Soil erodibility and rainfall erosivity are two important physical factors that affect the magnitude of soil erosion (Lal and Elliot, 1994). Soil erodibility, the resistance of the soil to both detachment and transport, is a function of soil texture, structure, permeability, organic matter content and also the management of the soil (Hudson, 1995; Morgan, 1995). Rainfall erosivity is the aggressiveness of the rain to cause erosion and is a function of the physical characteristics of rainfall (Morgan, 1995). It has been established that a few, very intense rainfall events are responsible for the largest part of the soil erosion and sediment delivery (Gonz ́alez-Hidalgo et al., 2007). It is also an established fact that soil erodibility represents the effects of soil properties and soil profile characteristics on soil loss by erosion (Romkens et al., 1996 cited in Fufa et al., 2002). However, the quantification of these two factors stated earlier is the basis to an understanding of soil erosion. To tackle the problem of gully erosion, the concept of rainfall erosivity as an erosion catalyst and predictor of erosion risk has been applied in some parts of Ghana (Baffour et al, 2012), in the Africa continent by Vrieling et al (2014) and in Europe by Panagos et al, (2015).
Soil erodibility is an estimate of the ability of a soil to resist erosion based on the physical and chemical characteristics of that soil. Generally, soils with faster infiltration rates, higher levels of organic matter and improved structure have a greater resistance to erosion (Wall etal.1987). Sometimes a soil with relatively low erodibility factor may show signs of serious erosion, yet a soil could be highly erodible and suffer little erosion (Nyakatawa et al. 2001).
This is because soil erosion is a function of many factors as stated in the universal soil loss equation (USLE). These factors include rainfall factor (R), soil erodibility factor (K), slope length (LS), crop factor (C) and control practice factor (P). This is represented in the universal soil loss equation as (Renard et al. 1997); A = R K LS C P.
13 According to Morgan (2001), one of the importance of quantifying the erodibility of soil materials is the prediction of the possible occurrence of gully erosion. The depth of erosion is very often determined by the soil depth. Soils below the plough layers are often compact and less erodible. Rills will develop in areas where the bedrock close to the surface is not resistant and if the parent material is unconsolidated such as sands and gravel (Morgan, 2001). Also erodibility is influenced by the organic and chemical constituents of the soil / rock as this determines the stability of the aggregates (Relf, 2001 and Morgan 2001). On the other hand, erosivity is a function of rainfall, a natural phenomenon which is outside human control and manipulation. Rainfall intensities can be high in Southeast Nigeria, thus the high incidence of gullies in this part of the country. Erodibility and erosivity are indispensable factors of soil loss (Bryan, 2000; Singh and Khera, 2009) as can be clearly seen in the USLE.
The interest of this study is on the stabilization of the soils / rocks prone to gullying in order to reduce their detachability (erodibility) and reduce the impact of the rainfall intensity (erosivity) on them. Therefore two major aspects of soil loss were harnessed and used as foundation concepts for this study, the concept of resistance and resilience.
Soil Resilience
Soil resilience has been defined as the capacity of a soil to recover its functional and structural integrity after a disturbance (Herrick and Wander 1998; see also Pimm 1984; Lal 1993a, 1993b, & 1994; Blum and Santelises 1994; Sombroek 1994; Blum 1998). This definition is consistent with the broader use of resilience as defined by Webster, which is
"the capability of a strained body to recover its size and shape after deformation caused especially by compressive stress." However, others have defined soil resilience as the capacity of a soil to resist change caused by a disturbance (Rozanov 1994; Lang 1994). This concept of "resistance to change," which differs from resilience, is an important component of ecosystem stability (Tilman and Downing 1994) and will be defined as a separate concept in this paper.
Functional and structural integrity are defined as a soil's capacity to perform vital soil functions such as those proposed by Karlen et al. (1997):
14 (i) Sustaining biological activity, diversity, and productivity;
(ii) Regulating and partitioning water and solute flow;
(iii) Filtering, buffering, degrading, immobilizing, and detoxifying organic and inorganic materials, including industrial and municipal by-products and atmospheric deposition;
(iv) storing and cycling nutrients and other elements within the Earth's biosphere; and (v) Providing support of socioeconomic structures and protection for archeological
treasures associated with human habitation. Structural integrity is linked to soil function and deals with the physical arrangement of primary soil particles and their aggregation.
Walker et al (2004) describe four critical aspects of resilience: latitude, resistance, precariousness, and panarchy. The first three can apply both to a whole system or the sub-systems that make it up.
1. Latitude: the maximum amount a system can be changed before losing its ability to recover (before crossing a threshold which, if breached, makes recovery difficult or impossible).
2. Resistance: the ease or difficulty of changing the system; how “resistant” it is to being changed (Walker et al, 2004).
3. Precariousness: how close the current state of the system is to a limit or “threshold”
(Peterson et al, 1998).
4. Panarchy: the degree to which a certain hierarchical level of an ecosystem is influenced by other levels. For example, organisms living in communities that are in isolation from one another may be organized differently from the same type of organism living in a large continuous population, thus the community-level structure is influenced by population-level interactions.
