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Vertisols

The soils along the White Nile alluvial plain are represented by the data in table 2.1 and profile 1. According to the data these soils are classified as vertisols.

The vertisols represented by profile 1 occupy the smallest area, not exceeding 2 per cent on the whole. On the basis of the physical characteristics these vertisols are classified as pellic vertisols (pellusterts). They are similar to the vertisols of the Gezira.

The chemical properties suggest a slight salinity in these soils. The pH values in the topsoil show signs of high alkalinity, which would be cause for concern if these soils were to be irrigated for commercial agriculture. So far no complaints about alkalinity have been reported in the area, probably because the agriculture practiced there is not intensive. Salinity, however, seems to have arisen in the irrigated schemes of Gummuiya since Briggs (1978) reported on the soil properties. The vertisols are also characterized by high calcium carbonate content, high cation exchange capacity and more than 45 per cent clay content in the topsoil. Exchangeable Ca+Mg are the dominant cations, suggesting clearly the influence of montmorillonitic clay minerals. Exchangeable K decreases with depth but is well above the 0.2 me/100 gm threshold value. No K deficiency is expected in these soils in the long run. Exchangeable sodium values give some concern, since they are relatively high at virgin sites. Irrigation could aggravate this problem. This is also supported by the EC values, which even in waterextracted 1:5 soil-to-water ratios are > 1 mMohs/cm in the topsoils and > 3mMohs/cm below 70 cm depth.

Organic carbon and organic matter show slight variability with depth. Calcium carbonate percentages are in accordance with the higher pH values in the surface soils, decreasing with depth also.

Except for the low permeability in these soils, most of the physical and chemical properties are agronomically favourable, making them comparable to the soils in the Gezira area. With proper management these soils could produce a variety of crops similar to those grown on the Gezira scheme.

Pedogenesis of the vertisols in the area began with the deposition of the White Nile alluvium. The factors of soil formation are mainly the traditional ones stated by Hans Jenny (1941), but the main processes involved result in the distinctive properties observed in the field as well as in the laboratory. Briefly, this comprises depletion of the less stable minerals from the sand, and in particular from the silt fraction, followed by increase in montmorillonite clay content, redistribution of calcium carbonate, and illuviation of clay. No exact age is given as most appropriate for their development, and this explains why some are designated as vertisols, whereas others with less than 40 per cent clay are vertic fluvisols.

TABLE 2.2a. Regosols: physical properties (profile 9: Umm Sidr-northern part of Qoz Abu Dulu)

Depth
(cm)
Percentage Colour
(Munsell)
Stoniness CS FS VFS Silt Clay LOI MC
0-10 0.00 1.02 50.25 41.78 2.84 4.11 0.85 0.44 5 yr 6/6
10-40 0.00 0.24 58.85 35.31 1.98 3.62 0.77 0.46 5 yr 616
40-100 0.00 1.01 42.00 39.35 8.03 9.61 1.60 1.01 5yr4/6

TABLE 2.2b. Regosols: chemical properties (profile 9: Umm Sidr-northern part of Qoz Abu Dulu)

Depth
(cm)
pH 1:5 EC mmhos/cm Percentages Exchangeable cations me/100 gm
CaCO3 OC OM K Na Ca+Mg CEC
0-10 7.77 0.15 0.00 0.02 004 0.10 0.03 5.17 5.30
10-40 7.66 0.17 0.02 0.14 0.25 0.09 0.03 11.47 11.59
40-100 7.60 0.16 0.04 0.14 0.25 0.08 0.05 5.30 5.43

Regosols

Soils developed on fixed sand dunes which are of Quaternary origin are classified as regosols (FAO-Unesco, 1974). Although they lack the morphological features of true soils, they are nevertheless very deep and permeable, a characteristic which makes them a good medium for millet growth. Generally they are composed of fine and very fine sand which make up more than 90 per cent of the particle sizes (tables 2.2 and 2.3). These sands are mainly fixed by the existing vegetation. Cross-bedding is apparent in these soils, indicating wind deposition.

The major physiographic feature formed by these soils is Qoz Abu Dulu. This is an elongated giant dune of about 2-4 km in width and more than 200 km in length. The origin of its formation is still in doubt, but it is the outcome of the northsouth-blowing winds of the Pleistocene and post-Pleistocene eras. Recently it has suffered intensive erosion and redeposition, especially during drought periods. The presence of minor barchans and self dunes on top of this giant dune is a common phenomenon these days, indicating increasingly dry conditions (i.e. desertification).

