Evaluation of surface soil quality and land suitability for agricultural soils affected by soil erosion

doi:10.21203/rs.3.rs-4817075/v1

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Evaluation of surface soil quality and land suitability for agricultural soils affected by soil erosion

https://doi.org/10.21203/rs.3.rs-4817075/v1

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Globally, soil erosion is an important environmental factor affecting most agricultural lands, particularly dryland areas where vegetation cover is limited. This study, based on two assessments, was aimed at addressing the physical impact of soil erosion in dryland areas of Kebbi State, Nigeria. Soil erosion was assessed from the visual appearance of different erosion types and their resulting deposition patterns. In the first assessment, six types of soil erosion were evaluated: sheet, rill, gully, winnowing, mass-movement and dry-mechanical. These six types of soil erosion have slight, moderate or severe degrees of impact. Additionally, eight different shapes of rill and gully erosion were classified based on their physical appearance and the nature of their surface soil textures: C, I, J, S, U, V, X and Y. The V, U and Y categories described gully erosion under a uniform sand-silt soil texture, whereas the C, I, J, S and X categories described rill erosion under a sandy loam soil texture. In the second assessment, erosion was classified according to its category and degree of impact for soil quality and land suitability descriptions under the proposed specific visual soil erosion approach (VSEA). The physical soil quality classes Sq1, Sq2, Sq3, and Sq4 or Sq5 describe the surface soil conditions of the study sites at very slight, slight, moderate and highly moderate or severe degrees of impact, respectively. Ls1, Ls2, Ls3, Ls4 and Ls5 are the land suitability classes of good land, moderately good land, poorly good land, very poor land and bad land, respectively. Erosion significantly affected the potential productivity of agricultural soils (226 cm to 555 cm width and 116 cm to 213 cm depth) in the study area. However, important social factors such as poverty, overgrazing, lack of management skills, corruption and poor environmental government policy likely contributed significantly to the initiation and occurrence of surface soil erosion at the affected sites. It was very clear that the physical surface soil was damaged and that sustainable management was necessary.

Soils

Evaluation

Soil erosion

Soil quality

Land suitability

In the Sudan savannah zone of Kebbi State, Nigeria, an important part of the dryland area has been affected by different forms of soil erosion [1]. Farmers from rural areas have voiced complaints regarding poor surface soil quality and soil fertility due to erosion and climate change in the region [2]. The global impact of soil erosion and its dimensional effects on land quality and food security are factors that must be considered when evaluating soil erosion impacts in a given area [3]. This evaluation has been recognised as a mechanism useful for understanding sediment transportation and how it should be managed under agricultural conditions [4]. Erosion causes anxiety, moves soil particles and damages surface soil by creating channels of different shapes and sizes [5]. Sedimentation decreases soil structural quality and causes a significant decrease in soil fertility [6]. Soil erosion is a global concern and is among the most serious environmental factors affecting environmental, soil and food security [7]. Examining the physical and quantitative impacts of soil erosion is a key to sustainable soil management [8]. Assessing how fast surface soil erosion occurs is helpful for future planning for the conservation and management of soil properties [9, 10]. To ensure agricultural sustainability, it is important to know how to evaluate soil erosion accurately [11]. This is primarily because the impacts of soil erosion on soil quality and land suitability create many imbalances in soil properties and soil processes [12]. Soil quality and land suitability play vital roles in ensuring better agricultural productivity [13]. Erosion decreases the quality and suitability of soil, leading to poor agronomic production and food insecurity [14]. Evans [11] noted that there is a need for field-based assessment and monitoring of soil erosion on cultivated land in affected localities. This is important because of the need to maintain surface soil cover, control soil erosion and runoff impacts, and improve soil quality, soil fertility, and soil function for sustainable agricultural production in the savannah region of northern Nigeria [2, 15]. This study was established with the specific objective of assessing and evaluating the soil quality and land suitability classes in an area affected by soil erosion in the Sudan Savannah Zone, Kebbi State, Nigeria.

2.1 Attributes of the study sites

The study was conducted in the dryland region of Kebbi State, Nigeria, covering six district areas within 12°46”N latitude and 4°7”E longitude. Prior to the commencement of the assessment of soil erosion in the field, 6 sites physically affected by erosion were selected following a specific field tour around the six district areas visited: Argungu, Birnin Kebbi, Bui, Fakon Sarki, Kamba and Tngar Dangwari (Fig. 1). These 6 sites were selected based on surface land resources relevant to agricultural activities in the study area. The specific surface land resources considered during the tour were cultivated lands, grazing lands and crops or farming activities. Evans [11] noted that there is a need for field-based assessment and monitoring of soil erosion on cultivated land in affected localities. This is important because of the need to maintain surface soil cover, control soil erosion and runoff impacts, and improve soil quality, soil fertility and soil function for sustainable agricultural production at the study sites [8]. The farmers in the region depend largely on agriculture as a major source of their livelihood. Therefore, the affected sites must be visually monitored and assessed according to the degree to which soil erosion impacts occur.

