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The Clay Plains
In addition to the well-known geomorphological advantages for irrigated agriculture on the clay plain of low angle, proximity to the White Nile, low transmission losses, and reasonably fertile soils, some characteristics of this landscape type do present problems for further agricultural development. The low infiltration capacity of the clays means that much heavy rainfall flows overland and collects in clay swales between dunes or in depressions on the clay plain itself. As a result runoff areas tend to have low salinity levels, whereas, conversely, run-on areas become more saline. The salinity levels of these latter areas are further exacerbated by the legacy of the late Quaternary history: they were formerly swampy depressions or maiya'a that became cut off from the White Nile as its discharge levels decreased during the Holocene; this in turn led to salt concentration (Williams et al., 1982). Thus surface depressions on the clay plain could all be expected to have higher salt concentrations than elsewhere.
Also, in the light of work by Williams (1968a) and Williams and Adamson (1974), it seems likely that the declining levels of the White Nile during the early Holocene led to concentrations of salts in the "littoral" zones. This could pose further problems for development, not only on the clay plain but also much farther to the west. Groundwater seepage from the White Nile passing westwards through such areas would seem unwise since the nature of the groundwater would aggravate the salinity problem. It must be stressed that salinity is not only a function of present day rainfall distribution but is ultimately bound up with the changing history of the White Nile. The dominant controls on salinity are soil texture, permeability and topography, all of which are inextricably linked to the depositional history of the river. Another related legacy of the late Quaternary history is the presence locally of underlying permeable beds of silts and sands. Unlined irrigation canals cutting across such areas could suffer considerable water losses through seepage.
Salinity poses problems here on the run-on areas of clay swales between dunes. The sands themselves naturally have high infiltration capacities and by comparison with other sediment types have poor water-retentive properties. There are, however, small but important differences between the pale and the red sands: the latter, owing to their haematitic coating, are slightly more water-retentive (Vail, 1982).
There is a considerable problem to agriculture on the qoz areas from sand movement. This is not so much a problem of dune encroachment, for their margins at least appear to be fixed, and only a few small, active barchan dunes of fine sand were observed in the area. Rather it is a problem of windblown sand moving across areas with perhaps a quite thin sand cover and accumulating around individual plants so that crops tend to become choked; at best their growth is impaired.
In such an arid marginal region it is clear the sands offer little potential, at least for annual crops. Any injudicious use of such lands for cultivation or heavy grazing would have serious repercussions, increasing the existing problems created by even small quantities of moving sand and by creating conditions conducive to the development of more active dunes to threaten irrigation schemes on the clay plains.
Hills and Pediments
Of the three landscape elements, these areas would seem to offer the least attraction for agricultural development. The underlying Basement Complex, which constitutes most of the rocks of this landscape type, yields comparatively thin, poor regoliths and groundwater supplies are likely to be confined to khor beds. The Nubian Sandstone Series, which underlies the pediments in the north, and the Umm Ruwaba Series of the central and southern parts of the research area both provide greater potential for groundwater development, though the characteristics of these deposits in some parts may render the water too saline for human or even animal consumption.
Clearly further work on the late Quaternary history of the area would be not only of academic interest but also valuable in assessing development potential. Excellent detailed work by M. A. J. Williams, D. A. Adamson, and associates on the nature of the sediments of the Gezira Plain and on the bank of the White Nile has made possible the reconstruction of the late Quaternary history of the area and demonstrated some important implications to future agricultural development, specifically with regard to salinity levels. A similar approach in the research area would have comparable results.
Detailed examination of the hydrology of the qoz area to the west would also have similar benefits in assessing development potential.
With further work in this area it would be possible to produce a useful geomorphologically based land-use capability map. A tentative one, based largely on soil character, is attempted for the research area north of 14 30'N, and extending further northwards as far as 15 371/2'N, in the next chapter. It is believed that the geomorphological characteristics discussed in this chapter are also applicable to this area to the north and that the comments upon soils in the next chapter relating to the more northerly area are also valid for the area discussed in the present chapter.
