Soil in Sudan

Wed, 27 Sep 2017



  Dr. Ibrahim Saeed - University of Khartoum

Abstract:

Sudan’s lands are consistent of an alluvial flat and slightly sloping plain, yet dotted with hills that cover fewer than 5% of their total area, the more important of which are: Red Sea Hills in Eastern Sudan, Nuba Mountains in South Kordofan and Jebel Midob and Jebel Marra in Darfur. However, the River Nile represents a more important geomorphological phenomenon in Sudan. Nevertheless, Sudan’s plains consist of various types of soil, the more important of which are the following:

Sandy soil: it is found in the desert and semi-desert regions in north and west of Sudan. This soil is fragile and vulnerable with low fertility, so it is exploited in cultivation of millet, groundnuts, sesame and hibiscus. Additionally, this soil provides a major pasture for camels, sheep and goats.

Clay soil: found in central and eastern Sudan. It represents important areas for cotton cultivation, mechanized and water-fed cultivation, as it similarly represents an important source for forest products, particularly fire woods and gum Arabic. Moreover, most of Sudan’s product of maize (Dura), the main food crop, is cultivated on this soil.

Group of silty loamy soils: found on banks of rivers, valleys and at both deltas of Gash and Toker. However, these soils are characterized with their high fertility due to their yearly renewal.

Volcanic fertile soil: found in south west Sudan, namely in Jebel Mara, and there is a small area of laterite soil in the far south-west Sudan.

 

 

 

Introduction:

Definitions of soil are numerous and varied according to differences in their utilizations, for example the civil engineer defines it as a crumbled layer that swarms up surfaces of solid rocks and whose thickness reaches many feet. As to geologists, they define soil as the layer at which operations or chemical and natural changes take place. On their part, agriculturists explain soil as the surface layer whose thickness ranges between 6 to 9 inches and where plant roots downwardly extend. This layer is naturally different in colour and plant from the underneath layer which is known as subsoil. However, the two layers, soil and under soil layers, are of paramount importance to soil scientists as we will address them in this paper.

We can generally say that soil is a mixture derived of crumbled rocks with different amounts of organic substances. This mixture composes surface layer on the crustal in a way that makes it in a continuous interaction with the atmosphere. As a result, this soil/ atmosphere interaction generates chemical decomposition and mechanical fragmentation and, thus, soil becomes a convergence zone of organic and inorganic substances. In the process, soil will be mixed with different rocky fragments, minerals, remains of plants and decaying animals, besides millions of organisms whether they are worms, creatures, roots of plant or bacteria. Soil nature is defined by its mechanical and chemical characteristics, in that soil texture is organized with its mechanical composition, i.e. the size of particles of which soil is composed. Even the degree of porosity of the soil and how it is permeable to water are identified with this size of its particles. Mechanically, soil is composed of sand or silt or of both. In their laboratory experiences, soil scientists agreed on united measures for defining soil components (gravel, sand and clay) and others which we can specify in the following:

Gravels: include all particles whose diameter is greater than 2 mm.         

Coarse sand: includes all particles whose diameter ranging between 0.2 – 2 mm.

Fine sand:  includes all particles whose diameter ranging between 0.2 – 0.02 mm.

Silt: includes all particles whose diameter ranging between 0.02 –0.002 Mm.

Clay: includes all particles whose diameter is less than 0.00 2mm.

Therefore, according to their mechanical composition, types of soil will be as follows:

Sandy soil: in which proportion of sand is greater than 70% and clay is less than10%.

Loamy soil: in which proportion of sand ranges between 40% to 70% and clay between 20% and 40%.

Clay soil: in which sand is less than 40% and silt ranges between 30% and 40%, and proportion of clay is more than 40%. Figure (1) explains the relationship between soil and its mechanical composition. 

 

Soil formations and s types: 

The outputs of mechanical and chemical weathering processes are considered as bases of soil formation. Similarly, there are organic substances mixed with mineral components of the soil and these substances are decisive for defining the magnitude of soil fertility and for development of its different profiles. It is notable that water and air are among the basic ingredients of these substances. However, Figure (2) represents a section in a mature soil with clear profiles, and every profile is distinguished by the following:

--- Profile O: represents the upper surface of soil and consists of fallen leaves of trees and remains of agricultural crops and other organic decaying substances.

--- Profile A:  contains organisms. In this zone, roots of plants extend downwards to absorb water and food stuff. Also, water moves through it towards bottom fields of soil while carrying the fine rocky fragments and total dissolved solids (TDS), so this zone is called the laundry zone. 

--- Profile B: in this profile, silt minerals and TDS which were transferred from the upper profiles get together, so this profile is called rally profile where rallied silt fragmentation keeps up water.

--- Profile C: this profile consists of relatively major rocky fragments posing ground base for other profiles. Often, there is no metal relationship between fragments of this profile and wreckages of other profiles which surmount the former profile. This indicates that the soil substance is transported from other places through mobile factors like water, air and ice.