15 Closely linked to resilience is adaptive capacity, which is the property of an ecosystem that describes change in stability landscapes and resilience (Gunderson, 2000). Adaptive capacity in socio-ecological systems refers to the ability of humans to deal with change in their environment by observation, learning and altering their interactions (Folke et al, 2002).
A disturbance is broadly defined as any event that causes a significant change from the normal pattern or functioning of an ecosystem (Forman and Godron, 1986). Whether an event is considered to "cause a significant change from the normal pattern or functioning"
depends on the temporal and spatial scale of interest. At geologic time scales, nearly every event can be considered to contribute to normal functioning. Many types of perturbations are actually necessary for ecosystem function. For example, formation of a single earthworm burrow is clearly a disturbance at the scale of the root system of a grass tussock when considered in terms of the lifespan of the plant. But, it may be considered part of the "normal pattern" at the field scale. A wide variety of disturbances are included in this broad definition, including those that are primarily natural in origin and others that are largely or wholly anthropogenic. Natural disturbances and causes of disturbance include fires, earthquakes, floods, landslides, and high-intensity storms. Nearly all human activities associated with land management and use can be classified as "disturbances," including logging, grazing, urban and industrial development, recreation, and annual cropping.
Agriculture itself is one of the greatest sources of stress and disturbance to the environment (Brussaard 1994). Common disturbances or stresses associated with agriculture include heavy load as a result of vehicular traffic; tillage; application of fertilizers and pesticides; and removal or exclusion of competing plant species (Bezdicek et al. 1996).
Soil Resistance
Soil resistance, which is distinguished from soil resilience, has been defined as the capacity of a soil to continue to function without change throughout a disturbance (Herrick and Wander 1998; Pimm 1984). The magnitude of decline in the capacity to function defines the degree of resistance to change. A small decline indicates a high resistance, whereas a relatively large decline indicates a low resistance to change throughout a disturbance.
Williams and Chartres (1991) distinguish the difference between the resilience and resistance
16 concepts with respect to soils: "The magnitude of the decline in the capacity of the soil to function (resistance) and the rate of recovery or the elasticity (resilience) are two key measures of sustainability."
The distinction between resistance and resilience can be illustrated further in an example using soil functions related to soil physical properties on an annual time scale. In temperate regions, many surface soils are resilient with respect to porosity changes following compaction. Frost action may serve as a recovery process. However, some soils are inherently resistant to porosity changes following compaction. These resistant soils can maintain their functioning capacity (e.g., hydrologic functions) at a higher level throughout the year than those that have a lower resistance but are resilient on an annual time scale.
The distinction between soil resistance and soil resilience also depends on the temporal scale of interest. For example, the capacity of soil to supply nutrients to plants (a soil function) can be degraded by nutrient removal through plant uptake into biomass. For short periods of time, the soil solution near the root surface can be replenished from supplies on exchange surfaces and through diffusion. Temporary reductions in nutrient availability may result.
However, because these reductions are generally undetectable in plant growth measurements, the soil is perceived to be resistant from a functional perspective. As exchangeable nutrients decline and diffusion distances increase, resistance to nutrient depletion processes decline and the importance of mineralization (recovery mechanism) increases. If mineralization rates are high, measurements made on an annual basis may indicate little or no change in nutrient availability, again suggesting high resistance, even though nutrients were limiting at one or more times during the year. The proportion of the year during which nutrients are limiting, then, depends on the rate of recovery or resilience.
The importance of soil resistance in sustainable management was demonstrated by Davenport et al. (1998) for Pinyon-Juniper (Pinus edulis, P. monophylla, P. cembroides-Juniperus monosperma, J. osterosperma, J. Occidentialis, J. scopulorum, J. deppeana) ecosystems in the western U.S. They found dramatic variation in the capacity of pinyon-juniper ecosystems to resist soil erosion as a result of reductions in ground cover. In sites that exhibited low resistance, a small reduction in ground cover was needed before a threshold
17 was reached and soil erosion increased dramatically. In sites with high resistance, thresholds sometimes did not exist, and if one existed, a large reduction in ground cover was needed before the threshold was reached.
Soil stabilization is a means of upgrading the engineering properties of soils to provide maximum return on investment in road construction or improvement. Although there is no precise definition of stabilization, a soil is said to be stable
when it resists change, particularly mechanical change, over long periods of time.
Conversely, an unstable soil is one that breaks up, shifts, or sinks when acted upon by the normal forces of load in an unfavorable slope (topography), under a critical climate or agent of erosion or mass movement, resulting in a premature deterioration of the surface area.
Three mechanical aspects are critical to a soil's ability towithstand loads: {a) cohesion, {b) friction, and (c) density.Cohesion refers to the ability of soil particles to stick together.Friction refers to the ability of particles to resist shifting oftheir position relative to each other. Density is the weight ofa material to its bulk. These three aspects when improved in a soil, improves the soil‟s resistance thus making the soil more stable.
The understanding of these concepts brings to mind the mechanism of the strength and weakness of a soil and how the soil can respond to erosion forces, thus creating a strong background to the actual exploration of the issues of grouting and soil stabilization as measure to control gully erosion through the improvement of the soil‟s resistance to erosion forces.