The financial support of the Inter-University Council (now Higher Education Division of the British Council) is gratefully acknowledged. Support from my own colleagues at the University College of Swansea and from the Department of Geography of the University of Khartoum was invaluable. The assistance of the Sudan Survey Department in agreeing to the release of the aerial photographs is acknowledged. I should like to thank Dr. D. S. Brown, who kindly identified the molluscs. Finally, I thank all the people I met in the research area for their unfailing generosity and hospitality.
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Vail, J. R.1978. Outline of the Geology and Mineral Deposits of the Democratic Republic of the Sudan and Adjacent Areas. Overseas Geology and Mineral Resources, 49. HMSO, London.
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2. Soil resources
A. S. Musa
The aims of pedological study in the Third World have been outlined by Moss (1968). These are, first, to understand the distribution and mode of formation of soils so that existing land use may be improved or new techniques introduced, and, second, to promote a greater knowledge of the land, and particularly of the soils, to enable predictions to be made about the behaviour of the land under proposed land-use changes. The prime aim of this study is to identify those characteristics of the soils of Western White Nile which would encourage or inhibit the introduction of rural change, bearing in mind the narrow environmental constraints within which the cultivator has to operate in such arid marginal lands.
The study area extended northwards from 14º30' to 15º371/2'N and from the White Nile westwards as far as far as 32 ºE (fig. 2.2). Although only part of the project area lies within this area of study the author had the opportunity of touring the project area to the south and believes that his findings are applicable to the whole of that area. The findings are based on visual examination and preliminary analysis in the field, aerial photography and satellite imagery, and laboratory analysis of field samples collected in September and October 1980.
In Sudan, pedological knowledge is very scanty, although the country's economy depends totally on crop and livestock resources. The vastness of the country has always been blamed as the cause of a lack of detailed research in many fields of study. In the present field, the only area which has been studied in relative detail is the Gezira, which accounts for not more than 1-2 per cent of the entire area of the country.
Compared with the Central Clay Plain of the Blue Nile, the alluvial plains of the White Nile, particularly on the west bank, have had little agricultural development until recently, and so very few semi-detailed soil surveys have been carried out; almost all of those that have have been confined to small agricultural schemes. Detailed information on these surveys is given by Ali (1970), Salih (1975), Younis (1977), and El Fadl and El Sayed (n.d.). In practice not all the soil survey reports are available.
The study area lies within the Sahel zone, which stretches from the Atlantic to the Indian Ocean between 11 and 15 N. According to Mensching and Ibrahim (1977) this zone is considered to be most vulnerable to the processes of desertification. At a conference held at Khartoum in September 1980, it was announced that some 700,000 square kilometres of semi-arid Sudan are threatened by desertification. Whether this figure is exaggerated or not, it still gives concern for the future of Sudan's soil resources. The apparently continuing decline in annual rainfall totals since 1969, in an area with a long dry season of eight to ten months, exposes soils to aeolian erosion, with the result that enormous quantities of fertile topsoil are blown away from the Sahel zone every year.
Increased cultivation and overgrazing of the desert margins encourages wind erosion by creating larger areas of minimal vegetation cover. Research in Northern Darfur Province in 1979 has confirmed that millet cultivation has extended desertwards beyond the normal climatic limits and has been the major factor in the process of desertification. Degradation of soil and a decrease in agricultural productivity have gone hand in hand. Though there is evidence to show that similar processes are occurring in the White Nile area, the results obtained in Northern Darfur must be considered of only limited applicability, as soil conditions differ appreciably between the two areas.
Physical Characteristics of the Study Area
Soil characteristics depend very largely upon the parent materials (see also chapter 1), the climate (ch. 3), the availability and movement of soil water, and the vegetation which develops (ch. 4).