However, soil is categorized according to its texture, minerals composition, and its origin or to it is suitability for cultivation. Yet, global classification of soil recognizes texture as a base of its category as is illustrated in Figure (1). This figure demonstrates an equilateral triangle in which every head represents an specific amount of rocky fragmentation while every side of this triangle is inserted in a scope as of zero up to a hundred. This triangle is divided into twelve fields, every one of which represents an specific kind of soil (Abdu, 2004).

        

       Figure 2:  A section in a mature soil        Figure 1: The global textural classification of soil

Source: Abid (2004)

Likewise, soils are classified according to climates that helped formulate them as follows:

  1. Tropical soil: where rainfall amount and rates in tropical zones are very high, this leads to intensive soil leaching process and denuding soil of mineral salts and entails fertilizing the soil.  
  2. Grass land soil: this kind of soil is found abundantly where there is a minimal amounts and rates of rain which is not sufficient for growing trees. In this kind of soil, profile (A) is marked with a dark brown color with a measure of blackness.
  3. Forest soil: it is a kind of complete profiles of soil that is found in humid zones.
  4. Desert soil: this kind of soil is found abundantly in dry zones, yet with unclear profiles and minimal thicknesses. It contains a high concentrations of the element calcium. 
  5. Arctic soil: its thicknesses are minimal and it is often frozen, except in short periods during the summer.  

 

Generally, soil fertility depends on the amount and availability of basic elements like nitrogen, carbon, potassium, phosphorous, calcium and magnesium.

 

 

 

Types of soil in Sudan:

Alluvial clay soil

It spreads out in vast areas of Sudan, from the far south to the far north, through desert and semi-desert regions. Similarly, this soil is limited to a narrow strip of the Nile and found at both deltas of Gash and Baraka while extending in central Sudan around White Nile and the Gezira plains. However, different factors played a role in forming this kind of soil, the more important of which are surface nature, climate and hydrological conditions. Physically, the regions surface is a flat plain extending from hillsides of south-east highlands and descends a little bit northwards to the extent that when rivers get away of these hills, they vanish among these plains. On these plains, water surplus of the neighborly highlands is pooled in, but, given the scarce mountainous slope besides a yearly seasonal distribution of rainfall, flooding water covers the earth’s surface. Also, drainage or disposal of water is not an easy process given that sides of water courses are always higher than the distant places. Also, though temperatures are generally high, relative humidity will also be high during this season and evaporation decreases accordingly. On the contrary to this, in the drought season, the ratio of humidity decreases in soil owing to rain stoppages on one hand and blowing of dry wind on the other hand besides the magnitude of solar radiation during this season when temperature of both air and soil are raised. Furthermore, what controls the level of water seepage in this kind of soil is the level of the soil humidity itself, because at the end of summer, the soil has deep cracks and early rain water will deeply infiltrates into it. Then, as soon as the outer layer gets imbibed with water, its particles will be swollen and they become impermeable to water and, thus, the problem becomes more complicated though the subsoil may be dry (Sayyad and Saudi, 1983).

Given this characteristic, the surface factor intervenes to act as a differentiator between subtypes inside this kind of soil. Yet, superficial differences are not big, as much as they are simple disparities of only centimeters. These differences may make a part of earth apt to be submerged throughout rain season while another part will not be submerged and a third part will be underwater for a relatively limited period. Again, given that this soil originates from adjacent highlands, the water that washed it away contains a high proportion of silica which were washed and removed from highlands by rain water. Hence, if water evaporates, these salts will remain on the surface and be predominantly marked with alkaline qualities. The following are the basic characteristics which are common among the different subtypes of soil, albeit with various levels.

Firstly: soil of medium altitude lands submerged during flood season

This soil is predominant in the floodplain and it is divided into two types:

  1. Cracked clay soils: it includes most of medium altitude lands in the floodplain. As to its chemical and mechanical characteristics, they are related to its hydrologic conditions and the way it is formed. This soil substance is of clay which often shrinks in the drought season and swells in the rainy season. However, submergence of soil, with no oxygen penetrating in, helps increase organic substances in this type of soil more than it does in soil which is not susceptible to submergence. Furthermore, submergence of soil also helps dissolve some nonorganic substances such as configurations of calcium and iron. But, in the drought season, water evaporates and leaves complexes of calcium carbonate on the surface.  

 

Natural characteristics: in this type of soil, clay ratio is greater than 50% and may sometimes reaches 70%. Clay in this sector is irregularly distributed, and it is a certainty that the high ratio of clay is responsible for differences of soil form from season to season. As we formerly mentioned, this clay ratio is extremely solid and compact in the drought season as it is, otherwise, extremely viscous but barely permeable to water in the rainy season.

Chemical characteristics: this soil is generally of alkaline nature as hydrogen concentration is over 8 degrees and sometimes reaches 9.5 degrees. But, usually, the upper layer is of lower alkaline concentration than the layers under it. As to the ratio of dissolved salts, it is naturally 0.5% while degree of salinity increases three or four feet away, yet water that submerges the soil is adequate to leach calcium carbonate which gathers on the lower layers to create white complexes. 