The vegetation cover of the study sites is mainly bare and characterised by few annual grasses and shrubs, which are short-length, dwarf (0.3 m – 2 m height) with long, thin, fine leaves growing wild and bushy plants of varying sizes and shapes (Table 1).

Table 1

Cropping, soil type and management characteristics of the study sites
Study Site	Vegetation type (m = metre)¹	Dominant soil type²	Agricultural land use²
Argungu	Shrubs (0.3– 1.2 m)	Aridisols	Intercropping
Birnin Kebbi	Shrubs (0.5–1.2 m)	Aridisols	Mono-cropping
Bui	Grasses (0.5–1.6 m)	Aridisols	Mono-cropping
Fakon Sarki	Grasses (0.3–0.6 m)	Anthrosols	Mono-cropping
Kamba	Shrubs (0.6–0.9 m)	Anthrosols	Intercropping
Tungar Dangwari	Shrubs (0.3–1.1 m)	Aridisols	Mono-cropping
¹Field-visit and survey (2008/2023); ² According to the FAO and USDA classification

The soil types fall under the USDA classification order ‘Aridisol’ and FAO classification order Anthrosols, which are physically low in organic matter (Table 1). The common attribute of the Anthrosols in the study sites is surface modification due to the addition of different organic materials by the landowners. The surface soil physical properties for the study sites are given in Table 2. However, all the sites have arid (dry) Sahelian climate conditions with monthly temperatures between 30°C and 45°C and annual rainfall of 650 mm/year to 850 mm/year based on the state climate data record. The major farming activities are monocropping of millet (Pennisetum glaucum) and intercropping of millet and cowpea (Vigna unguiculata) and of millet, cowpea and sorghum (Sorghum bicolor) annually (Table 1).

Table 2

Visual assessment of the surface soil properties at the study sites
Study Site	Soil texture*	Soil structure*	Soil consistency*
Argungu	Sandy loam	Grain	Slightly loose
Birnin Kebbi	Sandy loam	Grains	Loose
Bui	Sand-silt	Grains-massive	Very-loose
Fakon Sarki	Sandy loam	Grain	Loose
Kamba	Sandy-loam	Grains	Loose
Tungar Dangwari	Sand-silt	Grains-massive	Very-loose
*Classification according to USDA-NRCS [16]

2.2 USDA-NRCS and FAO International classification system

In the field, an international classification system of soil erosion was used to classify erosion at the affected sites according to type, category, shape and degree of physical impact (Table 3: [16, 17]). This international classification system is chosen because of its global appropriateness for different geographical locations where erosion is found to be damaging to surface soil (FAO, 2006).

Table 3

Classification used to describe surface soil erosion at the study sites
		a). Classification of erosion, by degree [17]
S	Slight	Some evidence of damage to surface horizons. Original biotic function largely intact.
M	Moderate	Clear evidence of removal of surface horizons. Original biotic functions partly destroyed.
V	Severe	Surface horizons completely removed and subsurface horizons exposed. Original biotic functions largely destroyed.
E	Extreme	Substantial removal of deeper subsurface horizons (badlands). Original biotic functions fully destroyed.
		b). Classification of erosion, by category [16]
Water erosion	Sheet Rill	Uniform removal of soil in thin layers from sloping land Detachment and transport of soil by concentrated flow of water
	Gully	An advance stage of rill erosion, deep like drainage channels
Wind erosion	Winnowing Mass-move	Soil detachment and transport by strong air (wind) Soil detachment and transport by strong air (wind)

The international classification system was applied to the study areas from the general overview of the physical and biophysical conditions of the affected sites during the field tours in 2008 and 2011. Figure 2 shows an exemplary relationship between the sample site affected by erosion and the broader landscape in the study area. From the general oversight of the study area, it is understood that the physical and biophysical functions of the surface soil in the affected portion are completely damaged – rill and gully erosion occurred. Deeper channels are caused by gully erosion, whereas smaller channels are caused by rill erosion (Fig. 2).

Therefore, the sites were carefully assessed and classified by direct visual assessment. The descriptions of each type of soil erosion in the USDA-NRCS [16] and FAO [17] were also used to further understand soil erosion and its categories at each site. This has also contributed to the understanding of the differences between areas affected by erosion slightly and those affected moderately or severely. Thus, wind erosion was assessed by observing how soil particles were typically moved by wind during the hammattan (dry) period in the months of January and February. During these two months, the nature and conditions of wind in the dryland areas of northern Nigeria were considered devastating and destructive due to poor vegetation cover [2]. Soil particles detached and transported by air in a strongly circulated manner commonly known as ‘guguwa’ in the Hausa language are classified as winnowing. Soil particles that move and are transported by wind in a uniform manner, commonly known as ‘tasowar hamada’ in the Hausa language, are classified as mass movements. Water erosion was assessed as sheet, rill or gully based on the nature and condition of the frequency of soil erosion in the field via direct visual assessment. However, with the visual assessment method described below, the classification of the physical impact of soil erosion was further extended at each site according to the observed degree of physical damage.

