Features of Some Semi‐arid Soils in the District of Khartoum, Sudan

Abstract

Two soil groups in the Khartoum district are described. The Red Sand-Ironstone Soils are characterized by a reddish sandy upper horizon, a second ironstone gravel horizon, and a third stony horizon, often consisting of kankar. They are dry soils, and contain very little soluble salts or plant nutrients, and the minerals are largely quartz and iron ores. The few heavy minerals show resemblance to those of the High Level Dark Clays in particular in their amounts of hornblende and epidote.The resemblance of the red sand to the 'God and the ironstone gravel to murram of laterite areas is noted and discussed, and evidence is produced for attributing the origin of both to the weathering of the rocks of the Nubian Series, and the soils are regarded as intrazonal.The Riverain Soils of the Blue Nile and Main Nile have very variable textures, but the White Nile Clays are remarkably uniform in texture and profile. The differences can be attributed to the difference in the regime of the rivers. Salinity is also variable, but usually much lower than in the High Level Dark Clays, and the more saline soils are heavy-textured and lying in slight depressions.The mineral content is similar in all the Riverain Soils except for the White Nile Clays which show some differences, and also similar to that in the High Level Dark Clays. The sources of the minerals are discussed, and a suggested mode of formation of the flood plain, by which islands link with the mainland, is proposed.IN a previous paper the main group of soils in the Khartoum district, the High Level Dark Clays, was described. Two other grou s, the Red Sand-Ironstone Soils and the Riverain Soils, occupying t R e west of the district and the flood plains of the rivers respectively, remain to be considered.The Red Sand-Ironstone Soils West of the Main Nile and the White Nile and lying on rocks of the Nubian Series are red sandy soils containing ironstone gravel (Fig. The land is nearly flat, but is dissected by wadis which deepen the river and often expose the underlying rock.These soils have little in common with the High Level Dark Clays reviously described. They consist fair1 regularly of a reddish-brown dark brown ironstones, followed by a stony layer which may be either buff-coloured kankar or rock rubble from the Nubian Series.A typical profile is as follows: Horzzon I : 20 in. red-brown loamy sand with scattered pea-iron gravel ; structureless; sharp to H. 2.Horizon 2: 30 in. dark brown pea-iron gravel, fairly uniform size, averaging 5 mm., loosely bound by calcareous fine earth and containing scattered pieces of ironstone and sandstone; fairly sharp to H. 3.Lamy sand top horizon, followed sharp r y by a layer of small rounded

Gypsum crystals in soils

“…These faces are often somewhat curved, giving a section resembling a biconvex lens. The curvature varies, so that the crystals range from an elongated diamond like a compass needle to a banana shape to almost round (Worrall, 1957). Inspection of Fig.…”