On the whole, the agricultural value of this soil depends on water conditions, because, except for rice, few yields can bear a lack of oxygen for a long period, besides the difficulty of cultivation of these yields. More or less, this type of soil will be the best one of other types if only the drainage problem is solved and nitrogen and phosphorus fertilizers are used (Saudi, 1983).

 

 Non-cracked clay soils: like the former type, albeit being lighter than it, it is found in medium altitude lands. This soil resembles the thick clay soil in several aspects except for the higher ratio of coarse substances in this one whose clay ratio ranges between 25% - 50% and ratio of coarse sand is greater than 20%. For these reasons, cracking process in this soil is less common and it occurs at the end of drought season. This kind of soil is similar to the former one in their chemical characteristics though alkaline nature of this one is apt to be lower, and hydrogen concentration ranges between 6.5 – 9 degrees and salts’ ranges between 0.03 – 0.01. Nevertheless, this kind of soil is fertile but its production and utility are affected by poor drainage.  

Secondly: high lands not submerged by water in floodplain

These lands are divided into two parts:

Clay soils: it is different from soil of medium altitude lands as there is no trace of floods with a few organic substances. On another hand, it is different from sandy soil due to its high clay ratio as this percentage can give it special natural and chemical distinctions derived from the glutinous characteristics of the clay. On the mechanical side, soil texture is different where clay ratio ranges between 15% - 40% and sand ratio are more increased on surface layer than it is on subsoil layers. As to its chemical characteristics, clay ratio here is defined by the level of acidity yet the soil is generally of an alkaline nature. The hydrogen concentration degree is high except for the superficial layer which maybe lightly acidic while salts ratio here ranges between 0.01% -- 0.15%. As regards its agricultural value, this type of soil is mechanically and chemically devoid of defects. After all, it is of debilitating types in floodplain, given that water does not submerge it in whatever season and, also, mixed farming can be practiced on this kind of soil.  

Sandy soils: this type occupies floodplain lands and is not affected by flood. Though it is often susceptible to being covered by flood, it can easily drain off water due to its coarse texture. These types of soil, however, are indebted for their advantages to original substances from which they were derived, the most of which are quartz.

This type of soil is characterized by the fact that the clay amount it contains is very low (1% - 4%). The soil is disjointed with a light alkaline degree and a small amount of salts (0.01% -- 0.05%) and also there is little organic substances (0.041 – 0.05%).

Thirdly: low land soil:

Some of this soil is saturated or submerged with water for most of the year and it remains saturated with water for the most part of the year. In this soil, clay ratio ranges between 20 - 60% and coarse sand may reach 40%. It is a disjointed soil whose thickness is 3 – 0.05 feet. From chemical part, it is acidic with a high ratio of salts (0.01% - 1.2%) and high organic substance between 30% - 50%. Hence, it is a fertile soil but, given to being overwhelmed with water most of the year, it is not possible to utilize it.

There are parts of this soil from which water recedes for a likely short period of time, because source of water here is rain and not river flood. The ratio of clay is high ranging between 40% - 70%, and it is normally an alkaline soil but salt ratio in it is moderate (0.05 – 0.5%). Agriculturally, this soil is fertile yet devalued owing to being submerged with water though, during the period of recession of water, it offers good grazing lands

 

 

Alluvial soil in northern Sudan:

It extends along the two banks of the River Nile north of Khartoum and down to the northern borders of the country. It may occupy vast areas in the relatively low areas close to the River Nile where it is submerged with water in flood seasons and, consequently, its fertility is yearly renewed due to the accumulation of clay on it. Here, this soil is defined as basins like Suleim basin in the Northern State and Salwa basin in River Nile State. However, table (1) illustrates a result of analyzing this sample of this soil.

 

Table 1: composition of flood soil in Northern Sudan

Hydrogen concentration degree

salts

clay

silt

Fine sand

Coarse sand

stone

Depth in inches

8.3

0.03

31

9

61

zero

zero

0 - 12

8.4

0.04

37

20

42

zero

zero

12 -24

8.4

0.03

25

15

60

zero

zero

24 -36

Source: (Saudi, 1983)

It is clear, from the table; this soil is of a silty nature varying from Gezira soil which has a high percentage of clay. Comparatively, this soil is nearer to the sandy one as average of clay in both is 25%. As to hydrogen concentration, it is 8.4 degrees, namely it is of alkaline nature though not that extremely alkaline.

In low parts of the area, clay ratio may appear high as a result of sedimentation of fine and soft substances in pools which are formed in the aftermath of flooding. Here, soil fertility largely depends on salts ratio, but if the ratio reaches 1%, it will have a bad impact on production.