2.3 Visual Soil Erosion Approach

To supplement the assessment of soil erosion by degree and category, a second more detailed visual assessment was made according to the surface soil conditions of the study sites. This approach is more useful for indicating the potential for physical soil erosion damage [6], although it provides little information on the actual quantitative impact [18]. Nevertheless, field-based assessments and monitoring of water erosion according to the actual extent of potential physical damage are needed [19]. This is important, particularly in sub-Saharan Africa, where daily soil erosion modelling is needed because of poor vegetation cover, climate change impacts and environmental insecurity [3]. Soil erosion is best monitored and described by visiting the selected sites and making a visual assessment of how erosion damage can be related to the condition of the soil for a particular agricultural use [20]. Therefore, soil erosion was assessed under a visual soil erosion approach (VSEA) adapted for the first time to explain and define the extent of the actual potential physical impact of soil erosion at the study sites.

The principles used in VSEA are the same as those used in all visual assessment methods [21, 22], but include some specific classes for describing the surface soil quality and land suitability conditions of the study sites (Table 4). These principles of visual assessment are direct evaluations of surface soil conditions by the naked eye and descriptions of surface soils according to their current status in the field [23]. Subsequently, the physical soil quality classes or P-Sq were used in the VSEA to describe the ability of soil to sustain production (physically) or to function in agricultural production under a given degree or category of soil erosion impact in the field. Similarly, the land suitability classes or P-Ls were used to describe the physical quality of surface land conditions (not affected by erosion) around the erosion-affected areas for crop production. These two definitions are useful for determining the suitability and potential of surface soil and land affected by erosion and sediment transport for a wide range of agricultural activities [24].

Table 4

Shapes, soil quality (P-Sq) and land suitability (P-Ls) description guide
a). Shape	Type	Upper case letters are used – I, J, S, U, V, X, and Y
b). P-Sq¹	Sq1	Very small-size channel of sheet erosion: 0–5 cm width and depth
	Sq2	Small-size channel of sheet erosion: 6–10 cm width and depth
	Sq3	Small-size channel of rill erosion: 11–20 cm width and depth
	Sq4	Large-size channel of rill erosion: 21–30 cm width and depth
	Sq5	Gully surface erosion: >30 cm width and depth
c). P-Ls²	Ls1	Good land: few indication of very small-size channels of sheet in the study sites at that time of assessment in the field
	Ls2	Moderately good land: few small-size channels of sheet erosion in the study sites at that time of assessment in the filed
	Ls3	Poor land: 20% of the site is affected by small channels of rill in the study sites
	Ls4	Very poor land: >20% of the site is affected by large channels of rill in the study sites at that time of assessment in the field
	Ls5	Bad land: significant portion of the land is affected by gullies in the study sites
^{1, 2}P-Sq and P-Ls classes are described based on VSEA in the field

The sets of P-Sq and P-Ls classes used under the VSEA at the study sites are given in Table 4. There are five sets of soil quality classes (P-Sq = Sq1, Sq2, Sq3, Sq4, and Sq5) related to the intensity of erosion and five sets of land suitability classes (P-Ls = Ls1, Ls2, Ls3, Ls4, and Ls5) related to the frequency of erosion features, indicating the suitability of each affected site for crop production according to the general background knowledge of soil and land quality discussed by Lal et al. [25]. The authors described the surface soil conditions of the study sites very well. The sets of soil quality and land suitability classes are derived based on the current status of surface soil conditions during the field tour. Therefore, the relative importance of P-Sq and P-Ls is considered according to the specific surface soil conditions in the eroded soils of the assessment sites based on quantitative impacts measured in the field. The measurements of the P-Sq were made using a measurement tape, and the data were recorded in centimetres (cm) and meters (m). The width (cm), depth (cm) and length (m) of the rill and gully erosion were measured.

Soil quality classes are attributes of surface soils and represent a specific soil area for a specific kind of erosion impact. Sq1, Sq2, Sq3, Sq4 and Sq5 refer to very small surface channels of sheet erosion, small surface channels of sheet erosion and surface channels of rill erosion, and large surface channels of rill erosion and gully surface erosion, respectively (Table 4). On the other hand, the land suitability classes are attributes of surface soils that can be suitable or not suitable for crop production. Ls1, Ls2, Ls3, Ls4 and Ls5 refer to good land, moderately good land, poorly good land, very poor land, and bad land, respectively (Table 4). Therefore, the difference between the P-Ls and P-Sq classes, as related to the first assessment of soil erosion by degree and category at the respective field sites, was assessed. The P-Sq classes are the substituted classes for different categories of water erosion but in a specified manner, whereas the P-Ls classes provide a historical description of land conditions according to the level of degree and extent of soil erosion damage at each of the study sites. Thus, P-Sq and P-Ls are proposed to help understand the extent of soil and land damage caused by soil erosion and the boundaries of their uses in the context of agricultural land use practices.

