Soil profile morphology as an indicator of geo-environmental processes and agronomic potential in tropical landscapes of Southern Ekiti, Nigeria

1 Introduction

Soils constitute one of the Earth’s most valuable natural resources, supporting food production, regulating water and nutrient cycling, storing carbon, sustaining biodiversity, and providing numerous ecosystem services essential for environmental and human well-being. As global demand for food, fibre, and bioenergy continues to increase under the pressures of population growth, climate change, and land degradation, the sustainable management of soil resources has become a major international priority. Achieving this objective requires a comprehensive understanding of soil genesis, spatial variability, and functional properties, which collectively determine the capacity of soils to sustain agricultural productivity and maintain ecosystem resilience. Consequently, accurate soil characterization and classification remain fundamental to sustainable land-use planning, precision agriculture, environmental conservation, and climate-smart agricultural development (1, 2).

Among the various approaches to soil characterization, soil profile morphology remains the cornerstone of pedological investigations because it provides direct evidence of the processes responsible for soil formation and landscape evolution. Morphological attributes such as soil colour, texture, structure, consistence, horizon differentiation, root distribution, mottling, concretions, and boundary characteristics reflect the combined influence of climate, parent material, organisms, relief, and time on soil development. These observable characteristics not only facilitate soil classification but also provide valuable insights into hydrological conditions, weathering intensity, redoximorphic processes, clay translocation, nutrient dynamics, and the overall functioning of terrestrial ecosystems. Recent advances in pedology have further emphasized the importance of integrating classical morphological observations with laboratory analyses, digital soil information, and geospatial techniques to improve the understanding of soil-landscape relationships and support sustainable land management across diverse agroecological regions (3, 4).

In recent years, significant progress has been made in understanding tropical soil development through the integration of soil morphology, physicochemical characterization, mineralogical investigations, and advanced statistical approaches. Modern soil classification systems, particularly the USDA Soil Taxonomy and the World Reference Base (WRB) for Soil Resources, have evolved to enhance international harmonization and facilitate meaningful comparisons of soils across climatic and geographical regions. Similarly, multivariate statistical techniques such as principal component analysis (PCA), hierarchical clustering, and digital soil mapping have increasingly been employed to reveal patterns of soil variability, identify dominant pedogenic controls, and improve predictions of soil behaviour under different land-use systems. These developments have strengthened the role of pedology as an interdisciplinary science that links geomorphology, hydrology, ecology, and agronomy in addressing contemporary environmental challenges (5).

Beyond understanding pedogenic processes, comprehensive soil characterization forms the scientific basis for evaluating land suitability for agricultural production. The productivity of tropical soils is strongly influenced by their physical and chemical properties, including drainage conditions, effective soil depth, texture, soil reaction, organic matter status, nutrient availability, and cation exchange capacity. These properties govern root development, water availability, nutrient retention, and ultimately crop performance. Consequently, integrating soil morphological observations with physicochemical analyses and internationally recognized land evaluation frameworks provides a more reliable basis for identifying production constraints, matching crops with suitable environments, and developing site-specific soil management strategies. Such integrated approaches are increasingly recognized as indispensable for achieving sustainable agricultural intensification while minimizing environmental degradation in tropical agroecosystems (5, 6).

Despite these advances, significant knowledge gaps remain regarding the relationships among soil morphology, pedogenic processes, soil classification, and agronomic potential in many humid tropical environments. Previous studies in tropical Africa have frequently focused on isolated aspects of soil characterization, such as physicochemical properties, fertility assessment, or taxonomic classification, with relatively few investigations integrating detailed morphological description, laboratory characterization, multivariate statistical interpretation, and crop-specific land suitability evaluation within a single geo-environmental framework. Furthermore, many existing studies have been conducted at localized scales without adequately examining how variations in geology, topography, drainage conditions, and landscape position collectively influence soil development and agricultural suitability. Consequently, the mechanisms linking observable soil morphological characteristics with pedogenic processes and land-use potential remain insufficiently documented for many tropical landscapes, particularly within the Precambrian Basement Complex terrains of southwestern Nigeria (7, 8).