Alluvial fan soil:

It is found in eastern Sudan at deltas of Gash and Toker on the end of khour Baraka. In this case, we can take the delta of Gash River as an example for this soil. The Gash soil is a rich one because it is derived from the basaltic highlands of Abyssinia and it is of great thickness and locally known as the soil of Lubbad. In both northern and western parts of the delta, this soil leans along the land that extends between Mikali and Hadalia till it is changed into thick clay where it is known as the soil of badoub. The soil gets cracked in the drought season and has even a scanty production of cotton. Anyway, all subtypes of Lubbad and badoub are existing here. Geologically, these new sedimentations are built on even elder deposits of old cracked undated-for clay.

On analysis of soil profiles of Gash, it was found that its alluvial texture is obvious as its layer sedimentation indicates that the soil was deposited by successive floods. Generally, soil of high parts of the delta is coarser than the case is in low lands. Yet, proportion of stone and sand is only scantily appearing in the Gash soil. Relatively, the ratio of fine sand, silt and clay is highly different, albeit silt ratio is no more than 50%. On another hand, there are no assembled salts in any layer of the soil while hydrogen concentration degree for the first five feet ranges between 7.1 – 8.3. Here, in flood season, humidity in lubbad soil reaches 18 feet in depth in whereas in Gezira land in which there is hardly any trace of humidity after the first five feet, regardless of the period during which this land is submerged with water.  

In the Gash soil, the thick clay layer preserves humidity in the lowest part of surface soil whereupon soil resembles a reservoir keeping up water to meet requirements of plants and people when light rain ends (Sayyad and Saudi, 1983).

 

Gezira soil:

The Gezira soil is another example for flood soil but it was thought that soil of Gezira plain was caused by Aeolian as there are no layers in the plain. Also, some other concerned soil scientists thought that the soil is an output of lake deposits, but both Andrew and Arkil confirmed that Sabaloka Gorge had once been as it is now, namely there is no dam collecting water behind it. Today, the overriding opinion is that this soil was deposited by the Blue Nile and dated back to the period 50,000 – 10,000 B.C.

Given the existence of the Gezira scheme in this land, many soil researches have been carried out for cultivation, particularly on surface layers. Table (2) illustrates the mechanical structure of a sample of Gezira soil.

       Table 2: The mechanical structure of Gezira soil

Clay

silt

Fine sand

Coarse sand

Gravels

Around 60%

13 - 20

12 -20

2 - 10

2.5

            Source: Saudi, 1983

In this soil, the more we travel northwards, the more the ratio of clay increases. However, predominant over Gezira soil is a surface layer with two feet in thickness, dark brown while this layer is laid upon another gray one with about two feet. Even, the latter layer depends on a third brown and slightly yellow coating. Structurally, inter-lines between layers are clear, albeit these lines extend as tongues of the upper brown soil to the gray layer. This overlap of the upper coating is ascribed to the nature of the soil itself which is cracked during the drought season, and this allows some of surface soil to reach the lower layers. Characteristically, Gezira soil is robustly alkaline and with huge quantities of gypsum. The soil proved to be suitable for cotton farming, provided the maintaining its fertility by utilization of fertilizers and agricultural rotations which entail long periods of respite for farming process. Even cotton should be preceded by a period of respite followed by another period of respite, because humidity has an excessive impact on soil texture as it makes it more expansive and, thus, makes it less possible for a plant to deeply strike its roots in the soil. In this regards, Jewitt says that continuous irrigation deteriorates soil whereas period of respite improves characteristics of the soil. Apparently, variety of cotton production in the Gezira land is ascribed to what soil contains of salts, particularly sodium carbonate. So, contradictorily, when the proportion of sodium carbonates increases in soil to 0.2%, production decreases accordingly. Also, experiences showed that fertilization with artificial fertilizers reflected that neither potassium nor phosphorous weakens the soil but, otherwise, nitrogen fertilizers boost production. It is noteworthy that in clay plains, some granite hills outcrop and the more we approach Ethiopian borders, the more the number of these hills increases (Sayyad and Saudi, 1983).

 

 

Laterite soil:

Buchanan was the first geologist who semantically gave the term ‘laterite’, in 1807, to a type of soil used in India for purposes of buildings, because this type of soil will become a solid block if it gets dried. In this context, researchers agreed that the word ‘laterite’ is used to imply local succession of rocks which were denuded due to washing and removal of most of the bases and silica while leaving remains containing various quantities of silica unmerged with alumina as well as iron oxides.  

Physically, formation of this type of soil is ascribed to factors of rain and temperature in that rain washes out and removes bases sesguixoids while hexagon oxides remains exchangeable in the soil as it is not diluted in water. For formation of laterite, some scientists argue that rainfall should be continuous or, at least, there should be no long periods of cessation of rain, because if the dry period remains long, silica may elevate from subsoil layers and, thus, the alteration process to laterite will be slow.   