2.4 Sampling frequency and size area used under the VSEA

Plots were sampled by considering the frequency and length of soil erosion at each study site. Each plot is approximately 100 m × 100 m (1 hectare of land or 2.471 acres) based on historical records from the State Land Survey (SLS). However, in accordance with a specific field survey carried out at each site, the sampling frequency in the main affected areas was readjusted to a specified size according to the maximum length of the gully and rill erosion measured in the field (Table 5). The sites were preliminarily assessed to determine the type of soil erosion to be assessed (Table 5). The field measurement transects were made by random selection within the typical occurrences of the undersized portion of the rill and gully erosion in the field (Fig. 3). All the selected positions were directly measured using a measuring tape for length (L), width (W) and depth (d). A total of 10 selections were assessed at each site and coded accordingly. The codes are divided into two: (a) gully erosion codes: a-gully to j-gully and (b) rill erosion code: a-rill to j-rill (Table 5).

Table 5

General information for the sampling frequency of soil erosion at the assessment sites
Study site	Land¹ size (hectare ha)	Frequency of erosion²	Number of measurement and their typical codes in each study site	Length (m)
Argungu	1 ha	Yes	a-rill, b-rill, c-rill, d-rill, e-rill, f-rill, g-rill, h-rill, 1-rill, j-rill	5.68
Birnin Kebbi	1 ha	Yes	a-rill, b-rill, c-rill, d-rill, e-rill, f-rill, g-rill, h-rill, 1-rill, j-rill	7.9
Bui	1 ha	Yes	a-gully, b-gully, c-gully, d-gully, e-gully, f-gully, g-gully, h-gully, 1-gully, j-gully	10.6
Fakon Sarki	1 ha	Yes	a-rill, b-rill, c-rill, d-rill, e-rill, f-rill, g-rill, h-rill, 1-rill, j-rill	6.5
Kamba	1 ha	Yes	a-rill, b-rill, c-rill, d-rill, e-rill, f-rill, g-rill, h-rill, 1-rill, j-rill	7.2
Tungar Dangwari	1 ha	Yes	a-gully, b-gully, c-gully, d-gully, e-gully, f-gully, g-gully, h-gully, 1-gully, j-gully	9.1

¹State Land Survey Record (KARDA, 1997).

² By typical overseeing the soil erosion affected sites in the field according to geomorphic approach by USDA-NRCS [16]

2.5 Statistical analysis

A simple statistical analysis of variance (ANOVA) was carried out using a Microsoft Office Excel sheet to evaluate the relationship of the site in terms of how soil erosion affected the surface soil conditions (i.e., the occurrence of rill and gully erosion). The average, sum, maximum, minimum and standard deviation were used as the statistical parameters in this evaluation. Differences in terms of these parameters were used to describe the relationships between the sites affected by soil erosion in the study area.

3.1 Physical impact of soil erosion by category and by degree

The results of the visual assessment of the physical impact of soil erosion based on international classification systems are given in Tables 6 and 7. The classification of soil erosion by category/degree of impact classes is given in Table 6, whereas the classification of soil erosion in terms of its physical shapes and structures is given in Table 7.

Table 6

Physical impact of soil erosion by category at the study sites
Study Site	Water¹ Erosion	Wind Erosion	Anthropogenic² Erosion	Degree Class
Argungu	Sheet, Rill	Winnowing of sand	Dry-mechanical	Slight/Moderate
Birnin Kebbi	Sheet, Rill	Winnowing of sand	Dry-mechanical	Slight/Moderate
Bui	Rill, Gully	Winnowing of sand	Dry-mechanical	Moderate/Severe
Fakon Sarki	Sheet, Rill	Mass-movement	–	Slight/Moderate
Kamba	Rill	Mass-movement	Dry-mechanical	Slight
Tungar Dangwari	Gully	Winnowing of sand	–	Severe
¹Erosion by category; ²Tillage practices

Table 7

Typical shapes and structures of rill and gully erosion at the study sites
Site	Erosion	Structure	Texture¹	Description²
Argungu	Rill	C-shape	Sand-loam	Moderate
Birnin Kebbi	Rill	S-shape	Sand-loam	Moderate
Bui	Gully	V-shape	Sand-silt	Severe
Fakon Sarki	Rill	I-shape	Sand-loam	Moderate
Kamba	Rill	I-shape	Coarse-sand	Moderate
Tungar Dangwari	Gully	U-shape	Sand-silt	Severe
¹Classification according to USDA-NRCS [16]; ²see Table 3

