**7. The influence of soil factor (erodibility)**

The erodibility of the soil is defined as the vulnerability or susceptibility of the soil to erosion. It is a measure of a soil's susceptibility to particle detachment and transport by agents of erosion. Igwe [20] remarked that a number of factors such as the physical and the chemical properties of the soil influence erodibility. In southeastern Nigeria, the nature and the long weathering his‐ tory of the soils parent material evident in the dominance of the clay mineralogy by non-ex‐ panding minerals and low soil organic matter concentration due to high mineralization rates and excessive leaching of nutrients could be linked to the worsening situation. The highly weathered soils contain high concentrations of Fe and Al oxides. Inappropriate land use and soil management options are also a common feature of agriculture in the region. Anthropogen‐ ic factors often combine to weakened soils to produce severe gullies. The soils are hence loose and slumps under high intensive rainfall that renders them easily detachable. Some of the soils have the tendency to slake and form seals under such intense rainstorms thereby resulting in considerable runoff and soil erosion. The soil erodibility factor has since been recognized as a contributing factor to soil erosion hazard. The erodibility of the soils in terms of soil indices that predict or promote soil erosion will be elaborated on. The contributions of soil factors to soil erosion in Nigeria have variously been discussed [21, 20, 22]. Igwe et al. [21] found that the soil clay content, level of soil organic matter (SOM) and sesquioxides such as Al and Fe oxides, clay dispersion ratio (CDR), mean-weight diameter (MWD) and geometric-mean weight diameter (GMD) of soil aggregates all influence soil erosion hazards in southeastern Nigeria. SOM, Al and Fe oxides control dispersion and flocculation of the soils. In the event of very aggressive rainfall, the soil inherent properties often combine with the physical forces of rainfall to pro‐ duce soil erosion in the soils.

**Figure 3.** Gully cutting

**6. The influence of geology**

162 Research on Soil Erosion Soil Erosion

The general influence of lithology on soil erosion processes is manifest directly by the resist‐ ance of the denuded bed rocks exposed to the flow of water and affected by the character of parent materials whose properties are given by the bed rock. The direct effect of bedrock is also manifest in the properties of the soil forming parent materials which conditions the principal properties. Some geological materials are vulnerable than others to aggressive en‐ ergy of the rainfall and runoff. High erosion risks match with units of weak unconsolidated geological formations. This is more pronounced when such geological units coincide with

In Nigeria, Ofomata [1] classified the potential erosion susceptible areas based on underly‐ ing geology. He indicated that areas of high susceptibility correspond to geological regions of weak unconsolidated sandy formations while least susceptible areas are within the con‐ solidated tertiary to recent sediments. Also in southeastern Nigeria, the classical gully sites are located in the False-bedded sandstone, Coastal Plain sands, Nanka Sands and the Bende-Ameki Formations. These are all sandy formations which have more gullies than their Shale formation counterparts. In these formations, there exist the sites of worst catastrophic soil erosion in the whole of sub-Saharan Africa. The geology therefore plays direct and indirect influence on the gully formation. The indirect effect is on the soil formation and the nature of soil which contribute significantly to erosion processes. The influence of soil process on soil erosion often referred to as erodibility is the subject of discussion in the next section.

The erodibility of the soil is defined as the vulnerability or susceptibility of the soil to erosion. It is a measure of a soil's susceptibility to particle detachment and transport by agents of erosion. Igwe [20] remarked that a number of factors such as the physical and the chemical properties of the soil influence erodibility. In southeastern Nigeria, the nature and the long weathering his‐ tory of the soils parent material evident in the dominance of the clay mineralogy by non-ex‐ panding minerals and low soil organic matter concentration due to high mineralization rates and excessive leaching of nutrients could be linked to the worsening situation. The highly weathered soils contain high concentrations of Fe and Al oxides. Inappropriate land use and soil management options are also a common feature of agriculture in the region. Anthropogen‐ ic factors often combine to weakened soils to produce severe gullies. The soils are hence loose and slumps under high intensive rainfall that renders them easily detachable. Some of the soils have the tendency to slake and form seals under such intense rainstorms thereby resulting in considerable runoff and soil erosion. The soil erodibility factor has since been recognized as a contributing factor to soil erosion hazard. The erodibility of the soils in terms of soil indices that predict or promote soil erosion will be elaborated on. The contributions of soil factors to soil erosion in Nigeria have variously been discussed [21, 20, 22]. Igwe et al. [21] found that the soil

medium to long and even very long slopes with marked gradients.