Southern Ekiti State represents an excellent natural laboratory for investigating these relationships because of its diverse geological formations, undulating topography, contrasting drainage regimes, and intensive agricultural activities. The area is underlain predominantly by Precambrian Basement Complex rocks comprising migmatite-gneiss, granite gneiss, schists, quartzites, and charnockitic intrusions, all of which exert strong controls on soil formation and landscape evolution. Combined with the humid tropical climate characterized by intense weathering and seasonal rainfall, these geological and environmental factors have produced considerable spatial variability in soil morphology, physicochemical properties, and agricultural potential. Although previous investigations have reported selected soil properties within parts of Ekiti State and southwestern Nigeria, integrated studies that simultaneously evaluate soil profile morphology, pedogenic characteristics, taxonomic classification, multivariate relationships, and crop-specific land suitability remain scarce.

The present study addresses this knowledge gap by integrating detailed field-based morphological characterization with laboratory physicochemical analyses, USDA Soil Taxonomy and World Reference Base (WRB) classification, multivariate statistical analyses, and FAO-based land suitability evaluation within a unified geo-environmental framework. Unlike previous studies that primarily emphasized either soil fertility or taxonomic classification, this research explicitly examines how soil morphological characteristics reflect dominant pedogenic processes and how these characteristics influence the agricultural suitability of contrasting tropical landscapes. The integration of classical pedological techniques with statistical interpretation and crop suitability assessment provides a more comprehensive understanding of soil variability and offers practical information for sustainable land-use planning, precision soil management, and agricultural decision-making in humid tropical environments.

Therefore, this study was undertaken to evaluate soil profile morphology as an indicator of geo-environmental processes and agronomic potential across selected tropical landscapes of Southern Ekiti State, Nigeria. Specifically, the objectives were to: (i) characterize the morphological, physical, and chemical properties of representative soils; (ii) classify the soils using the USDA Soil Taxonomy and the World Reference Base (WRB) systems; (iii) investigate the relationships between soil morphological characteristics and pedogenic processes using multivariate statistical techniques; and (iv) evaluate the suitability of the soils for maize (Zea mays L.), cassava (Manihot esculenta Crantz), and Bologi (Crassocephalum biafrae) production using the FAO land evaluation framework. It was hypothesized that variations in soil morphology are reliable indicators of dominant pedogenic processes and that integrating morphological observations with physicochemical characterization and land suitability assessment provides a robust scientific basis for sustainable land management in humid tropical landscapes.

2 Materials and methods

2.1 Study area

The study was conducted in selected locations within Southern Ekiti State, Nigeria. The area lies between latitudes 7°15′ and 7°45′ N and longitudes 4°45′ and 5°30′ E, covering parts of Gbonyin, Ikere, and Ekiti South-West Local Government Areas (Figure 1). Southern Ekiti State experiences a humid tropical climate characterized by distinct wet and dry seasons. The mean annual rainfall ranges from 1,200 to 1,500 mm, with a bimodal distribution extending from March to October, while the dry season spans from November to February. Mean annual temperature ranges between 25 and 30°C, with relative humidity generally exceeding 70% during the rainy season.

The vegetation of the area is predominantly derived savanna interspersed with secondary rainforest species, resulting from prolonged anthropogenic activities such as cultivation, bush burning, and logging. Geologically, the area is underlain by Precambrian Basement Complex rocks, consisting mainly of migmatite-gneiss, granite gneiss, quartzite, schist, and charnockitic intrusions. These parent materials significantly influence soil formation and variability within the landscape (9, 10).

2.2 Field survey and soil sampling

A preliminary reconnaissance survey was conducted across the study area to identify representative soil-landscape units based on variations in parent material, topography, drainage conditions, vegetation, land use, and observable soil morphological characteristics. Information obtained during the reconnaissance survey guided the selection of sampling sites to ensure that the major pedogenic environments within Southern Ekiti State were adequately represented. Such reconnaissance-based surveys are fundamental in conventional soil mapping because they facilitate the identification of relatively homogeneous soil units before detailed characterization (11, 12).