The rocks of the study area (fig. 1.1) mainly belong to the Nubian Sandstone Series, which overlies the Pre-Cambrian Basement Complex. The former comprises quartzites, yellowish brown sandstones, mudstones, and basal conglomerates. Its mineralogy is mostly quartz with feldspars and a variety of ferruginous and manganic minerals. This series provides one of the most significant water-bearing aquifers in the Sudan wherever it exists in substantial thickness, and contributes significantly in this area as the parent material to its soils. The Basement Complex comprises an assemblage of igneous and metamorphic rocks outcropping in only a very few localized areas. Basement Complex outcrops are believed by some to be the "parent material" of the sediments that later formed the Nubian Sandstone Formation (Hussein, 1976). Superficial deposits of qoz sands and river and wadi alluvia are the most recent geological units in the area. Stabilized sand dunes such as Qoz Abu Dulu cover the entire western part, extending beyond longitude 32° E.
Six physiographical units of pedological significance may be
- White Nile alluvium of the flood plain.
- Wadi alluvial plains.
- Sand dune areas (the qoz).
- The inland plains (with "red soils").
- Partially weathered outcrops
- Hilly areas and gravel mounds of the Nubian Sandstone. A schematic cross-section from west to east explains the association of these geomorphic units and their contribution to soil development (fig. 2.1). Immature soils prevail on steeper slopes, whereas the lowlands have experienced more advanced development in soil profiles. (Further details of geomorphology are considered in chapter 1.)
The study area's regional position means that the climate is hot and dry for most of the year (see figs. 3.1 and 3.2). The number of months in which precipitation exceeds (potential) evapotranspiration is less than one throughout the whole area (Davies, 1973). Such a climatic regime suggests a transition southwards from desert detritus in the far north through varieties of arid and semi-arid soils with improved maturity of development towards the south. (Climate is discussed in chapter 3.)
Apart from the waters of the White Nile, two other resources exist: surface waters confined mainly to wadi flood plains, and groundwater from the Nubian Sandstone aquifers. The Nubian Sandstone aquifers provide an alter native source to surface water. Today, full use is made of groundwater at the El Rawakeeb and Gadain mixed farming schemes.
The pilot scheme at El Rawakeeb, which started in the early 1970s, continues to be successful. Besides helping the nomads with their livestock, this scheme enables cheap poultry to be reared for sale in the Three Towns. At first water was found to be moderately saline and very hard, but continual pumping has proved the salinity to be only superficial. With careful pumping and proper selection of freshwater aquifers this water resource in the Nubian Series could prove most useful in developing agriculturally areas to the west of the White Nile. Such possibilities, together with the use of Nile waters for irrigation, make a proper assessment of these soils imperative.
The different geomorphic units are characterized by different combinations of grass and tree species; that is, the units are ecological as well as pedological, though the former have less sharp boundaries than the latter. Field observations show clear signs of degradation of the vegetation. Worrall (1960) concluded that vegetation in the semi arid and arid zones of the Sudan is determined mainly by soil moisture conditions. Kassas (1956) demonstrated clearly, in the desert and semi-desert areas to the west of the Nile, the detrimental effect of man's cultivation and the grazing of his animals upon the natural vegetation, especially the grass cover, and hence the detrimental effect upon the soil.
Soil Characteristics General Description of the Soils
The geomorphic units identified from aerial photography and satellite imagery have been considered as basic soil units. Some workers regard them as comparable to soil series and as such they have been widely used in many parts of Africa (Areola, 1977; Moss, 1968: Tomlinson, 1970).
The soils west of the White Nile have developed on flood plains and recent terraces of former courses of the White Nile (Williams and Adamson, 1974). Other soils further west include those formed on stabilized sand dunes and on wadi alluvia. Soils derived from Nubian Sandstone parent material are quite extensive and fall into two units: the "red soils" and the "immature lithomorphic soils." The delineated units are described below.