Although the physical and chemical properties of these soils indicate infertility, which gives them a dystric regosol classification (FAO-Unesco, 1974), they are quite suitable for millet and often for groundnut cultivation, even with very low amounts of precipitation. This is particularly obvious in the southern part, as exemplified by site 4 (Umm Usheishat),, compared to site 9 in the northern part. Although they belong to the same order and suborder, slight differences do suggest differences in moisture regimes. Properties indicative of variations in colour, organic carbon, other organic matter, and moisture content all show gradual variation with latitude and consequently with precipitation. Calcium carbonate and CEC also increase southwards with the increase in precipitation (tables 2.2 and 2.3).

Apart from the threat posed by the mobile sands, which could bury them, there is no immediate reason why these soils should not be utilized for subsistence millet production by the local inhabitants. If more care is taken to prevent sand blowing once the land is cleared for cultivation, the future will not look so gloomy for the seminomadic people living there.

Lithosols

These soils are formed on the hilly slopes and escarpments of the weathered Nubian Sandstone formation. They are mainly confined to the westernmost part of the area with some occurrences further east. Erosion by the intermittent but frequently heavy rains has resulted in shallow soils which are akin to the Rankers of the temperate zones with AC horizons. Being coarsely textured and weakly developed they qualify as lithosois (FAO-Unesco, 1974). The influence of parent material is very clear (table 2.4), with stoniness ranging from 1 per cent in the surface layers to45 per cent in the subsoil. The relatively moderate CEC (1927 me/100 gm) tells a great deal more than the actual situation in the field. The data reveal typical desert conditions of general nutrient deficiency, and hence very little can be hoped for from agriculture. Rough grazing is the only possible activity here.

TABLE 2.3a. Regosols: physical properties (profile 4: Umm Usheishat-southern part of Qoz Abu Dulu)

Depth
(cm)
Percentages Colour
(Munsell)
Stoniness CS FS VFS Silt Clay LOI MC
0-40 0.00 0.90 73.02 20.87 2.06 3.15 0.71 0.68 5 yr 6/8
40-50 0.00 0.77 67.48 22.61 4.25 4.89 0.88 0.68 5 yr 4/6
50-130 0.07 0.64 62.83 25.69 501 5.82 0.85 0.48 10yr5/6

TABLE 2.3b Regosols: chemical properties (profile 4: Umm Usheishat-southern Dart of Qoz Abu Dulu)

Depth
(cm)
pH
1:5
E.C.
mmhos/cm
Percentages Exchangeable cations me/100 gm
CaCO3 OC OM K Na Ca+Mg CEC
0-40 7.50 0.14 0.08 0.41 0.71 0.09 0.02 3.09 3.20
40-50 7.56 0.13 0.08 0.41 0.71 0.07 0.02 1.75 1 84
50-130 7.63 0.12 0.06 0.12 0.21 0.06 0.03 11.11 11.20

Flavisols

These are the soils formed along wadi courses, represented here by profile 13. They are generally sandy clay loams with stones averaging more than 5 per cent throughout the profile. They are alkaline (pH 7.9-8.15) with calcium carbonate ranging from 1.66 to 4.12 per cent and increasing with depth. Cation exchange capacity averages 32.33 me/ 100 gm, with K just above the threshold value of 0.2 me/ 100 gm on average but quite substantial in the topsoils. No signs of salinity are observed and table 2.5 suggests that these are fertile soils with good agricultural potential.

Samples in the vicinity of the present site have been studied by Williams (1974). The topsoils have been dated using C14 and the ages of the shells ranged between 6,990 100 and 8,400 150 years B.P. The deposits were interpreted as a result of local lakes which were full during that period. No dating was performed on the present samples to confirm Williams' findings but the presence of pure carbonate accumulation on the surface of these soils and the abundant Ca and Mg in the exchange surfaces probably support in part the likelihood of lacustrine conditions during the Pleistocene. Without the proper methods of dating these soils it is difficult to differentiate between wadi deposits and lake deposits in this area. In the present survey both are classified as fluvisols.

Yermosols

These soils comprise the major part of the delineated units and cover about 40 per cent of the area. They range from those developed on Holocene alluvium and lacking any evidence of carbonates or salinity to those with moderate carbonate content. In the vicinity of the Three Towns (Khartoum, Khartoum North, Omdurman) they have been mapped previously as "red soils" (Worrall, 1957). Their natural fertility, as suggested by Briggs (1976), is very limited. Other studies by Younis (1977) on the El Rawakeeb pilot scheme have come to similar conclusions. The favourable physical properties they possess and the presence of adequate groundwater in the area have rated these soils high in use potential. Minor salinity hazards were noticed in the early years of this scheme, but through pumping this danger has disappeared.