Following the reconnaissance survey, nine representative benchmark pedons were selected using a purposive stratified sampling approach. The benchmark pedon concept emphasizes the detailed characterization of representative soils that typify the dominant soil-landscape relationships within a region rather than exhaustive sampling of every soil occurrence (11). Consequently, the selected pedons were distributed across contrasting geological formations, drainage conditions, and landscape positions to capture the principal pedogenic variability within the study area. A total of 43 horizon samples were collected from these pedons according to natural genetic horizons. Sampling by genetic horizons provides sufficient vertical resolution for evaluating soil morphology, physicochemical properties, pedogenic processes, and taxonomic classification while ensuring that each diagnostic horizon is adequately represented (13). Although the study was not intended to quantify fine-scale spatial variability statistically, the selected pedons adequately represent the dominant soil types and environmental conditions relevant to the objectives of this study.

2.3 Soil sampling and laboratory analysis

Disturbed soil samples were collected from each identified genetic horizon for laboratory analyses. A total of 43 soil samples were air-dried, processed, and analyzed at the Soil Science Laboratory, Department of Soil Resources and Environmental Management, Faculty of Agricultural Sciences, Ekiti State University, Ado-Ekiti, Nigeria, following standard analytical procedures.

Particle-size distribution was determined using the hydrometer method after dispersion with sodium hexametaphosphate (14). Soil pH was measured potentiometrically in a 1:2.5 soil-to-water suspension using a glass electrode pH meter (15). Organic carbon content was determined by the Walkley and Black wet oxidation method (16), while total nitrogen was analyzed using the micro-Kjeldahl digestion method (17).

Available phosphorus was extracted using the Bray-1 method and determined colorimetrically (18). Exchangeable calcium (Ca), magnesium (Mg), potassium (K), and sodium (Na) were extracted with 1N ammonium acetate (NH₄OAc) at pH 7.0. Calcium and magnesium concentrations were determined using atomic absorption spectrophotometry, while potassium and sodium were measured using a flame photometer (19). Cation exchange capacity (CEC) was determined using the ammonium acetate saturation method, and base saturation was calculated as the percentage of the sum of exchangeable bases relative to the CEC.

2.4 Soil classification

The soils were classified according to the USDA Soil Taxonomy and correlated with the World Reference Base for Soil Resources (WRB) (20). The use of both classification systems enhances the scientific robustness and international applicability of the study. USDA Soil Taxonomy provides a hierarchical framework based on diagnostic horizons, soil moisture and temperature regimes, and measurable soil properties, thereby facilitating the interpretation of pedogenic processes and soil genesis. In contrast, the WRB offers an internationally harmonized classification system that enables comparison of soils across different climatic regions and geographical settings using Reference Soil Groups and diagnostic qualifiers (4, 20).

The identification of diagnostic surface and subsurface horizons, together with field morphological observations and laboratory analytical data, enabled reliable classification of the investigated pedons. The combined application of USDA Soil Taxonomy and WRB successfully captured the observed variability in soil morphology, drainage conditions, horizon development, and weathering intensity, thereby providing a comprehensive framework for interpreting soil-landscape relationships and facilitating comparison with similar tropical soils worldwide.

2.5 Land suitability evaluation

Land suitability evaluation was conducted following the FAO Framework for Land Evaluation (21, 22) and crop requirements proposed by Sys et al. (23). The suitability of the soils for maize (Zea mays L.), cassava (Manihot esculenta Crantz), and Bologi (Crassocephalum biafrae) production was assessed by matching soil characteristics with the ecological requirements of each crop.

Land characteristics considered in the evaluation included drainage condition, effective soil depth, texture, soil reaction (pH), fertility status, rooting conditions, and nutrient retention capacity. Suitability classes were designated as Highly Suitable (S1), Moderately Suitable (S2), Marginally Suitable (S3), Currently Not Suitable (N1), and Permanently Not Suitable (N2). The most limiting factor approach was adopted in assigning suitability subclasses.