**7. The influence of soil factor (erodibility)**

Erodibility varies with soil texture, aggregate stability, SOM contents and hydraulic proper‐ ties of the soil. Igwe [22] claimed that the soil dispersion ratio (DR) and the clay dispersion ratio were good indices of erodibility. The soils with high water-dispersible clay (WDC) in southeastern Nigeria often create problem in that in tilled land use, mud flow and soil loss from runoff cause major alteration in the stream flow within watersheds causing severe en‐ vironmental challenges. Soil crusting, sealing resulting from aggregate breakdown are sec‐ ondary problems arising from deposited sediments. The large particle sizes are resistant to transport because of the greater forces required to entrain these large particles while the fine particles are resistant to detachment because of their cohesiveness. Aggregate stability and associated indices have been shown to be most efficient soil properties that predict the ex‐ tent of soil erosion.

In other parts of the world the use of aggregate stability indices in predicting soil erodibility have shown reliable information on the extent and degree of soil erosion [23, 24]. In Western Europe, Le Bissonnais [25] indicated that the mean-weight diameter (MWD) of soil aggre‐ gates was a very reliable soil property that could show the erosion potential of the soil in the sense that MWD predicts soil erodibility. Therefore aggregate stability and MWD are very reliable properties in explaining, quantifying or predicting soil erosion and other soil prob‐ lems such as crusting and sealing.

therefore are more effective than aluminium hydroxide in cement effectiveness except for

Gully Erosion in Southeastern Nigeria: Role of Soil Properties and Environmental Factors

http://dx.doi.org/10.5772/51020

165

An important factor which contributes significantly to soil erosion problem in southern Ni‐ geria is anthopogenic influence arising from misuse of land. Poor farming systems have con‐ tributed to collapse of soil structure and thus encouraging accelerated runoff and soil loss due to erosion. In the event of uncontrollable grazing caused by the nomads has resulted in deforestation of the landscape while indiscriminate foot paths created on the landscape has helped the incipient channels on the landscape to form. These channels eventually metamor‐ phose to gullies especially when they are not checked at inception. Road constructions in‐ cluding uncontrolled infrastructural developments have contributed significantly in gully developments. Some road networks under construction have been abandoned in the region

soils undergoing frequent oxidation-reduction processes.

**8. Anthropogenic influence**

due to gully formation.

**Figure 5.** Gully about cutting an asphalt surfaced road

**9. Identification of gullies and erosion sites**

Soil erosion sites in southeastern Nigeria have been identified through various methods. In the 1960s and 1970s gullies were enumerated through natural resource surveys but this method proved to be very cumbersome and often do not actually represent the actual situa‐

Again other soil properties encourage structural failure, sliding and mass movement of soils. These soil factors are the mineralogy of the clay and even the soil chemical properties. The sta‐ bility of the soil mass is therefore depended on the clay minerals present. Illite and smectite more readily form aggregates but the more open lattice structure of these minerals and the greater swelling and shrinkage which occur on wetting and drying render the aggregates less stable than those formed from kaolinite. Soils in which either kaolinite or illite clay predomi‐ nates but contains small amounts of smectite are easily dispersive. Smectitic soils are more erodible than the soils that contain only small amount of smectite. Conversely, soils that do not contain smectite are more stable, less erodible and less susceptible to seal formation.

**Figure 4.** Gully site in association with interill and rill erosion

The sodium dithionite extractable Fe oxide is a soil chemical property which relates signifi‐ cantly with erodibility of the soil. This particular property affects the soil structure and the soil fabric, often being responsible for the formation of soil aggregates and cementation with other major soil components [26, 27]. The mechanism of aggregation of soils in southeastern Nigeria in the presence of Fe (Hydr) oxide has been demonstrated [8, 26, 27]. The presence of OH-Al polymers may lead to a reduction in the swelling and expansion of clay particles by bonding adjacent silica sheets together and by displacing interlayer cations of high hy‐ dration power and thus promoting aggregation. Well crystallized aluminium hydroxide may also be able to act as cementing agent in acid soils such as in southeastern Nigeria but its magnitude may be negligible as compared with non-crystalline materials. Iron oxides therefore are more effective than aluminium hydroxide in cement effectiveness except for soils undergoing frequent oxidation-reduction processes.