Studies west of the White Nile have shown that these salinity hazards at El Rawakeeb are caused by a localized hydrogeological basin (Muse, 1973). Other areas further west and south do not have this problem. The application of nitrogenous fertilizer or the cultivation of leguminous crops are essential to ensure adequate crop growth with irrigation if commercial agriculture is pursued.

The process of soil formation under these semi-arid conditions starts with the weathering of the Nubian Sandstone and the aeolian deposits. Low precipitation limits the development of soil horizons. The soil's red colour results from the oxidation of Fe minerals, and high temperatures contribute to the lack of organic matter. The high chrome in these soils does not necessarily indicate maturity: erosion by wind and heavy storm rains have often resulted in truncated and compacted subsoils. Table 2.6 summarizes the physical and chemical properties of these soils.

TABLE 2.4a. Lithosols: physical properties (profile 24: north of Jebel Surubit)

Depth
(cm)
Percentages Colour
(Munsell)
Stoniness CS FS VFS Silt Clay LOI MC
0-10 0.95 9.51 40.08 37.26 6.80 6.35 1.51 0.75 6 yr 5/6
10-25 12.29 10.81 30.23 32.25 14.15 13.19 2.49 1.15 5 yr4/6
25-35 45.20 11.40 14.86 15.64 20.14 37.96 6.36 3.12 5yr3/6

TABLE 2.4b. Lithosols: chemical properties Iprofile 24: north of Jebel Surubit)

Depth
(cm)
pH
1.5
EC
mmhos/cm
Percentages Exchangeable cations me/100 gm
CaCO3 OC OM K Na Ca+Mg CEC
0-10 7.71 0.06 0.06 0.60 1.03 0.16 0.06 19.06 19.28
10-25 7.70 0.03 0.06 0.48 0.83 0.10 0.08 19.85 20.03
25-35 7.77 0.06 0.06 0.46 0.79 0.11 0.13 26.96 27.20

Land Capability Classification

An attempt is made to classify the delineated land units into land capability classes using the data from their respective representative profiles. The factors used in the classification are those pertaining to the soils and topography. Water availability, road access, cost, and marketing factors are not considered, so that this classification is strictly pedological.

The classification is arranged in a descending order of suitability for agriculture. Six classes are recognized, with Class 1 having the fewest physical or chemical limitations (fig. 2.3).

The Classes

Class 1. Very Little Limitation

These are lands with insignificant limitations. In this category are included only the vertisols of the White Nile alluvium and some of the fluvisols in the vicinity of the White Nile. They are excellent for agricultural purposes if properly managed. Salinity is the only hazard, and might rate them in some places lower than Class 1.

Class 2. Little Limitation

These are lands with minor but significant limitations, often of water availability, susceptibility to erosion, and undulating topography. Wadi alluvial soils (flovisols) come into this category. The limitations are mainly physical rather than pedological. If water could be raised to implement irrigation in these areas then some parts would raise their rating. Their potential fertility is sometimes better than the vertisols.

Class 3. Moderate Limitation

This class comprises land of moderate agricultural potential The soils are vulnerable to nutrient deficiencies, wind and water erosion, or sand burial. Groundwater is the suggested means of irrigation if fertilizers can be used. The salinity hazard from the groundwater is another limitation, but with careful selection of fresher aquifers this can be overcome. Recently the UNDP, in collaboration with the Sudan Ministry of Agriculture, Food and Natural Resources, has introduced jojoba, an important oil-seed of great economic potential value, which has done well on these soils.

Class 4. Moderate to Severe Limitation (Dune Soils)

These are the dune soils with low agricultural potential. They are deficient in mineral and organic nutrients. Intense evaporation and very fast infiltration coupled with the unreliable precipitation might put them into a lower class still, especially in the northern part, except for the fact that they are good for millet cultivation and rough grazing. Lack of water for domestic and animal use is often a handicap for development in these areas.

Class 5. Severe Limitation

These are soils with weak development and shallow depth (regosols and lithosols), lacking in fertility, moisture, and good physical properties. They are too stony to support more than a little vegetation suitable for rough grazing.