2.6 Statistical analysis

Descriptive statistics, including means, standard deviations, and ranges, were used to summarize the physico-chemical properties of the soils. One-way Analysis of Variance (ANOVA) was employed to determine significant differences in selected soil properties among the study locations. Where significant differences were observed, means were separated using Duncan’s Multiple Range Test (DMRT) at P ≤ 0.05. Pearson correlation analysis was performed to examine relationships among selected soil properties. Statistical analyses were conducted using IBM SPSS Statistics version 25.0.

3 Results

3.1 Soil morphological characteristics

The morphological properties of the studied pedons are presented in Tables 1, 2, 3. The soils exhibited considerable variability in profile development, drainage characteristics, texture, structure, and horizon differentiation across the study locations.

Table 1. Morphological properties of study area 1 (GBONYIN LGA) – GBY.

TEXTURAL CLASS: SL, Sandy loam; SCL, Sandy clay loam; LS, Loamy sand; S, Sand; STONINESS: f.s, fairly stony; v.s, very stony; Ʃ: V., Very.

CONSITENCY: n.st, not sticky; s.st, slightly sticky; f, firm; l, loose; v.f, very firm; s, sticky; d.l, dry loose; s.h, slightly hard.

Table 2. Morphological properties of study area 2 (IKERE LGA) – IKR.

TEXTURAL CLASS: LS, Loamy sand; SL, Sandy loam; L, Loam; SCL, Sandy clay loam; SiL, Silt loam; STONINESS: f.s, fairly stony; s, stony; v.s, very stony; Ʃ: V., Very; CONSITENCY: fr, friable; s, sticky; h, hard; f, firm; s.h, slightly hard; v.h, very hard.

Table 3. Morphological properties of study area 3 (EKITI SOUTH WEST LGA) – ESW.

KEYS. TEXTURAL CLASS: LS, Loamy sand; SL, Sandy loam; SCL, Sandy clay loam; STONINESS: s, stony; v.s, very stony; f.s, fairly stony; Ʃ: V., Very.

CONSITENCY: f, firm; St., Sticky; l, loose.

Profile depths ranged from 94cm to 150cm, indicating generally deep soils suitable for agricultural production. Surface horizons were predominantly characterized by loamy sand (LS) and sandy loam (SL) textures, while subsurface horizons varied from sandy loam (SL) to sandy clay loam (SCL) and silt loam (SiL). The increase in clay content with depth observed in several pedons, particularly IKR 3 and ESW 3, suggested the occurrence of clay illuviation and profile development.

Soil colors ranged from dark brown and brown in surface horizons to yellowish brown, grayish brown, and gleyed colors in subsurface horizons. Mottles of varying abundance and contrast were observed in GBY 1, IKR 1, ESW 1, and ESW 2, indicating fluctuating groundwater conditions and periodic water saturation. Conversely, well-drained pedons such as IKR 3 and ESW 3 exhibited relatively uniform matrix colors with little or no mottling.

The soil structure varied from weak fine granular in the surface horizons to moderate medium and coarse subangular blocky structures in subsurface horizons. Root abundance generally decreased with increasing depth and was more restricted in poorly drained pedons. Horizon boundaries ranged from clear smooth to gradual smooth transitions across the profiles.

3.2 Physico-chemical properties of the soils

Selected physico-chemical properties of the studied pedons are presented in Table 4. Soil pH ranged from 5.0 to 6.1, indicating slightly acidic to moderately acidic conditions. The highest pH value was observed in IKR 3 (6.1), while ESW 2 recorded the lowest value (5.0).

Table 4. Physico-chemical properties of selected soils.

Organic carbon content ranged from 0.91 to 1.88%, with IKR 1 exhibiting the highest value and GBY 3 recording the lowest. Total nitrogen varied from 0.08 to 0.15%, following a similar trend to organic carbon distribution. Available phosphorus ranged from 6.2 to 15.6 mg kg^-¹, with higher concentrations observed in IKR 3 and ESW 3.