Table 6 shows the classes of soil erosion by category and by degree: sheet, rill, gully and dry-mechanical; physical disintegration of soil particles due to unsuitable tillage practices (Fig. 4); and mass movement and winnowing of soil particles. Sheet, rill and gully are three categories of soil erosion classified under water erosion; however, according to the degree of physical impact of these three categories of water erosion, each category was classified into slight, moderate and severe degrees of impact. Wind erosion was also classified into slight and moderate impact classes (Table 6). For dry-mechanical erosion, moderate and slight impacts are classified as water erosion and wind erosion, respectively, at the study sites. Differences in these two degree impact classes of dry mechanical erosion are judged according to how water and wind move soil particles from one area to another during the rainy and hammatan (dry) periods at the study sites.

The movement of soil particles in wide water channels and slight surface impacts are regarded as sheet erosion. The extension of the sheet surface channel places the same area under moderate impact and rill erosion. Deeper cut and larger channels and gully erosion are classified as having severe degrees of impact. The differences accounted for by these three types of water erosion based on physical damage is very clear (Fig. 4). Sheet erosion noticeably affects only the upper surface soil layer, while rills affect both the upper and immediate subsurface layers, in contrast to gullies, which largely include subsurface soil layers (Fig. 4). The movement and transport of very light soil particles of sand by wind are common phenomena in dry periods at the study sites and are classified as winnowing.

The typical appearance of the shapes and structure of the rill and gully erosion detected by visual assessment are given in Table 7. These shapes more fully described the degree and physical impact class of each site. The shapes and structures of each soil erosion type described the outwards appearance and arrangement of the rills and gully channels at each study site. They also portrayed the frequency of soil erosion and the differences in its occurrence at an individual study site. Thus, the shapes of the erosion structures at the study sites vary considerably (Table 7). For example, the particles removed from U-shaped or V-shaped gullies are much larger than those from Y-shaped gullies (Fig. 5). Similarly, more soil particles were removed from the X-shaped and S-shaped rill channels than from the C-, J- or I-shaped channels. The X-shaped mode indicates that rill erosion occurs by producing an inter-rill surface through crossing another rill channel, forming an interlocked position where the two channels meet. This X-shaped structure of rill erosion has a complex frequency that differs from that of I-, S-, or U- and V-shaped erosion. However, the portraits of X-shaped rill erosion and Y-shaped gully erosion can be explained by both being formed by an inter-channel, positioned within the X-rill and Y-gully channels, but differ in the way that this position appears and widens.

3.2 Physical impact of soil erosion by VSEA

The results of the VSEA classification are given in Tables 8 to 13. The results provide detailed descriptions of soil erosion impacts in terms of physical soil quality and land suitability classes corresponding to slight, moderate and severe degrees of soil erosion impact at the study sites.

Table 8

Soil quality and land suitability classes at the Argungu site
Argungu: typical Measurement	Width (cm)	Depth (cm)	P-Sq Class	P-Ls class
a-rill	18	9	Sq3	Ls3
b-rill	17	6	Sq3	Ls2
c-rill	14	6	Sq3	Ls2
d-rill	14	7	Sq3	Ls3
e-rill	16	3	Sq3	Ls1
f-rill	12	5	Sq3	Ls2
g-rill	17	5	Sq3	Ls1
h-rill	10	6	Sq2	Ls2
i-rill	14	4	Sq3	Ls2
j-rill	19	3	Sq3	Ls2

Table 9

Soil quality and land suitability classes at the Birnin Kebbi site
B/Kebbi: typical Measurement	Width (cm)	Depth (cm)	P-Sq Class	P-Ls class
a-rill	19	9	Sq3	Ls3
b-rill	19	6	Sq3	Ls2
c-rill	10	6	Sq2	Ls1
d-rill	10	6	Sq2	Ls1
e-rill	17	7	Sq3	Ls2
f-rill	18	6	Sq3	Ls2
g-rill	17	5	Sq3	Ls3
h-rill	13	4	Sq3	Ls2
i-rill	13	5	Sq3	Ls1
j-rill	19	8	Sq3	Ls3

Table 10

Soil quality and land suitability classes at the Bui site
Bui: typical Measurement	Width (cm)	Depth (cm)	P-Sq Class	P-Ls class
a-gully	253	55	Sq5	Ls3
b-gully	247	46	Sq5	Ls3
3-gully	375	155	Sq5	Ls4
c-gully	369	134	Sq5	Ls4
e-gully	500	143	Sq5	Ls5
f-gully	290	113	Sq5	Ls4
g-gully	290	61	Sq5	Ls4
h-gully	360	85	Sq5	Ls4
i-gully	405	94	Sq5	Ls5
j-gully	241	46	Sq5	Ls4

