Studies on the Genesis of Soils in Jong River Basin in the Northern Province of Sierra Leone

The soil resources of Sierra Leone remain an integral part of sustainable agricultural development but knowledge of the suitability of soils and their management requirements for a variety of land uses is still lacking to a greater extent. In this view, a detailed soil survey of three chiefdoms in Northern Sierra Leone was undertaken to evaluate the genesis of soils and interpret soil-landscape properties and their relationships using the toposequence and free style approach of soil survey. Three major soil types, viz, gravelly, gravel-free over gravel and gravel-free soils, formed either directly or indirectly from weathered and disintegrated parent materials of two geologic origins namely Rokel River Series (comprising of sandstone, shale and mudstone), and Granite and Acid Gneiss were identified. The gravelly soils are formed from sandstone and sandy shales of the Rokel River Series, which vel-free over gravel soils are mostly located in the summit and back slopes and show dark brown to dark yellowish-brown hue and moderate chroma, weak to moderate structure and absence of coarse fragments in the 0 - 40 cm layer, and a strong brown hue and high chroma, very weak fine angular blocky structure having abundance of coarse fragments from 40 cm and above. In these soils, the compactness of ironstone and sandstone gravels increases with depth. The gravel-free soils are found on three landscape positions, viz, back slopes, foot slopes and toe slopes. On the back and foot slopes, the soils are characterized by very dark brown to dark yellowish-brown hue and high chroma, sandy loam to sandy clay texture, coarse angular to moderate sub-angular blocky structure and absence of coarse fragments throughout the horizon. In these soils, the clay content and sand grain sizes were observed to increase with depth. On the toe slopes, the soils show black to greyish-brown hue and low chroma, silty loam to sandy loam, structureless and high water holding capacity.


Introduction
Five soil orders namely, Oxisols, Inceptisols, Entisols, Spodosols and Ultisols, have been discovered in Sierra Leone [1]. Depending on the physiography, factors and processes of soil formation, these were further subdivided into sixteen natural regions called "soil provinces" to guide land use planning. Within these subdivisions, three widely distributed soil types are gravelly soils, gravel-free over gravel soils and gravel-free soils. The occurrence of these soils was first reported in the Njala area of Southern Sierra Leone by Dijkerman and Breteler (1969) and van Vuure et al. (1973) [2] [3], and in the Jagbwema area of Eastern Sierra Leone by Sutton et al. (1980) [4]. In Northern Sierra Leone, earlier studies by van Vuure and Miedema (1973) [5] reported the presence of these soils in the Makeni area. This was further confirmed by Blokhuis (1979) [6] in the Robis Polder area of Mambolo Chiefdom along the coastal plains between the estuaries of the Great Scarcies and Little Scarcies rivers. The origin of these soils is attributed to the integrated effect of soil-forming factors, of which climate (i.e. rainfall and temperature) is most influential. These soils are formed from primary minerals, which undergo rapid weathering due to instability in environmental conditions such as high rainfall and temperature during the rainy and dry seasons respectively [1]. This seasonal variability in rainfall and temperature is a recipe for the formation of iron-rich reddish materials commonly known as "plinthite" in soils of Sierra Leone, especially in gravelly soils and sub-surface layers of gravel-free over gravel soils.
In the upland gravelly and gravel-free over gravel soils, iron-rich reddish ma- Open Journal of Geology terials from overlying horizons or higher adjacent areas are added to sub-surface horizons, which harden irreversibly when exposed to recurrent wetting and drying thereby forming "plinthite glaebules". About 80% of the total soil mass of these well-drained gravelly upland soils contain hardened plinthite glaebules of variable gravel sizes [7], which develop from gravelly colluvium parent materials usually overlying residual materials over weathered bedrock (or saprolite) of about 122 cm thick. In colluvial foot slopes and stream terraces, gravel-free over gravel soils contain very high gravel content in the subsoil, but gravel-free surface layers. In some uplands, gravelly soils that develop from sedimentary rocks such as sandstone, siltstone, and mudstone, a gravel-free layer of about 25 cm thick may overlay a gravelly layer as a result of termite activities.
Conceptually Hence, the present study was undertaken to interpret soils-landscape relationships and evaluate the genesis of soils in the study area.