As to temperature factors, it is indispensable, given that organic substances are abundant in moderate zones. Again, due to the lack of decomposition and the demise of these substances, organic acids which will be united with them are abundant enough to dissolve hexagon oxides while silica remains unaffected. As to tropical zones, high temperature decomposes organic plants and, consequently, organic acids and bacterial activity diminish. Furthermore, high temperatures help increase the speed of oxidation of iron and alumina and make them more resistant to dissolution with water. For this reason, silica diminishes but iron oxides and alumina still exist, hence temperature helps form soil of laterite.

Sometimes, formations of laterite in Sudan immediately cover basement complex or they are found above surface formations known as iron stone. As regards the predominant soil of laterite which covers iron formations, they are not directly derived from original basement complexes. Geologists see that this type of soil passes two stages:

First stage: some soils were formed on vast areas of basement complex at a time when drainage was worse and water somewhat washed away the surface layer, but as to subsoil, it had been full of iron oxides.

Second stage: here, conditions of drainage had improved and valleys were created. In this stage, running water carries small grains while leaving behind big grains which consisted of quartz besides complexes of iron which were solidified due to being exposed to climate. By so exposition, these complexes became stony and were designated as Iron Rock, and soil of laterite was derived out of this latter complexes.

To this point, oozing water at the bottom of soil carries some atoms of iron, manganese and some aluminum down to lower layers. These atoms are deposited on this layer in a coating form or, otherwise, they come together in the form of peairon, so we find there a layer whose thickness is ranging between 15 – 30 centimeters (6 – 12 inches) scattering in a coating form of iron formations under the surface. As regards formations of iron stone, their thickness ranges between 3- 5 meters and they are crowned with a sandy loam with an iron knot.  

The effect of the compact underlying layer is reflected in the lack of soil absorption of abundant rainwater. So while flowing, water sweeps tiny grains before it. Hence, soil texture will be more different on the surface of the plateau than in valleys and this is what is meant by local succession of groups of soils as a result of catena associations of soils. In this case, more than one type of soil emerges in the same sector succeeding as follows:  

Eluvial Complex: it emerges on high slopes where tiny atoms were removed while the remains are mere silt with a high proportion of sand. But if is soil still there, the plateau irony rocks may appear on the surface in the form of a lateritic shield.

Colluvial Complex: it means medium slopes of valleys in which there is found some substances deposited from higher layers while other substances were removed to the lower layer. Therefore, their surface formations vary from silt and stone to only stone.

Eluvial Complex: found at the rock bottom of the slope where tiny substances were deposited. In fact, high levels of this layer are of highly fertile soils, albeit their lower levels were affected by submergence in water and bad drainage. For this reason, laterite soil types in Sudan differ according to terrain nature on one hand and an abundance of rainfall on another hand.

Red sandy silt: it is apparent where rainfall is abundant in the far south-west of Sudan where there is a plentiful amount of rainfall and there is a light surface layer. So, clay ratios in these types range between 17% and 38%, as can be shown in the following type (Table 3) and this is why this soil allows roots of trees to strike deep into the earth. Though this silt is not rich with organic substances, forests can grow and live in it because the amount of phosphate and bases the plant absorbs from soil can be again restored to it by vegetation and leaves that fall onto the surface. So, if a forest is cut and replaced by farming, deterioration of soil will take place. Here, impact of rain increase will be shown in growing of forests and in the increase of the proportion of organic substances and percentage of acidity, while the degree of nitrogen concentration ranges between 5– 6. On the whole, fertility of the soil depends on how deep it is and how it is affected by factors of erosion on the surface layer above on which organic substances assemble (Saudi, 1983).

 

Table 3: laterite soil type composition

Nitrogen

(ppm)

Dissolved salts

 Degree of concentration of nitrogen

clay

silt

Fine sand

Coarse sand

Gravel

Depth in inch

0.131

0.2

5.5

26

10

25

27

zero

0-6

0.150

0.1

5.3

27

6

23

26

zero

6 -12

Source: Saudi, 1983

 

Dark silt: sometimes, thick soils of dark silt emerge on the plateau but they pose pockets inside rocks. This type is different from the former one in that the ratio of clay is higher in this type than in the former one and, also, iron knots are not found on the surface but rather under the soil. This soil is acidic and speedily exhausted due to shortages of foodstuffs.  

Fertility of laterite soil: it was mentioned that processes of leaching and removal of silica are responsible for the removing a great proportion of bases and, thus, soil will be poor source of foodstuffs if it is compared with soils which are formed in moderate zones or in zones which are lightly exposed to this leaching. Out of chemical analysis of these soils, apparent shortage of potassium and phosphoric acid is abnormally exposed. Here the proportion of nitrogen, albeit meager, is not with the same shortage of potassium and phosphoric acid, yet acidity is dominant on this soil. After all, if this soil is well fertilized, it will be well cultivable because it has good mechanic qualities. Incidentally, oil palm which is an important yield in West Africa is grown in many areas whose soil is derived from laterite there. Also, there is cacao of Ashanti in Ghana and sisal hemp, cane and coffee are grown in Malawi on laterite soil, as is most of the tea in Assam, cane, banana and coconut from Java. As to fertilizers, this soil requires large production; they are organic ones along with those of phosphate and potassium.