TABLE 2.5a. Fluvisols: physical properties [profile 13: Wadi Abu Heliefi)

Depth
(cm)
Percentages Colour
(Munsell)
Stoniness CS FS VFS Silt Clay LOI MC
0-20 3.96 15.36 22.81 18.06 18.62 25.16 4.60 3.94 10yr3/3
20-35 8.16 17.46 25.24 18.23 14.56 24.51 4.66 3.80 10yr3/3
35-75 7.43 15.81 24.88 18.56 14.73 26.03 5.43 3.81 10yr4/3

TABLE 2.5b. Flovisols: chemical properties (profile 13: Wadi Abu Heliefi)

Depth pH EC Percentages Exchangeable cations me/100 gm
(cm) 1:5 mmhos/cm CaCO3 OC OM K Na Ca+Mg CEC
0-20 7.91 0.07 1.68 0.48 0.83 0.48 0.20 30.31 30.99
20-35 8.08 0.11 2.94 0.41 0.71 0.18 0.28 34.10 34.56
35-75 8.15 0.13 4.12 0.31 0.53 0.23 0.83 30.38 31.44

Class 6. Very Severe Limitation

These are lands with hilly surfaces, too barren to support even rough grazing. They are confined to the western part of the area. Settlement areas near the White Nile (not mapped in this study) and some semi-mobile sands also being to this category

Summary

From field and laboratory studies the area is apparently suitable for intensive agriculture where irrigated or for less intensive agricultural use where reliance is upon rainfall. Land classified as vertisols and fluvisols has good agricultural potential. These soils are mainly clayey in texture and occupy the flood plains of the White Nile and the wadis. Soils with more limited fertility and minor problems of water quality are suitable for less intensive agriculture. Irrigation by groundwater can support cultivation of those crops that tolerate severe conditions of aridity and low soil fertility in such areas. The successful El Rawakeeb scheme is a good example of the use of such soils. Because these areas are vulnerable to erosion and burial by blowing sands, means of defence against wind are necessary if the area has been cleared of trees and shrubs. Teras cultivation has been practiced in this area, as in Khor Barok. These bunds slacken the water movement to allow enough surface water to seep into the subsurface layers. Such lands with adequate water can yield considerable crops of sorghum and vegetables.

The regosols and the lithosols are of more limited value and are suitable only for millet cultivation and rough grazing. Outcrops of hills, lands around settlements, and semi-mobile dunes have no agricultural value.

Recommendations

In this study both genetic and functional explanations have been used to identify the origin and development of the landscapes delineated and to predict their behaviour under specified use conditions. Accordingly the following recommendations are made:

1. Land units delineated from satellite imagery and aerial photography could well be applicable elsewhere, especially in neighbouring areas having similar soils and landscape characteristics.
2. More detailed investigation of the soils, water resources, pasture, and forestry of these units is essential if the areas are to be used wisely.
3. To combat desertification and avoid soil degradation, proper measures to control land use and to prevent treecutting for fuel are needed.
4. Irrigation by groundwater needs to be encouraged in suitable areas, using the El Rawakeeb scheme as an example
5 Efforts should be made by the appropriate government departments and water authorities in the Sudan to raise the reservoir level of the Jebel Aulia Dam to permit irrigation of the extensive and fertile lands north and northwest of Ed Dueim and the lands at present under rain-fed agriculture along the wadi flood plains.
6. Embankments could be built across wadi beds to make better use of the otherwise wasted waters of the intermittent floods, for irrigation as well as for animal and human consumption. Socio-economic studies should start prior to such schemes in the area.

TABLE 2.6a. Yermosols: physical properties (profile 20: El Rawakeeb)

Depth
(cm)
Percentages Colour
(Munsell)
Stoniness CS FS VFS Silt Clay LOI MC
0-10 1.64 4.54 10.53 18.89 34.91 31.13 4.96 2.79 5yr4/8
10-55 1.29 10,69 20.81 23.57 18.51 26.42 4.13 2.91 5 yr 316
55-150 0.93 12.17 21.98 25.57 10.78 29.50 3.93 3.82 5yr4/3

TABLE 2.6b. Yermosols: chemical properties (profile 20: El Rawakeeb)

Depth pH EC Percentages Exchangeable cations me/100 gm
(cm) 1:5 mmhos/cm CaCO3 OC OM K Na Ca+Mg CEC
0-10 7.15 0.07 0.06 0.31 0.53 1.05 0.25 21.68 22.98
10-55 7.20 0.06 0.08 0.26 0.45 0.37 0.16 29.43 30.25
55-150 7.35 0.14 0.06 0.26 0.45 0.10 0.13 30.63 30.86

7. In the sandy soils, cultivation of legumes should be encouraged to restore fertility to the nutrient-deficient soils (regosols and lithosols).

8. In the southern parts near Jebel Arashkol and beyond, where rainfall is relatively more abundant, rain-fed cultivation can be successfully carried out. Sorghum, peanuts, and vegetables are suitable for these soils. Rich pastures exist and the milk products, including cheese, bear witness to their productivity.