Study Area
The study area is located in the Jong river basin, and lies in the domain of 8.4˚N -8.4667˚N and 11.8833˚W -11.8167˚W covering a total area of 1810.8 ha ( Figure 1). This area comprises patches of land situated in the Upper Massakong, Petifu Mayawa A and Yele Manowo chiefdoms in Tonkolili District, Northern Province of Sierra Leone. The mean annual rainfall for the district is about 1979 mm [9]. The physiography is interior plains with very gentle undulating plains, and a geology that has developed on the resistant sediments of the Rokel River Group [10].

Soil Survey and Mapping
A detailed soil survey was carried out using LANDSAT ETM+ imageries, digital elevation model (DEM) and land system map [11] as base maps for field work.
The imageries, DEM and digitized land system map were geocoded, and subsets were created in ArcMap 10.5 on a 1:12,500 scale. After processing the LANDSAT imageries, standard False Colour Composites (FCC) were produced by a combination of bands 4, 3 and 2, and these were used for on-screen visual interpretation of landforms. The isolated landforms were intensively traversed and the different land facets were identified and studied to establish soil-landscape relationships.
A digital soil mapping model using a high resolution DEM of 12.5 m, which assigns the same colour attributes to soils located at same elevation was used to speed up the survey process. Ground truth observations were conducted in order

Soil Analysis and Classification
From each pedon, horizon-wise soil samples were collected, processed and analyzed at the Njala University Quality Control Laboratory (NUQCL) following standard analytical procedures. Based on the morphological, physico-chemical and chemical properties, the soils were classified using the "Keys to Soil Taxonomy" [14]. The taxonomic classes of soils were correlated with the UNDP/FAO Land System classification [11] and soils were mapped as gravelly, gravel-free over gravel and gravel-free soils.

Landscape Characteristics and Physiography-Soil Relationship
The relationship between physiography and landscape elements (land facets) of an area and soils has been widely recognized as the factors involved in the physiographic processes corresponding close to that of soil formation. This relationship between landscape features and soil conditions makes it possible for predicting the nature and distribution pattern of different soils in a landscape. In the present study, three major physiographic units, namely upland, undulating midlands and lowlands, with land facets comprising of crests, side slopes (shoulder and back slopes), terraces (foot slopes and toe slopes) and adjacent inland valley swamps were identified using on-screen visual interpretation of satellite images and in-situ terrain feature characterization. The landscape characteristics and physiography-soil relationship attributes are presented in The side slopes are gently to very gently sloping from the upper to the lowest parts along the topo-sequence of the interfluves, comprising of slopes ranging from 2% -5%. In some areas, the lower side slopes of the interfluves are very gentle, with a level to nearly level topography that is shaped by a trough of undulation having slopes of not more than 2%. Adjacent to the Taia River, is a long and narrow stretch of nearly level terrace soils ( Figure 2) of about 1% slope that is believed to have been formed by the deposition of colluvial materials. Within these areas, IVS are also evident. These are generally narrow, elongated and nearly level valley bottoms, drained by seasonal streams, which are largely waterlogged for much of the year. At some points, these IVS occur as narrow and elongated and total runoff water are directly proportional to slope percent. In conformity with this proposition, soils found on gentle slopes, having thick vegetation cover were less eroded than soils without vegetation.

