A study has been made of the features associated with repeating bands of vegetation in the Butana region of the Sudan. Aerial observations show that they have similar features to those described by MacFadyen in British Somaliland except that the curved bands lie on the interfluves and are convex downslope.The patterns consist entirely of grasses and herbs, and lie on very gentle slopes of about I in zoo; there is no measurable difference in elevation between the grasses and bare areas. Slopes of less than I in 500 have no patterns. The width of the bands is variable, but commonly between 8 and 12 m., and the bare ground about twice the width of the grass.The profile consists of a grey-brown clay loam, with an A horizon slightly lighter in colour and texture. No difference in profile, composition, or properties between the soil under grass and under bare ground has been detected, except in pH, nitrate, and moisture. There are differences in soil surface, the bare areas being smooth and the grassed areas broken by cracks and potholes which form at the beginning of the rains and concentrate just upslope of the previous season's grass. Experiments to simulate on the bare ground the moisture and growth conditions obtaining under the grass bands were partially successful.The current season's grass concentrates at, and in advance of, the previous season's front, and there is an annual migration of the grass bands upslope.The vegetation consists dominantly of the grasses Aristida spp., Schoenfeldtia gracilis, Cymbopogon nervatus, and Sehima ischaemoides. Species of grass may have a restricted position in the grass band, the most striking being Schoenfeldtia. Herbs are more ephemeral, but are prominent during the rains in the old grass areas.The origin of the patterns appears to be related to water supply, topography, and soil, but not to wind. Comparison with gilgai is made, but reasons are given for considering that the patterns have a different mode of origin, and an initiation due to a type of soil slip or to a rhythmic deposition of vegetation is considered possible.
Summary Bands of acacia trees rhythmically disposed occur in the Sudan in a form resembling grass patterns. They have been found only on the red sand belt of Western Sudan, and aerial and ground observations and measurements seem to indicate that they all lie on gentle slopes very roughly on the contour, that they are as much as zoo m. wide with similar or wider spaces between them usually occupied by ground patchily covered by grass. Though sometimes tree bands may have a man‐made origin, the cause is generally natural and is probably the same as that producing grass patterns.
Summary The topographic, climate, geological, and vegetational setting of khartoum is briefly described. The field characterstics of the mail soil group, the High Level Dark Clays, are given, and the secondary minerals, the curved structures, and the interpenetrating horizons are described in some detail and their origins discussed. It is suggested that some features may be fossil. Laboratory analyses show that the distribution of moisture, soluble salts, and sodium in the profile is uniform, with maxima in the second or third foot. Values in the top 6 in. are always low. The salts consist mainly of sulphate and chloride in the subsoil and of bicarbonate near the surface.The soil is strongly alkaline and consistently verylow in organic nitrogen in all horizons. The coarsest fractions of the soil consist chiefly of secondary minerals with some quartz, but as the fractions become finer, quartz grains become increasingly abundant. The chemical and minerals composition of the calcareous concretions is given, and they are shown to have formed by replacement. The habit and crystallography of the fibrous and lens gypsum is described and its mode of formation discussed. Microscopic examination of the minerals of the fine fraction showed that, as the content of the light minerals consisted almost entirely of quartz and that the percentage of heavy minerals in the total fine sand averages only 4.3 percent., considerable weathring of the primary minerals must have occurred. From the species and proportions of the heavy minerals present, it is suggested that much of the material may have come from Sudan sources, and that colluviation may have played a large part in its formation. The classification of the soil is discussed, and the name ‘Solonized Sub‐tropical Black Earth’ proposed.
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
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.