Exchangeable bases varied among pedons, with calcium ranging from 2.4 to 6.2 cmol kg^-¹, magnesium from 1.2 to 2.8 cmol kg^-¹, potassium from 0.14 to 0.32 cmol kg^-¹, and sodium from 0.08 to 0.15 cmol kg^-¹. Cation exchange capacity (CEC) ranged from 8.7 to 18.3 cmol kg^-¹, while base saturation varied between 38.9 and 51.6%.

Particle-size distribution showed that sand content ranged from 60 to 84%, silt from 8 to 18%, and clay from 8 to 25%. The relatively high sand fractions observed in several pedons indicate coarse-textured surface horizons with varying nutrient retention capacities.

3.3 Descriptive statistics and variability of soil properties

Descriptive statistics for selected soil properties are presented in Table 5. Mean soil pH was 5.49 ± 0.37, reflecting the generally acidic nature of the soils. Organic carbon and total nitrogen averaged 1.57 ± 0.29% and 0.13 ± 0.02%, respectively.

Table 5. Descriptive statistics of physico-chemical properties across pedons.

Available phosphorus had a mean value of 10.22 ± 3.01 mg kg^-¹, whereas CEC averaged 14.26 ± 2.87 cmol kg^-¹. Base saturation ranged from 38.9 to 51.6%, with a mean value of 46.60 ± 4.11%. Sand content dominated the particle-size distribution, averaging 70.56 ± 7.66%, while clay content averaged 16.89 ± 5.92%.

3.4 Soil classification

The classification of the studied pedons according to USDA Soil Taxonomy and the World Reference Base (WRB) systems is presented in Table 6. The soils were predominantly classified as Ultisols, represented by Typic Hapludults, Aquic Hapludults, Typic Paleudults, and Aquic Paleudults.

Table 6. USDA and WRB classification of the studied pedons.

Pedons GBY 1 and IKR 1 were classified as Aquic Hapludults and Aquic Paleudults, respectively, due to evidence of periodic saturation and aquic conditions. IKR 3 was classified as Typic Paleudults, reflecting advanced profile development and clay accumulation. GBY 3 was classified as Typic Udipsamments because of its sandy texture and weak horizon differentiation. ESW 1 and ESW 2 were classified as Aeric Endoaquults and Typic Endoaquults, respectively, due to their pronounced gleyic features and poor drainage. The corresponding WRB classifications included Haplic Acrisols, Gleyic Acrisols, Gleyic Plinthosols, Dystric Gleysols, and Haplic Arenosols.

3.5 Suitability evaluation for maize production

The suitability evaluation for maize production is presented in Tables 7 and 8. Pedons IKR 3 and ESW 3 were classified as highly suitable (S1) for maize cultivation due to favorable drainage conditions, adequate profile depth, and balanced textural properties.

Table 7. Suitability evaluation of soils for maize production and limiting factors.

Table 8. Summary of limiting factors affecting maize production.

Suitability classes were assigned according to the FAO Framework for Land Evaluation (24), considering soil drainage, texture, effective depth, soil reaction, fertility status, and rooting conditions relative to maize production requirements in humid tropical environments.

GBY 2, IKR 2, and ESW 1 were rated as moderately suitable (S2), with moderate fertility constraints and drainage fluctuations constituting the major limitations. GBY 1, GBY 3, and IKR 1 were marginally suitable (S3) owing to poor drainage, coarse textures, and restricted rooting conditions. ESW 2 was classified as currently not suitable (N1) due to persistent wetness and poor aeration.

3.6 Suitability evaluation for cassava production

The suitability assessment for cassava production (Tables 9, 10) indicated that GBY 2, IKR 2, IKR 3, ESW 1, and ESW 3 were highly suitable (S1). The adaptability of cassava to moderately acidic soils contributed to the improved suitability ratings observed across several pedons. GBY 1, GBY 3, and IKR 1 were classified as moderately suitable (S2), while ESW 2 was rated as marginally suitable (S3) due to severe drainage limitations.

Table 9. Suitability evaluation of soils for cassava production and limiting factors.

Suitability classes were assigned according to the FAO Framework for Land Evaluation (24), considering soil drainage, texture, effective depth, soil reaction, fertility status, and rooting conditions relative to cassava production requirements in humid tropical environments.