Many yields like tea are adapted to the high degree of acidity, though other yields such as cotton and millet are not cultivatable if the concentration of nitrogen is lower than 5.5 degrees, so it is necessary to add lime to it.

As regards laterite soil in Sudan, chemical fertilizers are rarely used and, instead, it is dependent on organic substances particularly from the dung of cows which graze on that soil. Therefore, inhabitants practice shifting cultivation to restore soil fertility. In this context, if the problem of fertilization is added to the annual problems of fires which sweep through trees and plants that help fix soil and make it resistible to factors of erosion, we find out that the problem of sweeping of surface soil in laterite zone is also a problem. It is noteworthy that the separation of Southern Sudan has caused Sudan to lose most of the area of laterite soil as only fewer than 10% of the soil remained in Northern Sudan.

 

 

Sand dune soil, Goz soil:

It is an aerobic soil found in a large domain west of the Nile in Central Kordofan and East Darfur. Physically, this soil appears in a form of stable sandy dunes. It is likely that this type ascribed to deposits of Nubian gravel layers lying in the north where northern trade winds might have deposited it in a drought period occurred at the end of Pleistocene epoch, in a period different from the present one. In view of its direction, it is apparent that the soil, in general, extends along axils from north to south. Today, this soil is fixed by welding substances which maybe a thin layer of iron oxides on surface or fixed by the plant itself. In the process, rainfall sweeps the tiny particles in holes and depressions, so a thin clay layer is formed in them.

On the whole, the Goz soil is of little fertility, albeit it keeps up rain water till water is consumed by plant. Nevertheless, Goz lands are a main regions of rain farming as they afford good crop of millet in Kordofan and Darfur. However, Goz soil is deep and tends to be yellow in color or red brown colour. Out of mechanical analysis, it is apparent that ratio of clay in this soil is not, at any rate, more than 5% while coarse and fine sands represent about 80% and coarse sand is particularly dominant, but the gravels of more than a millimeter is rarely found as shown in Table 4. Also, the soil is chemically poor since proportions of phosphates, calcium and organic substance get lower in it.

The Goz soil is poor in organic substance but with high permeability of water, so it contains water in dry seasons and natural plants grow there, particularly perennial and seasonal flora which thickly grow in relatively low areas where the proportion of clay increases in the soil. Excessive human activity, mainly over cultivation and overgrazing, caused the removal of vegetation from some areas of this soil, mainly areas around towns and villages, and this again dismantled soil and rendered it exposed to wind erosion.  

 

   Table 4: Composition of Goz Soil

Degree of nitrogen concentration

salts

clay

silt

Fine sand

Coarse sand

Gravels

Depth in inch

7.3

0.5

3

1

41

55

zero

6-12

7.4

0.5

4

1

42

63

zero

6-12

7.4

0.5

4

1

43

52

zero

12-18

7.4

0.5

4

2

27

67

zero

18-24

         Source: Saudi, 1983.

 

Local soils:

They are groups of soils scattered throughout various districts of Sudan, and they differ from each other in their compositions and characteristics. The followings are examples of these soils:

a) Gardoud soil:

It is found in many places on the basement complex, whether this complex is alkaline igneous rocks as in Butana and Nuba Mountains, or extrusive rocks in Gedarif or in far west of Darfur. Geographically, soils here range from clay depressions to silty-sandy soil adjacent to river courses. In particularly, high districts standout with a kind of sequence of crumbled soils where plants barely grow on high slopes because there is no soil to strike their roots in. Under these slopes, there is a red layer on which thorny trees grow, and then comes out another brown and kind of thick layer. This latter layer is sometimes dotted with stones, devoid of salts, calcareous with a few azotates and it tends to be alkaline. These two layers are known to Sudanese people as gardoud which, otherwise, means good drainage lands (Green’1984).  

b) Highland soil:

It is formed in highland districts through valleys and khours. It is an interzonal soil, namely it is formed and recurred on highlands. Geologically, the actual causes of formation of such soils are the existence of certain inherent substances or slopes where water courses and valleys spread out like wadi Kutum and wadi Azum of Jebel Marra area. These substances run from the summit of the Jebel down towards low areas while deriving a vertical force from water power and waterborne materials insofar as to perpendicularly deepen and horizontally widen this water course. When these substances reach the slightly sloping areas, they start depositing their load to form the fertile sedimentary soil along the two sides of the water course.