Table 11

Soil quality and land suitability classes at the Fakon Sarki site
F/Sarki: typical Measurement	Width (cm)	Depth (cm)	P-Sq Class	P-Ls class
a-rill	12	5	Sq3	Ls1
b-rill	12	5	Sq3	Ls2
c-rill	16	5	Sq3	Ls2
d-rill	16	6	Sq3	Ls2
e-rill	15	4	Sq3	Ls2
f-rill	16	7	Sq3	Ls2
g-rill	13	6	Sq3	Ls2
h-rill	13	8	Sq3	Ls1
i-rill	15	9	Sq3	Ls2
j-rill	14	9	Sq3	Ls3

Table 12

Soil quality and land suitability classes at the Kamba site
Kamba: typical Measurement	Width (cm)	Depth (cm)	P-Sq Class	P-Ls class
a-rill	14	9	Sq3	Ls3
b-rill	19	4	Sq3	Ls2
c-rill	13	7	Sq3	Ls3
d-rill	17	7	Sq3	Ls3
e-rill	15	7	Sq3	Ls2
f-rill	19	5	Sq3	Ls2
g-rill	17	3	Sq3	Ls3
h-rill	13	6	Sq2	Ls2
i-rill	13	4	Sq3	Ls2
j-rill	12	5	Sq3	Ls2

Table 13

Soil quality and land suitability classes at Tungar Dangwari
Tungar/D: typical Measurement	Width (cm)	Depth (cm)	P-Sq Class	P-Ls class
a-gully	466	143	Sq5	Ls4
b-gully	399	149	Sq5	Ls4
c-gully	344	143	Sq5	Ls4
d-gully	226	116	Sq5	Ls3
e-gully	555	116	Sq5	Ls5
f-gully	533	180	Sq5	Ls5
g-gully	421	180	Sq5	Ls4
h-gully	375	174	Sq5	Ls4
i-gully	375	134	Sq5	Ls4
j-gylly	384	213	Sq5	Ls4

According to the information presented in Tables 8 to 13, the Sq2, Sq3 and Sq5 soil quality classes correspond very well to slight, moderate and severe degrees of impact of soil erosion, respectively. This means that soil erosion may also be categorised in existing classification schemes, such as rills or gullies, even under the same surface soil conditions [26], but that the degree of impact and the severity of its occurrence will differ greatly [27]. In contrast, the land suitability classes show large differences between rill and gully erosion. Most of the gully affected areas are very poor (Ls4) and bad (Ls5) lands and are not suitable for agriculture. This is contrary to the case for rill erosion, i.e., Ls3. On the other hand, in the case of rill-affected areas, most of the land area is good (Ls1) or moderately good (Ls2). The direct interpretation of these evaluations is that areas characterised by poor soil quality classes (Sq4, Sq5) and poor land suitability classes (Ls4, Ls5) are at high risk of further damage compared with those characterised by moderate/good soil and land classes (Sq2, Sq3, Ls2, Ls3).

Tables 14 and 15 show how the soil erosion counts in terms of the average, sum, maximum and minimum at the study sites. The results were used to evaluate the relationships between the sites for these parameters. There was no significant difference in the width or depth of the rill erosion site between the rill erosion site and the gully erosion site (**), where a crucial change existed (Tables 14, 15). In each case, reasonable damage to surface soil between the means was identified as indicated by the sum and standard deviation values. Unlike rill erosion, gully erosion was much greater at the Tungar/D site (STDV = 94.76380 cm) than at the Bui site (STDV = 79.25907 cm), although both were identified as bad land (Sq5, Ls4 or Ls5).

Table 14

Average, sum, maximum, minimum and STDV width (cm)
Site (width)	Average (cm)	Sum (cm)	Maximum (cm)	Minimum (cm)	STDV (cm)
Argungu^*	15.1	151	19	10	2.806738
B/Kebbi^*	15.5	155	19	10	3.471311
Fakon Sarki^*	14.2	142	16	12	1.536229
Kamba^*	15.2	152	19	12	2.616189
Tungar/D^**	407.8	4078	555	226	94.76380
Bui^**	333	3330	500	241	79.25907
^Rill erosion and ^* Gully erosion

Table 15

Average, sum, maximum, minimum and STDV depth (cm)
Site (depth)	Average (cm)	Sum (cm)	Maximum (cm)	Minimum (cm)	STDV (cm)
Argungu^*	5.4	54	9	3	1.837873
B/Kebbi^*	6.2	62	9	4	1.400000
Fakon Sarki^*	6.4	64	9	4	1.685230
Kamba*	5.7	57	9	3	1.828782
Tungar/D^**	154.8	1548	213	116	31.23673
Bui^**	93.20	9320	155	46	39.21173
^Rill erosion and ^* Gully erosion