Soil Morphological Features
Morphological studies were conducted on five representative pedons and the results (   The structure of gravelly as well as gravel-free over gravel soils is fine to medium, granular to sub-angular blocky but weak to moderate ped for gravelly soils and moderate to strong ped for gravel-free over gravel soils. In colluvium interfluve and alluvium gravel-free soils, the structure is angular to sub-angular blocky with fine and moderate to strong ped while hydromorphic gravel-free soils are structureless. The angular and sub-angular blocky structure is an indication of slickensides formation in these soils. In either soils, with the exception of hydromorphic gravel-free soils, the structure of the subsurface horizons of all pedons irrespective of physiography is more developed compared to surface horizons. This is an indication of how the factors of soil formation had influenced soil development over time in the study area. With reference to resistance of soil material to rupture, gravelly soils exhibit a slightly hard to hard (dry), very friable to friable (moist) and non-sticky and non-plastic to slightly sticky and slightly plastic (wet) consistence whereas gravel-free over gravel soils are slightly hard to hard (dry), firm to very firm (moist) and non-sticky and non-plastic to slightly sticky and slightly plastic (wet).
In gravel-free soils, consistence vary with depth from hard to very hard (dry), firm to very firm (moist) and non-sticky and non-plastic to moderately sticky and moderately plastic (wet). The increase in stickiness and plasticity could be re-  [19]. Surface horizons exhibited lower class consistency while sub-surface horizons exhibit higher class consistency, which is possibly due to clay eluviation from surface layers and illuviation into subsurface layers [20]. According to Rudramurthy and Dasog (2001) [21], this physical behaviour of soils as influenced by dry, moist and wet conditions could be attributed to the textural make-up and clay mineral composition.
The horizon boundaries are abrupt to gradual and smooth to wavy in gravelly soils, abrupt to clear and wavy to irregular in gravel-free over gravel soils, and abrupt to gradual and smooth to irregular in gravel-free soils. In hydromorphic gravel-free soils, horizon boundaries are more distinct but with wavy and irregular topography, which is an indication of Gleization due to the persistent waterlogged condition to which these soils were subjected.

Soil Physical Properties
The physical properties of the pedons are presented in Table 3. The textural properties of soils revealed that the soils contain high sand content compared to silt and clay fractions, which may permit high infiltration rate, thus making them fairly good for the cultivation of most arable crops. The soil depth vary from deep to very deep. In gravelly soils, the content of coarse fragments increases with depth, whereas in gravel-free over gravel soils, an irregular trend ranging from non-gravelly in surface horizons to very gravelly in subsurface horizons was observed. The gravel-free soils are non-gravelly except in some areas, the surface is covered with patches of partially decomposed plant residues. The content of coarse fragments in the Bt and AC horizons is less than the Ap and Cr horizons due to Argilli pedoturbation in these horizons.
The clay content in the gravel-free over gravel and gravel-free soils is higher compared to their gravelly soils counterpart. In general, increase in finer materials and clay content with depth is observed in gravel-free soils. This could be at-