FIG. 2.1.Schematic cross-section from Umm Usheishat to El Quweiz (lat.14°52'30"N)
Soil Units (fig. 2,2)
White Nile Flood Plain Unit
Soils developed on this landform are of very fine texture with clay-sized material forming the dominant particle size. The matrix is generally non-calcareous, although dark-grey inert calcium carbonate concretions are often encountered throughout the profile. Field investigation reveals that the soils are dark grey to dark brownish-black, slowly permeable, cracking clays. Their high shrink-swell properties suggest a dominant montmorillonite clay mineral. These soils are comparable with the Central Clay Plain soils of the Blue Nile region. Minor differences in physical, mineralogical, and chemical properties exist. No attempt was made to differentiate between the recent and older terraces of the White Nile flood plain in this study. The soils generally belong to the vertisol group (FAO-Unesco, 1 974 ).
Wadi Alluvial Plains
These are soils formed along the wadi flood plains west of the White Nile river. They frequently show many features associated with vertisols, such as shrinking and swelling, formation of deep cracks, slickensides and gilgai features. These phenomena could be attributed to the presence of montmorillonitic clay minerals, but their clay percentages are insufficient for them to qualify as vertisols. They are somewhat stony, but otherwise fertile, being extensively utilized for dry cultivation of sorghum and vegetables in particular These dark brown, silty clay loams could qualify as fluvisols. Wadi Afu, Wadi El Hal and Waarat El Hamdab are examples from the southern part, while from the northern part Khor Barok and Wadi Abu Heleifi are considered classic examples. Substantial variations in texture, nutrient content, and colour exist between these soils in the various parts of the study area.
Sand-dune Areas (Qoz)
These are mainly associated with stabilized sand dunes but where cultivated are frequently exposed to wind erosion. They occur mainly in the western part of the study area and more precisely along the giant dune of Qoz Abu Dulu. Although the variation in climate is not abrupt, noticeable differences in soil and vegetation exist when the northern and southern parts are compared. The soils on this zone are fully utilized for millet and groundnut cultivation despite the scarcity of precipitation (<200 mm per annum). This is because plants are able to take full advantage of the wet season moisture accumulated at depth by the high permeability of these sandy soils. Scarcity of groundwater for domestic animal and crop use makes this zone vulnerable to crop failure during drought periods. Termites have some influence on soil development in the area, most probably in a retrogressive manner. This is because they play a part in completely destroying the vegetation and hence inducing desertification. These soils are classified as regosols.
FIG. 2.2. Soils
The Inland Plains
These "red soils" are the dominant soils north of 14 45'N. They are mostly sandy or silty clay loams similar to the wadi alluvia in clay content but quite distinct as far as colour and other physical and chemical properties are concerned. They probably form a complete catena with the immature and hilly soils mentioned below. The factor of topography plays an important role in the formation of these soils. The agents of erosion, wind and precipitation, have also influenced their development. The underlying strata is usually the Nubian.
General field observations show that the landscape is flat with minor undulations covered with a thin film of mobile sand. A cracking crust, often surficial, frequently occurs where wind erosion is intense and the sand removed. The soils are easily permeable by water, but lower in the profile moderately hard layers of ironstones and gravelly sandstones occur at variable depths. They have been classified as yermosols.
Partially Weathered Outcrops, and Hilly Areas and Gravel Mounds of the Nubian Series
These are extensive in the western part of the northern section, but are of little agricultural consequence. They may be classified as lithosols.
Methods of Investigation
In the field at least one profile was selected to represent each of the delineated units and these 25 profiles were described according to the current procedures used in the Sudan Soil Survey Administration (El Fadl and El Fadl, 1973; Nykango, 1973). Three samples were taken from each profile, air dried and packed in polythene bags. They were analysed for their chemical and physical properties in the pedology laboratory of the Department of Geography, University College of Swansea. The location of each profile is indicated in figures 2.2 and 2.3.
Six soil profiles have been selected for description as representative of the soil units delineated.
White Nile Flood Plain
Site 1: Vertisol
Location: El Alaqa riverine alluvial plain.
Parent material: White Nile alluvium.
Natural vegetation: Nile reeds and a few acacia trees.