Sometimes, confined to mountainous zone, volcanic soils stand out as the case is in Jebel Marra. As well, they may appear in the form of silt as in deltas of water courses and strips of valleys which slope down the Jebel.  These soils are deep and rich as shown in Table 5 which represents analysis of a soil type of Jebel Marra. The area of this soil in Sudan reaches 1.290km2 and the yearly amount of rain ranges between 600 – 1000mm and it is dominated by the Mediterranean climate of a cold rainy winter. This climate makes this soil cultivable for crops like apples, strawberry and grapes whose cultivation is unsuitable in other regions of Sudan. Moreover, planting of trees and grazing are practiced on this soil area and also, though narrowly, mechanized farming is practiced. Incidentally, there are steppe clay lands in the middle of the Nuba Mountains whose area is about 65,000 km2. These lands are categorized among highlands soil and suitable for grazing and agricultural mechanized production.

 

  Table 5: Jebel Marra's soil composition

Nitrogen(ppm)

phosphoric acid

Degree  of nitrogen

clay

silt

Fine sand

Coarse sand

gravel

Depth in inch

785

750

7.5

19

20

30

4

5

0 -11

Source: Saudi, 1983

 

 

 

Desert areas soil:

In an accurate sense, desert areas are devoid of soils, in that the prevailing climate of deserts does not normally allow formation of full-grown soils (mature profile). Therefore, all parts of desert zones are devoid of mature soils with suitable depths and definite vertical horizons. In this respect, desert area earth surface is mostly covered with dismantled sands or with a mixture of sand and gravels or, otherwise, it stands out in a rocky form free of any dismantled assemblages, except for rivers, beaches/banks, seasonal valleys and inside oases, some of which may experience the formation of full-grown soils or they are apt to be developed through local environmental conditions surrounding them. Unluckily, such disability in providing necessary conditions for the formation of mature soils is witnessed in all parts of the northern region, vast areas of northern ends of the central region and in both eastern and western regions of Sudan (Saudi, 1983). These types of soils are shown in Figure 3.

 

 

 

 

Agricultural utilization of soil in Sudan:

Agriculturally, the clay soil of central Sudan is a more important one. This soil extends from far eastern Sudan (Kasala district) through central Sudan, over to the districts of South Kordofan. Geologically, it is known as a cracked clay soil because it is cracked in the drought season, and this enables the soil to maintain its porosity and, thus, a measure of water and air which improve its biological and physical characteristics. The soil is utilized for irrigated farming in the districts of Gezira and Khashm al-Qirba. Also, there are large areas east of the Blue Nile, i.e. Gedarif and Semsem and west of the Blue Nile, i.e. Dali and Mazmoum, where the soli is utilized for mechanized rainy farming for production of huge quantities of crops, particularly for maize, sesame and sunflower.

As to west of the White Nile, this soil is used for traditional farming for production of maize, sesame and groundnuts, besides cotton crop in Nuba Mountains districts. Alternatively, sandy lands in semi-desert areas in the states of North Kordofan and North Darfur are reserved for grazing, where natural plant grows to that end. On another hand, in the south of the two former states besides West Darfur State, sandy soil is known as Goz soil. Normally, the prevailing activity in these districts is rearing of cows in addition to production of some important cash crops like millet, sesame, groundnuts and melon seeds. More importantly, sandy soil, namely Goz soil, is considered as the main zone for production of gum Arabic which is drawn from the acacia tree.

In relation to laterite soil, 90% of its area was devolved to South Sudan (in the regions of Equatoria and Bahr el Ghazal). Accordingly, what has remained of this laterite soil in Sudan is only a small area mostly covered with trees and some pools are formed during autumn. So, these areas are left as nature reserves like Radum’s in the far south-west Darfur. For this reason, they are exploited in tourism aspects as well they are often utilized for the production of crops in some of the scattered parts of the land. Negatively, one of the characteristics of this soil is that it quickly loses its fertility in comparison with other types of soils in Sudan (Tothill, 1948).

 

 

Factors of soil deterioration in Sudan:

Deterioration of soil in Sudan is one of the problems acutely and immediately affecting natural, economic and social environments in Sudan. In a sense, the deterioration of soil means negative change inn the characteristics of soil, whereupon this leads to continual decline of its productivity. As a result, if this is not addressed and redressed, soil may once and for all lose its fertility and thus become desert. Hence, if soil is deserted and desertification takes place, this will have a negative effect on the country, given that agriculture represents the first productive sector in Sudan. In general, deterioration of soil is caused by an increase of requirements of inhabitants (population) who will be pressed to meet these needs through some actions which finally debilitate, consume and deteriorate soil resources. In this context, there are a number of reasons that led to deterioration of soil in Sudan, which are:

1. Over cultivation and improper farming:     

Over cultivation and improper farming involves farming in marginal areas, irrigation that helps increase salinity of soil, debilitation of soil and misuse of agricultural chemicals. By such definition, improper Farming spreads all over the country, mainly around towns and groundwater-irrigated areas.