Table 10. Summary of limiting factors affecting cassava production.

Suitability classes were assigned according to the FAO Framework for Land Evaluation (24), considering soil drainage, texture, effective depth, soil reaction, fertility status, and rooting conditions relative to cassava production requirements in humid tropical environments.

3.7 Suitability evaluation for Bologi (Crassocephalum biafrae) production

The suitability evaluation for Bologi production is presented in Tables 11 and 12. Pedons GBY 2, IKR 2, IKR 3, ESW 1, and ESW 3 were classified as highly suitable (S1) for Bologi cultivation.

Table 11. Suitability evaluation of soils for Bologi (worowo) production and limiting factors.

Suitability classes were assigned according to the FAO Framework for Land Evaluation (24), considering soil drainage, texture, effective depth, soil reaction, fertility status, and rooting conditions relative to cassava production requirements in humid tropical environments.

Table 12. Summary of limiting factors affecting Bologi production.

Suitability classes were assigned following the FAO Framework for Land Evaluation (24), considering drainage status, texture, effective soil depth, soil reaction, fertility status, and rooting conditions relative to the ecological requirements of Bologi (Crassocephalum biafrae) production.

GBY 3 was rated as moderately suitable (S2) due to excessive sandiness and moderate nutrient retention limitations. In contrast, GBY 1, IKR 1, and ESW 2 were classified as marginally suitable (S3) because of drainage constraints and poor rooting conditions.

3.8 Summary of crop suitability classes

The overall suitability assessment (Table 13) revealed that IKR 3 and ESW 3 consistently exhibited the highest agronomic potential, being highly suitable for maize, cassava, and Bologi production. Conversely, ESW 2 displayed the lowest suitability ratings across the evaluated crops due to severe wetness limitations. The remaining pedons exhibited varying degrees of suitability depending on crop-specific requirements and prevailing soil limitations.

Table 13. Summary suitability table.

3.4 PCA and heatmap analysis

The PCA biplot (Figure 2) shows that Dim1 (37.9%) and Dim2 (22.0%) explain 59.9% of the variation in GBY morphological properties. Cluster 1 (GBY1) is positively associated with soil water state, concretions and textural class, indicating wetter soils with higher concretion levels and finer/variable textures. GBY1_A and GBY1_E also relate to stronger consistence and structure. Cluster 2 (GBY2) is mainly positioned on the negative side of Dim1 and positive Dim2, strongly associated with mottles and pores, suggesting more mottling and larger pore development. Cluster 3 (GBY3) is linked with roots and drainage, indicating better rooting conditions and drainage status. Stoniness lies near the origin, showing weak influence on sample separation. Overall, GBY profiles differ mainly by moisture-concretion-texture gradients and mottling-root-drainage conditions.

The PCA biplot (Figure 3) for Ikere LGA exhibits that Dim1 (37.4%) and Dim2 (21.4%) explain 58.8% of the variation in soil morphological properties. Cluster 1 is clearly separated on the positive side of Dim1 and is strongly linked with soil water state, mottles, and soil structure, suggesting wetter soils with more visible mottling and better structural expression. Cluster 2 is spread across the plot, indicating mixed morphological conditions, with some profiles associated with concretions and consistence, while others relate to roots. Cluster 3 is positioned negatively on Dim1 and is mainly influenced by stoniness and textural class. Drainage and pores contribute more to the lower part of the plot. Overall, variation in Ikere soils is mainly controlled by moisture, mottling, structure, stoniness, and rooting conditions.

The PCA biplot (Figure 4) for Ekiti South West LGA shows that Dim1 (47.0%) and Dim2 (21.5%) explain 68.5% of the total variation in soil morphological properties. ESW1 plots mainly on the positive side of Dim1 and upper Dim2, showing strong association with concretions, root abundance and mottles, suggesting soils with more visible pedogenic features. ESW2 is clearly separated on the negative side of Dim1 and relates more to soil water state, consistence and soil structure, indicating wetter and more structurally distinct soils. ESW3 falls mostly in the lower right quadrant and is linked with drainage, stoniness and textural class. Overall, the soils are chiefly differentiated by moisture-related traits, rooting, concretions and drainage conditions.

