Soil Structure in Vertisols of the Blue Nile Clay Plains, Sudan

Summary A survey of 1·5 million hectares of the Blue Nile clay plains has shown the majority of soils to belong to the order of Vertisols. The morphological characteristics of these soils are described and compared with similar soils in other parts of the world. Soil cracking, structure, surface mulching, and micromorphology are discussed in detail. The use of crack width is considered to be unsatisfactory as one of the criteria for differentiating Vertisols and Entisols. Cracking may prove to be a useful criterion for land suitability classification. Three different types of surface mulch have been distinguished. A separation between soils possessing surface mulches and hard crusts is recommended at the family level. The unequal stresses and strains which result from the swelling and shrinking processes are responsible for the aggregation and structure formation within these soils. Platy layers were found to restrict water penetration and root development. The term‘bicuneate’ is proposed for the description of the wedge‐like structure of the soils. The micromorphology appeared to be strongly affected by pressure: numerous slickenside faces were recognized.