it merges into the southern clay plain, while northwards it gives way to the red non – cracking clays, termed "Gardud". After that we encounter the sandy plains in the "Goz country" towards Northern Kordofan. 1 Previous studies of South Kordofan clays were made to characterize the soils and propose selected sites suitable for mechanized agriculture. High and reliable rainfall in this region renders it suitable for extensive mechanized crop production in the deep clay soils. As these previous studies were for a specific purpose they did not undertake deep characterization of the physical, chemical and mineralogical features of the clay soils so as to determine the conditions under which they were formed. The previous studies did not particularly emphasize the investigation of the clay mineral types and the environment in which they were formed. The present study is primarily concerned with the joint factors and the environmental conditions under which these soils were formed, and the relationship between the soil properties and the topographical sites in which they exist. With this understanding, an approximately 300 kilometers – long transect was selected. It follows a toposequence extending from Habila, representing the upper intermountain clay plain, through the Abbasiyia – AL- Terter intermediate clay plain, down to Megeinis lower clay plain and ending at the White Nile upper terraces. Seventeen (17) representative sites were chosen along this transect, where soil profiles were dug, described and sampled. The samples were analyzed to determine the physical, chemical and mineralogical soil properties. Routine laboratory analyses were used to determine the physical and chemical properties. X – Ray diffraction technique was run on soil bulk samples and the < 2μ clay fraction for the qualitative and quantitative characterization of the clay minerals. The morphological field studies revealed deep clay soils with wide cracks in the dry state (1-7 cm wide and 50-100 cm deep), with a surface mulch and weak blocky structure in the topsoil. Moderate drainage and slow permeability in the wet state. Slickensides and wedge-shaped structures exist in the subsoil, with massive structure below that. Whitish irregular calcium carbonate concretions are commonly scattered throughout. The soil color is a homogenous very dark grayish brown (10YR 3/2) in the moist surface and subsurface. Particle size distribution indicated clay content ranging between 33.2 – 71.35%, silt 8.2 – 34.72% and sand 2.73 – 32.08% in the different sites. Bulk density varies between 1.27 – 1.76 g\cc increasing with depth. The soil reaction is neutral to moderately alkaline pH 6.28 - 8.69, the soils are non - saline (ECe < 1 dS/m), the exchangeable sodium percentage (ESP) ranges between 0.56– 36.07, higher in the upper intermountain clay plain (Habila area). The soils are slightly calcareous with CaCO3 ranging between 3.2– 9.9 %. Organic matter is low (2.4 - 0.363 %), decreasing with depth and so is the nitrogen content (0.08 – 0.02%). X-ray differaction of the clay fraction in all four topo - sequence sites shows the dominance of smectite clay minerals with different amounts, present as high as 84.6 - 85.11%. The smectite content at the Habila upper intermountain area ranges between 49.5 - 84.6 %. This is higher than in the intermediate clay plain (Abbasiyia - AL-Terter areas) 36.1 - 71.3% and the lower clay plain (Megeinis) 37.01 – 41%.The White Nile upper terraces contains high amounts of smectite (85.11%). Kaolinite clay mineral was found in all the investigated clay samples ranging between 8.2 – 46.45 %. The lower amount of kaolinite occurred at Habila, higher at Abbasiyia - AL-Terter and Megeinis. The Investigated samples also contain low amounts of chlorite, illite and interstratified minerals including layers characteristic of smectite/chlorite. It is now evident from the results of this study that further research of clay soils in other parts of South Kordofan is needed to identify in detail the soil properties. This would render information towards suitable practices for the management of these soils under mechanized Agriculture.