Land use: None at time of visit due to high reservoir level; during the dry season gerf (river-bank) cultivation is maintained. Sorghum and vegetables are grown. Cotton can do well here.
FIG. 2.3. Land capability
0-30 cm.10 yr 2/1 (moist), dark brownish-black, cracking loamy clay, heavy, with a few white specks on the surface of the peas; granular, sticky, low permeability, plastic; cracks are 1-4 cm wide and become tapered downwards with depth. Abundant rootless and animal burrows; boundary is not sharp.
30-70 cm.10 yr 2/1 (moist), dark brownish-black, maintains some physical characteristics as above except for the decrease in size of cracks; very few rootless, nonpermeable; slickensides are apparent; rather massive and much less permeable than above.
70 cm +.10 yr 2/1 (moist), dark brownish-black, sticky, plastic; few thin cracks, very few rootless, nonpermeable.
Wadi Alluvial Plains
Site 13: Flavisol
Location: Wadi alluvium of Wadi Abu Hileifi.
Parent material: Wadi alluvium and probably old lake deposit of White Nile origin.
Relief: Gently undulating surface; 0-2 per cent slope towards the White Nile.
Natural vegetation: Very few Acacia orfota (La'ot), short (1m tall) rich grass along the wadi.
Land use: Sorghum cultivation, vegetables, and rough grazing.
0-20 cm.10 yr 3/3 (dry), dark-brown sandy to silty loam; granular structure, coarse sand grains are abundant, sticky but non-plastic, no organic matter, no cracking; some larger pebbles of the reworked Nubian Sandstone are encountered; fairly permeable.
20-35 cm.10 yr 3/3 (moist), dark-brown sandy loam; harder than above layer, moderately sticky and weakly plastic, no organic matter, gravelly; fairly permeable; diffused boundary.
35-75 cm.10 yr 4/3 (moist), dull yellowish-brown, sandy to silty loam; weakly structured, fissile, harder than layer above; white quartz grains are frequent; sticky, rather plastic; fairly permeable.
Sand-dune Areas (Qoz)
Site 9: Regosol
Location: Umm Sidr-northern part of Qoz Abu Dulu.
Topography: Undulating; top of fixed sand dune, sloping east and west, but site area generally flat.
Natural vegetation: A few grasses dominantly (haskanit, 30 cm tall) and a few scattered small acacia trees.
Parent material: Sand deposited by wind action, some locally but mostly from the north.
Climate: Semi-arid, but much drier than the rest of the area under study.
Land use: Millet cultivation is the dominant activity in the surroundings, with rough grazing in spite of the dry climate.
0-10 cm. 5 yr 6/6 (dry), orange, friable sand with no horizonation; lamination due to wind deposition is apparent; no organic matter, no rootless; some dark mineral grains; at the base a concentration of coarser sands, indicating basal deposition on top of a moist surface, is encountered; faint but well-defined boundary; very permeable and very porous.
10-40 cm. 5 yr 6/6 (moist), orange, loamy sand, structureless, non-plastic and non-sticky, easily destroyed; no rootless or organic matter; very permeable.
40-100+ cm. 5 yr 4/6 (moist), reddish-brown, friable loamy sand, structureless, no rootless, non-sticky, non-plastic, very permeable.
Site 4: Regosol
Location: Um Usheishat-southern part of Qoz Abu Dulu.
Topography: Gently undulating to the west with slopes of 2-4 per cent.
Natural vegetation: Rich grass cover and a good tree growth with less haskanit, termite mounds very common.
Parent material: Aeolian deposits, mainly sands and fine sands.
Land use: Millet cultivation on sand dunes and rough grazing along the inter-dune areas; this land has been cultivated continuously for more than 30 years.
0-40 cm. 5 yr 5/8 (moist), bright reddish-brown; soft, laminated, showing wind deposition; structureless, entirely fine and very fine sands; friable, non-plastic, very permeable; diffused boundary but quite distinct in places.