References

Ali, M, A, 1970. "Semi-detailed Soil Survey of Sixteen Irrigation Schemes in the White Nile Area." Soil Survey Department Report, no. 20. Soil Survey Department, Wad Medani.

Areola, O. 1977. "A Review of Land Facets as Soil Mapping Units." Savanna, 6(1): 85-89.

Bridges, E. M., and C. P. Burnham. 1980. "Soils of the State of Bahrain," Soil Sci., 31: 689-707.

Briggs, J. A. 1978. "Farmers' Responses to Planned Agricultural Development in the Sudan." Trans. Inst Brit Geog., 33: 464-475.

Davies, H. R. J. 1973. Tropical Africa: An Atlas for Rural Devolopment University of Wales Press, Cardiff.

El Fadl, A. I., and R. H. El Sayed. N.d. "A Preliminary Report on Rainfed Agricultural Areas West of the White Nile." Sudan Soil Survey Department, Wad Medani.

El Fadl, A., and K. O. El Fadl. 1973, Methods of Soil Analysis Used in the Soil Survey Department. Wad Medani. Part l: Chemical Analysis. Sudan Soil Survey Department, Wad Medani.

Farwa, A. G. 1978. "Geology and Structure of the Gezira Area as Deduced from Gravity Measurements." M.Sc. thesis. University of Khartoum, Khartoum.

FAO-Unesco. 1974, Soil Map of the World. Vol.1: Legend. Unesco, Paris.

Hussein, M. T. 1976, "Contribution to the Sedimentology of the Nubian Sandstone Formation in Khartoum Area." M.Sc. thesis. University of Khartoum, Khartoum.

Jenny, H. 1941. Factors of Soil Formation: A System of Quantitative Podology. McGraw-Hill, New York.

Kassas, M. 1956. "Landforms and Plant Cover in the Omdurman Desert, Sudan." Bull. Soc. Geog. d'Egypte, 24: 5-58.

Kheiralla, K. M. 1976. "Determination of Recharge Potentialities Based on Environmental Isotopes Techniques," Paper presented to the 3rd Conference on African Geology, Khartoum.

Lebon, J. H. G. 1965. Land Use in the Sudan, Geographical Publications, Bude.

Mensching, H., and F. Ibrahim. 1977. "The Problem of Desertification in and around Arid Lands." Applied Sciences and Development (Tubingen), 10: 7-43.

Moss, R. P., ed. 1968. The Soil Resources of Tropical Africa: A Symposium of the African Studios Association of the United Kingdom. Cambridge University Press, Cambridge.

Muse, A. S. 1973. "A Hydrogeological Study West of the White Nile." B.Sc. thesis. University of Khartoum, Khartoum.

Nykango, J. T. 1973. Methods of Soil Analysis Used in the Soil Survey Department, Wad Medani. PartIl: Physical Analysis. Sudan Soil Survey Department, Wad Medani,

Omer, M. K. 1975. "Genesis and Diagenesis of the Nubian Sandstone Formation in Khartoum Province." Geol. Min. Res. Dept. Bull., 27.

Salih, T. M. 1975. "A Report on a Detailed Soil Survey of El Rawakeeb Scheme, Khartoum Province." Sudan Soil Survey Department, Wad Medani.

Shirlaw, D. W. G. 1967. A Practical Course in Agricultural Chemistry. Pergamon, Oxford.

Tomlinson, P. R. 1970. "Variations in the Usefulness of Rapid Soil Mapping in the Nigerian Savanna." J. Soil Sci.. 21: 162-172.

Van der Kevie, W. 1973. "Climatic Zones in the Sudan." Preliminery report. Sudan Soil Survey Department, Wad Medani.

---. ed. 1976. Manual for Land Suitability Classification for Agriculture. Sudan Soil Survey Department, Wad Medani.

Whiteman, A. J. 1971. The Geology of the Sudan Republic. Clarendon Press, Oxford.

Williams, M. A. J., and D. A. Adamson. 1974. "Late Pleistocene Desiccation along the White Nile." Nature, 248: 584-586.

Worrall, G. A. 1957. "Features of Some Semi-arid Soils in the District of Khartoum, l and II."J. Soil Sci., 8: 193-210.

---. 1958. "Soils and Land Use in the Vicinity of the Three Towns," Sudan N. and R., 39: 2-12.

---. 1960. "Patchiness in Vegetation in the Northern Sudan." J. Ecology, 48: 107-115.

Younis, M. G. A. 1977. A Study of the Effect of Well Water on Some Morphological, Chemical and Physical Properties of El Rawakeeb Soils. Ministry of Agriculture, Food, and Natural Resources, Khartoum.

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