Soil Chemical Properties
The chemical properties of the pedons are presented in Table 4. The soil pH measured in 1:2.5 soil:water extract was strongly acidic (5.05 -5.64) but very strongly acidic to extremely acidic (4.25 -4.65) when measured in 1:1 soil:KCl extract.   The organic carbon ranges from very low to high (0.06% -2.07%) and is observed to either increase or decrease with depth or follow an irregular trend, which might be associated with the physiographic-soil relationships and land use patterns. In addition, the organic carbon content of the surface soils is higher than the subsurface soils in most of the pedons. This could be attributed to the high amount of litter and crop residues and the rapid mineralization at the surface layers. However, most pedons exhibit low organic carbon content, resulting from the induced rapid rate of organic matter oxidation due to the high temperature, with resulting decreasing trend tending towards accumulation of crop residues every year, without substantial downward movement [24].
Nutrients have a profound effect on soil fertility and crop yield. Nitrogen, for example, contributes to an increase yield and after-harvest residue, thus preventing the loss of soil organic matter [25]. In this study, the  [28]. In African soils such soils of Sierra Leone are highly weathered soils [7], and thus pH is usually low [26]. Under such conditions, phosphorus levels less than 15 ppm Bray P1 puts crops at the greatest risk of yield loss from a deficiency of phosphorus. Potassium is moderate, ranging from 4 to 91 mg•kg −1 .
The exchangeable bases are in the order of Ca 2+ > Mg 2+ > Na + > K + on the exchange complex, ranging from 20 -395, 5 -85 and 5 -24 mg•kg −1 , for Ca, Mg and Na respectively. The trend shows that the exchange complex is saturated with Ca 2+ followed by Mg 2+ , Na + and K + . This order of abundance is in accordance with Jenny's proposition that leaching caused preferential losses of Na + and K + [29]. The high content of exchangeable Ca/Mg is an indication of decreasing extractable magnesium content in these soils. From the distribution of and Na + ), it is noted that Mg 2+ content is lower than Ca 2+ in these soils due to the high mobility of Mg 2+ . The low value of exchangeable monovalents compared to divalent cations is as a result of the preferential leaching of monovalents than divalents in these soils. The exchangeable acidity (H + ) content is also high varying from 0.17 to 0.92 mg•kg −1 , and this is probably due to the weathering of acidic parent material originating from acid rocks and granitic magnetite complexes [11].
The CEC is low, ranging from 1.13 to 9.71 cmol•(p+)•kg −1 . These values are critically low to medium, which might have serious implications on the overall productivity of these soils because soils with CEC less than 5 cmol(+)/kg generally have a low clay and organic matter content, low water holding capacity, requires more frequent lime and fertilizer additions, and are subject to leaching of NO 3 , B, NH 4 , K and most probably Mg [30]. Jenny (1941) [29] stated that soil formation is a function of five factors, viz., climate (cl), vegetation (v), relief (r), parent material (p) and time (t), which he expressed mathematically as S = f (cl, o, r, p, t). The pedogenic studies reveal that two passive factors including topography (relief) and parent material, and one active factor, climate are the dominant factors of soil genesis in the study area. The active pedogenic processes involved in the genesis of gravelly and gravelfree over gravel soils are rubification, ferrugination and braunification, whereas in gravel-free soils, pedoturbation is the most active pedogenic process. These pedogenic processes together with climate, topography and parent material had played a greater role in the formation of soils, as explained below.

Influence of Topography and Relief on Genesis of Soils
The influence of topography and relief on soil formation in the study area exhibits a topo-sequence and soil catena character, which vary with slope steepness and severity of surface water runoff and erosion. Gravel-free soils are located in lowlands at the footslopes and valleys, gravel-free over gravel soils in the undulating midlands within the back slopes and gravelly soils in the crest and pediments regions of the uplands. Gravelly and gravel-free over gravel soils, due to their position in the landscape, show well developed soil profiles having coarsetextured, deep to very deep and brown to dark-brown soils with eroded surfaces.
The A horizon of these soils is slightly shallow due to constant surface disturbance resulting from cultivation and erosion. In addition, the external and internal drainage conditions as well as the differential transport of eroded materi- Gravel-free soils are slightly eroded, having thicker A horizon, high organic matter content, dark colour, deep to very deep and moderately drained soils with slow to moderate permeability. This is related to the frequent transport and enrichment of finer materials that are eroded from the uplands and undulating midlands into the lowlands. The moderate to poor drainage conditions and differential deposition of eroded materials in the toe slopes is a measure of the effect of topo-function on the genesis of these soils, which over time might have resulted in the formation of associated gravel-free over gravel soils in substantial amount ( Table 5).