Additionally, horizontal expansion of cultivation at the expense of grazing lands and forests is also an illegal farming technique, since conservation of the base of natural resources of soil; rainfall and local weather are not observed. Thoughtfully, the overriding concept that the cultivation of food and cash crops in vast areas is an optimum way for the utilization of soil is conflicting with the nature of environmental variance, from which Sudan enjoys the way it benefits from its location in the dry belt of the African continent. In this context, the gum Arabic belt in Sudan produces for the country and the inhabitants of the region greater economic proceeds of acacia trees than of any other cultivated crop. This is simply because acacia trees, without any notable agricultural inputs, produce the third cash crop in Sudan, i.e. gum Arabic, along with provision of fodder and appropriate environment for grazing. Moreover, acacia trees provide native inhabitants with firewood of dry branches and perennial trees. Still, the more important thing than what was mentioned is that this kind of utilization of soil brings about sustained development since the generosity of the land remains permanent and stable to the exclusion of desertification and the deterioration of the productivity of soil. This is merely an example, and there are large areas of marginal environments which are not appropriate for production of agricultural crops but for a short period after which they become desertified and maybe desertified once and for all (Abdel Latif, 1993). 

2. Decreased of tree cover:

Trees are cut off for several reasons, the main of which are improper expansion of farming and the production of charcoal, firewood, timber and building materials. The improper expansion of farming involves cultivation in marginal areas which are exposed to drought and erosion of soil. As well, it involves the expansion of mechanized rainy farming to the exclusion of other kinds of traditional production such as grazing, traditional farming and wildlife in the same area and without taking necessary measures for protection of the soil which is considered a pillar of agriculture. That is what happened in the late ‘1970’s and 1980’s when the then government distributed millions of acres for the benefit of foreign and national companies. In the interim, even unplanned cultivated area exceeded the planned area and, even worse, preparing ground for mechanized farming had been initiated by uprooting trees, burning them and/or selling them to producers of charcoal and firewood. In so doing, the local environment will be deprived of sustainability and denuded of elements of soil protection and, likewise, this leads to continuous deterioration of soil productivity due to erosion of soil while making crop incapacitated against bad climate conditions such as drought, high temperature and low humidity, besides diseases and insects that spread out without natural barriers.  

 3. Mismanagement of grazing lands:    

In fact, misdistribution of water stations and expansion of farming at the expense of grazing lands are two factors that help increase the numbers of livestock whereby pastures will be incapacitated and their grass overgrazed. Furthermore, uncontrolled spreading of fires leads to destruction of tree cover. Therefore, both overgrazing and spreading of fires lead to a retraction of vegetation as well as disintegration and erosion of soil with the inevitable result of desertification.

4. Lack of tree belts :   

More than half the area of Sudan lies in desert or semi-desert zones susceptible to mobile sands and soils. So, with the absence of tree belts both farming and housing areas will be also susceptible to continuously being covered with sands and soils whereupon they will be transformed into inappropriate districts of farming and housing and this leads to its eventual desertification.

These movable sands and soils don’t only threaten farming and housing areas but also threaten the main water resources like the River Nile and its tributaries which are apt to be buried by sand dunes. At any rate, this is one of the more dangerous environmental problems of Sudan because it threatens the whole Northern State, more than two thirds of areas of the states of Kordofan, Darfur, Khartoum, Eastern region and more than half area of Central State besides the River Nile course. 

 

 

References:

  1. Arabic references:

-- Sayyad, Mohammed Mahmoud. Saudi, Mohammed Abdel Ghani (1983) – Sudan, a study on physical situation, human entity and economic building – Anglo-Egyptian library – Cairo. 

-- Saudi, Mohammed Abdel Ghani (1983) – Geography of the Sudan -- Anglo-Egyptian library – Cairo. 

-- Al Tom, Mahadi Amin and Abdel Rahman, Babikir Abdalla (2010) – Aspects of natural and human geography of Sudan.

-- Abdel Latif, Isa Mohammed (1993) – Environmental perspective of development in Sudan – Sudanese society for protection of environment.

-- Abdel Rahman, Siddig Nurein Ali and Magzoub, Al Tahir Ahmed (2007) – Agricultural utilization for the soil of wadi Kutum basin – North Darfur, Sudan.

--Abdel Rahman, Siddig Nurein Ali (2007) – Georphology of wadi Kutum basin, unpublished PhD thesis – University of Nilein, Faculty of Arts.

-- Abid, Abdel Gadir and others (2004) – Basics of Ecology: Wa’il House for printing and publication, Oman, Jordan.

 

  1. English references:

-- Harrison, M.N. and Jackson, J.K. (1985) – Ecological classification of the 

   Vegetation of the Sudan       

-- Andrew, G. (1948) – Geology of the Sudan: in Agriculture in Sudan ed.

    S.D. Tothill Landan.  

-- Green, H (1948) – Soils of Sudan: In Agriculture in the Sudan ed. J.D    

    Tothile Landan.

-- Sharts, H.L. and Marbut C.F. (1923) – The Vegetation and Soils of Africa   

    Res.  Ser .Amer geagr SOC No 13.                                                           

-- Tothill, G.D, (1948) – Agriculture in Sudan, London.

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