The assessment and classification of surface soil erosion into sheet, rill, gully, dry-mechanical (human-anthropogenic) and winnowing of soil particles correspond very well to previous studies, as comprehensibly reviewed by Salumbo [9] and Gebrie et al. [7]. The effects of these types of soil erosion on the surfaces of agricultural soils are physical (soil removal, aggregate disorientation), chemical (organic matter and important chemical loss), and biological (a decrease in microbial biomass) [24, 28, 29, 30]. The main causes were attributed to the lack of proper surface soil protection against raindrop impact, decreased aggregate stability, long and steep slopes, intense rainfall, and decreased infiltration by compaction or other means [20, 24]. These changes might have led to complete surface soil layer disappearance and subsequent subsurface soil layer damage [Figure 4; 31]. Similarly, a decrease in soil fertility and soil quality and an increase in crop yield may result because disintegrated aggregates and high soil particle erodibility enable easy detachment and transport of soils by runoff and wind [11, 3], thus leading to surface soil deterioration characterised by different surface soil erosion structural shapes (Table 7). Such deterioration can result in a noticeable decrease in organic matter in agricultural fields and ultimately leads to partial or even complete destruction of soil productivity by damaging soil structure and aggregate stability and reducing porosity, water infiltration rates, soil biological functions, and the storage and availability of nutrients [Figure 4; 5, 32].

Generally, a more meaningful assessment and evaluation of surface soil erosion can be made if the precise objective of such actions is known [6]. For instance, if the main objective is to assess only surface rill and gully erosion, the overall properties, dynamic nature, and classes of soil affected by rills and gullies could be identified in more detail [33]. In the present assessment, the different types of soil erosion were only considered preliminary. Although the target is to evaluate the types and classes of soil erosion, the results of these findings may not be sufficient for future sustainable soil management. However, the assessment provided an overview of the physical surface soil conditions in terms of soil quality and land suitability at erosion-affected sites. Further important information on soil erosion at the same site may still be needed if the complete soil management package is to be applied at the affected site [12]; however, it is believed that the shape pattern after the removal of soil particles of the specified type of soil erosion (Table 8–13) explains the level of soil quality and land suitability for agricultural production. This removal of soil particles corresponding to the stated land quality and land suitability classes was attributed to the weakened structure and aggregates of the surface soils [34]. Thus, areas affected by rills (I, C, S, X shapes) have less impact (Ls1, Ls2, Ls3) and better surface soil quality (Sq2, Sq3) than those affected by gullies (U, V, Y shapes – Ls4, Ls5, and Sq4, Sq5). These classes are tallied with the risk classes of soil degradation defined by Lal et al. [25]. According to their definitions, Ls1, Sq1, Ls2, and Sq2 represent the slight and moderate impact classes because the original surface soil biotic function is still intact, but full restoration of the impacted areas is needed. On the other hand, the Ls3 and Sq3 classifications fall under the poor land class characterised by small channels of the rill because most of the surface area has been degraded. Sq4, Ls4, Sq5, and Ls5 correspond to the extreme class, where re-establishment of the landscape components (soil, vegetation) is needed for some level of agricultural productivity and biotic function (Fig. 6).

Although several methods for assessing overall soil quality and land suitability have been developed based on different criteria, including soil structural quality [21, 35, 36], the procedure used in the present study is important for defining both the inherent and current productivity of the soil at the time of soil assessment in the field [19]. This assessment might also be useful for determining the ability of soil/land to support plants, organisms, and soil nutrients or for determining fertility [3]. Further research revealed that the assessment sites within the surroundings of the affected areas were physically well-drained sand, sandy-loam and sandy-silt textural soil particles (Table 2). These soil particles are physically very loose and can be easily removed by rains, causing sheet, rill, gully and dry-mechanical erosion (Fig. 4; 1, 15, 20a). Based on this observation, the surface soil erodibility at the study sites might be attributed to the impact of raindrops and runoff, which simultaneously enhanced the breakdown of soil aggregates and decreased the strength of the soil structures [20]. Thus, physically, the permanent reductions in crop performance in the affected soil erosion sites are caused primarily by erosion impacts that might have led to changes in soil physical conditions: soil structural quality and aggregate stability (Fig. 4). These problems are related to poor agricultural surface soil management, poor vegetation cover leading to mass movement of sol particles [38]. Sharma et al. [39] considered poor soil management to be one of the factors contributing to the initiation of soil erosion and deterioration of surface soil quality. Poor soil management is also regarded as one of the major threats to food security in most sub-Saharan African regions [15, 40, 41].