40-50 cm. 5 yr 4/6 (moist), reddish-brown, fine and very fine sands, distinct horizon of dark minerals, probably iron; structureless, no organic matter or animal activity, very permeable, boundary changes sharply.
50-130+ cm. 10 yr 5/6 (moist), bright reddish-brown, no lamination, structureless, sandy fleshy rootless from cultivated crops.
Site 20: Yermosol
Location: El Rawakeeb mixed farming scheme.
Topography: Flat surface sloping north-east towards Khor Barok, 0-1 per cent slope.
Parent material: Nubian Sandstone and sands deposited earlier by winds.
Natural vegetation: Some sparsely distributed acacia trees becoming denser towards wadi beds.
Land use: Rough grazing at the moment but the surrounding area is an irrigated mixed farm (vegetables, citrus fruits, jojoba experimental plantation, poultry, etc.).
0-10 cm. 5 yr 4/8 (dry), reddish-brown fine silty loam; wellstructured with crumb and columnar structures, easily destroyed, no organic matter, gradually merges into:
10-55 cm. 5 yr 3/6 (dry), dark reddish-brown silty loam, permeable, columnar structure, weathers easily to sand; few rootless, no animal activity, rather plastic but non-sticky.
55-150 cm. 5 yr 4/3 (moist), dull reddish-brown silty loam; harder than horizon above, quartz grains and some opaque minerals (iron or manganese) are encountered; difficult to penetrate by auger, non-plastic and nonsticky, fairly permeable, no fissures or organic matter.
Partially Buried Outcrops and Hilly Areas
Site 24: Lithosol
Location: North of Jebel Surubit.
Topography: Hilly and undulating with 8-15 per cent slope towards the east and south-east.
Parent material: Nubian Sandstone Formation.
Natural vegetation: Small Acacia orfota and some other scattered acacia species; thin grass cover.
Land use: Rough grazing by nomads.
0-10 cm. 5 yr 5/8 (dry), bright reddish-brown sand with platy structure deposited by wind, no vegetation or rootless, soft, easily crushed by hand, non-soil.
10-25 cm. 5 yr 4/6 (dry), reddish-brown, gravelly loamy sand, no organic matter, no soil horizonation, no animal burrowing, rather permeable but stony.
25-35 cm. 5 yr 3/6 (dry), dark reddish-brown gravelly weathered sandstone, iron-cemented, angular quartz, pisolitic, ironstones 0.5 to 1.5 cm in diameter, very hard to penetrate by auger.
Results of Soil Analysis
Only those properties required for interpretation of the inherent soil characteristics, and consequently their development, were selected for laboratory testing. The determinations included pH, particle size analysis, loss on ignition, moisture content, electrical conductivity, calcium carbonate, organic carbon and organic matter content, cation exchange capacity, and exchangeable cations.
Soil pH was determined by shaking a soil-water mixture (soil to water ratio 1:5) for two hours and taking pH readings with a digital pH-metre. Particle size analyses were performed by the pipette method. Loss on ignition, moisture content, electrical conductivity, organic carbon and organic matter, calcium carbonate and the rest of the chemical analyses were performed following the procedures used by the Sudan Soil Survey Administration, Wad Medani. The results obtained were employed to classify the soils as well as to determine tentatively land capability and potential productivity.
TABLE 2.1a. Vertisols: physical properties (profile 1: Alaga)a
a. CS = coarse sand;
FS = fine sand;
VFS = very fine sand;
LOI = loss on ignition;
MC = moisture content.
TABLE 2.1 b. Vertisols: chemical properties (profile 1: Alaqa) a
|Percentages||Exchangeable cations me/100 gm|
a. EC = electrical conductivity;
OC = organic carbon;
OM = organic matter;
CEC = cation exchange capacity.
These properties are relevant to soil use and have a practical application in the field as well as for classification. Soil orders identified in this study are according to the FAO-Unesco system of 1974 and imply the various processes dominant in their genesis. Soil classification was limited to orders and subgroups only, as a reconnaissance-level survey does not warrant more detailed classification.
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