Influence of Parent Material on Genesis of Soils
Many difficulties arise when recognising the material from which a soil originates. Jenny (1941) [29] defined parent material as the initial state of the soil system, which consist of consolidated rocks as well as unconsolidated deposits such as river alluvium, lake or marine sediments, glacial tills, loess (silt-sized, wind-deposited particles), volcanic ash and organic matter (such as accumulations in swamps or bogs). According to Aide (2022) [31], he influence of parent material is integral to the discerning soil profile differences. Depending on the mineralogical composition, texture and stratification, soil formation in any area can be influenced by the kind of parent materials. Dark-coloured ferromagnesian (iron-and magnesium-containing) rocks, for example, can produce soils with a high content of iron compounds and of clay minerals in the kaolin or smectite groups, whereas light-coloured siliceous (silica-containing) rocks tend to produce soils that are low in iron compounds but contain clay minerals in the illite or vermiculite groups. The pedogenesis of soils in the study area revealed that these soils have developed over time from weathered schist and acid rock and granitic magnetite complexes under prevailing humid tropical climate and topography, as exhibited by toposequence variability [1].
Under favourable climatic conditions, the coarse-textured granitic rocks have undergone weathering to form coarse and loamy-textured gravelly soils with well-developed E horizons while the fine-textured rocks have weathered to form soils, which accord them their characteristic well-drained nature due to percolation of water to greater depths. The presence of high content of plinthite materials (i.e. iron-rich reddish materials) in the gravelly soils is an indication that their genesis is associated with the dark-coloured ferromagnesian (iron-and magnesium-containing) rocks of sedimentary origin, which upon weathering might have produced soils with a high content of iron materials. The distribution of iron gravels is also observed to be high throughout the profiles of gravelly soils with the size and content of gravels increasing with depth, which further reveals that these soils have undergone higher degree of weathering and laterization. The occurrence of gravel-free over gravel soils is due to the variation in mineralogical composition of the parent rock, which is an indication of soil development from a transported parent material over weathered residual parent material. The finer texture of gravel-free soils denotes their formation from transported parent materials, which become deposited at lower elevations, thus forming deep to very deep gravel-free soils over time.

Influence of Climate on Genesis of Soils
According to Amara

Soil Classification and Mapping
After laboratory analysis, soil correlation was done based on the morphological, physical and chemical properties and soils of the study area were classified into four taxonomic classes and mapped as follows: gravelly soils are classified as Plinthic Paleudult, gravel-free over gravel soils as Eutropept over plinthic paleudult, colluvium/alluvium gravel-free soils as Inceptisol, and hydromorphic gravel-free soils as Psammaquents. The five major soil types which are representative of the study area have been mapped and named after the communities where their occurrence is greater (  Figure 2).

Conclusion
Gravelly soils with dark brown to red hue, having high chroma and abundance of coarse fragments; gravel-free over gravel soils with dark brown to dark yellowish-brown hue and moderate chroma, having weak to moderate structure and absence of coarse fragments in the 0 -40 cm layer, and a strong brown hue and high chroma, very weak fine angular blocky structure and abundance of coarse fragments from 40 cm and above; and gravel-free soils characterized by very dark brown to dark yellowish-brown hue and high chroma, having silty and sandy loam to sandy clay texture, coarse angular to moderate sub-angular blocky structure and absence of coarse fragments throughout the horizon and structureless conditions in some cases with high water holding capacity. These soils were formed either directly or indirectly from weathered and disintegrated parent materials of two geologic origins: 1) Rokel River Series (comprising of sandstone, shale and mudstone) and 2) Granite and Acid Gneiss. The gravelly soils were formed from sandstone and sandy shales of the Rokel River Series, which upon weathering released iron oxides and hydrated oxides that harden into plinthite upon drying. The gravel-free over gravel soils were formed from the acid gneiss series containing acid and intermediate igneous and metamorphic rocks, which upon weather gave rise to the sandy clay texture nature of these soils. The amount of coarse fragments found in gravel-free over gravel soils was less when compared to gravelly soils. The gravel-free soils were derived from the sedimentary rocks of the Rokel River Series such as shales and mudstones, which upon weathering and action of organisms gave these soils their characteristic sandy clay loam to sandy clay texture. The active pedogenic processes involved in the formation of the gravelly and gravel-free over gravel soils were rubification, ferrugination and braunification, whereas, in gravel-free soils, pedoturbation was the most active pedogenic process. The results of present study are informative