Other important factors contributing to initiating and spreading soil erosion in the region might also include poverty and overgrazing, as well as poor environmental government policy [15]. Poverty is important to consider in this observation because the relationship between social factors such as poverty in areas such as Kebbi State, Nigeria, is physically more complex than the lack of good management practices [8]. Critically, poverty may be associated with initiating most, if not all, surface soil erosion occurrences in some parts of the affected sites. For example, in the struggle to sustain their daily livelihood, the poor people in the study sites sometimes have no choice but to fall trees around them, thus causing deterioration of natural vegetation cover and surface soils and leading to erosion and desertification [2]. This is one of the reasons why poverty is considered one of the key factors that exacerbate soil erosion in poor regions of the world [3]. Therefore, if the disappearance of surface soil cover due to sheet, rill, or gully erosion might be due to poverty at the study sites, as reported in the present study, it jeopardises most agricultural soils and annual crop production in the region. This evidence is a good indication of strong relationships between poverty and soil erosion occurrence at most of the affected sites. Overgrazing might also contribute to the occurrence of soil erosion at the study sites. For example, as one of the northern Nigerian regions that supplies meat for national consumption, Kebbi State has become dominated by a nomadic and cattle-rearing community that depends on farm residues for animal feed [1]. The behaviour of these animals is considered to hinder plant canopy development and soil structural strength, which provides protection to surface soils against soil erosion [8], however, the organic manure they supplied to soil during grazing period is a good compost which may be useful to bind the surface soil layer [42].

Despite all the properties of soil erosion observed in this assessment, there is a crucial need to further understand soil erosion in terms of its volumetric impact [3]. The classes and types of soil erosion defined as corresponding to the land quality and land suitability classes in the respective study areas can be used to understand the physical aspects of soil erosion impacts, as suggested by previous studies [6, 11, 19, 43]. It is very likely that this new approach of assessing the physical impact of soil erosion in the field will provide even more direct definitions of surface soil quality and land suitability around soil erosion-affected areas under similar environmental conditions in sub-Saharan Africa or elsewhere in the world.

Soil erosion has been identified to explain the level and extent of its physical impact based on type, shape, soil quality and land suitability in the field. As with all environmental degradation, soil erosion was visualised and observed as challenging to surface soil retention at the study sites. It affects both the potential and functional development of surface soil, soil properties and economic performance of land quality. The physical and biotic components of the soil at the affected sites were also affected. The level and extent of these physical impacts were defined as slight, moderate or severe. Photographs of typical examples of these impact classes showed the need to take immediate action to control erosion at the sites described as Ls4, Sq4 and Ls5, Sq5 (land suitability and soil quality classes). The sites Ls3 and Sq3 also need to be maintained under sustainable soil management to prevent their extension to Sq4 and Ls4. The proposed practically adapted visual soil erosion approach (VSEA) provides a simple way of explaining and understanding the physical impact of soil erosion in the field. Although this approach may require substantially more information on several other soil erosion-related factors, such as rainfall data, history of environmental changes and complete socioeconomic information of the given study area, it is clear that the approach provides primarily an improved understanding of soil erosion and its physical impact at the respective study sites. This approach also demonstrates the benefit of visual soil assessment and its adaptability and suitability for different soil and environmental field studies. There is much to learn in adapting this approach to other aspects of soil degradation at the study sites, in Nigeria and sub-Saharan Africa in general.

Funding

This research was funded by the Kebbi State Government, Nigeria for Soil base-line data collection in the State. We acknowledge the efforts of the former state governor Alhaji Saidu Usman Nasamu Dakin-Gari (KB/STATE-S.USMAAN-PROJECT].

Ethical Statement

This work is mainly on soil and soil resources and does not involve any information on humans or animals; thus, an ethical statement is not applicable to the context of the manuscript.

Credit authorship contribution statement

Suleiman Usman designed the study and collected the data in the field. He also provided the writing and detailed analysis of the original draft. James O. Jayeoba supervised the study, reviewed the collected data and updated the information. Sani Mathew Amana provided an introductory updates and also run the simple analysis of the results.

Declaration of competing interest

The authors declare that they have no known competing financial interests regarding the information presented and that there are no personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

To Kebbi State Government Nigeria, for funding of this research in 2009/2023, and to the State Government Ministry of Education who have organised for the successful take-up of the research activities in the State. As part of this salutation, we also thank the comments and suggestions received from Dr. Bruce Ball of Scotland’s Rural College (SRUC), Edinburgh, UK.

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Evaluation of surface soil quality and land suitability for agricultural soils affected by soil erosion

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1 Attributes of the study sites

2.2 USDA-NRCS and FAO International classification system

2.3 Visual Soil Erosion Approach

2.4 Sampling frequency and size area used under the VSEA

2.5 Statistical analysis

3. Results

3.1 Physical impact of soil erosion by category and by degree

3.2 Physical impact of soil erosion by VSEA

4. Discussion

5. Conclusion

Declarations

References

Additional Declarations

Status:

Version 1