The heatmap (Figure 5) demonstrates clear variation in the morphological properties of soils in Gbonyin LGA (GBY). Most GBY1 profiles cluster together and are generally characterized by higher soil water state, consistence, and concretions, but lower pores, root abundance, and drainage. GBY1_B and GBY1_C also show relatively high textural class values. GBY2_Atii stands out as a distinct profile, with notably high concretions and pores but low soil structure. In contrast, many GBY3 profiles, together with some GBY2 samples, show higher drainage and root abundance, indicating better rooting and moisture movement. Overall, the main differences among GBY soils are driven by moisture condition, concretions, pores, rooting, and drainage.

The heatmap (Figure 6) shows clear differences in the morphological properties of soils in Ikere LGA (IKR). A group made up mainly of IKR1_A, IKR1_Eti, IKR1_O, and IKR2_A is characterized by higher soil water state, soil structure, root abundance, mottles, and consistence, but generally lower drainage, textural class, and stoniness. In contrast, IKR1_Bii, IKR3_Btii, and IKR3_C stand out for higher textural class and stoniness, with relatively lower root abundance and soil structure. Samples such as IKR2_Bii, IKR1_Bi, and IKR2_Bi are more associated with higher concretions. Overall, the main variation in IKR soils is controlled by moisture condition, structure, roots, stoniness, texture, and concretions.

The heatmap (Figure 7) exhibits noticeable variation in the morphological properties of soils in Ekiti South West LGA (ESW). The ESW2 profiles cluster together and are mainly characterized by higher consistency, soil water state, and soil structure, but lower values for mottles, pores, stoniness, and drainage. ESW1 profiles show higher mottles, concretions, and root abundance, suggesting stronger visible soil development and rooting conditions. In contrast, ESW3 profiles are more associated with stoniness, drainage, and pores, indicating better drainage but more coarse fragments in some horizons. Overall, the heatmap suggests that ESW soils are mainly differentiated by moisture condition, structure, mottling, root abundance, drainage, and stoniness.

4 Discussion

4.1 Soil morphology and pedogenic processes

The marked variability in soil morphological characteristics among the investigated pedons reflects the combined influence of parent material, topography, drainage regime, vegetation, and long-term weathering processes that characterize humid tropical environments (22, 25, 26). The progressive increase in clay content from surface to subsurface horizons, particularly in pedons IKR3 and ESW3, provides strong evidence of clay illuviation (lessivage), one of the dominant pedogenic processes responsible for argillic horizon development in highly weathered tropical soils (5, 27). Such clay translocation is facilitated by alternating wetting and drying cycles that promote the downward movement of dispersed clay particles and their subsequent deposition within subsurface horizons (28, 29). The presence of well-developed argillic horizons therefore indicates prolonged pedogenic stability and advanced soil development.

Conversely, the occurrence of gleyed matrix colours and abundant mottles in GBY1, IKR1, ESW1, and ESW2 demonstrates the influence of fluctuating groundwater tables and periodic waterlogging. These redoximorphic features result from repeated oxidation–reduction cycles involving iron and manganese compounds under alternating aerobic and anaerobic conditions (30). Such hydromorphic processes not only influence soil morphology but also affect nutrient availability, microbial activity, and root development, thereby reducing agricultural productivity where drainage remains poor (31). Similar relationships between redoximorphic features and seasonal saturation have been reported for tropical soils in West Africa (32, 33), Southeast Asia (34), and humid South America (35).

The PCA and hierarchical clustering analyses further support these interpretations by demonstrating that soil water state, drainage condition, texture, root abundance, and concretions accounted for much of the observed morphological variability. Rather than acting independently, these attributes collectively describe distinct pedogenic environments ranging from well-drained, highly weathered uplands to hydromorphic depressions influenced by prolonged saturation. Similar applications of multivariate statistics have successfully differentiated pedogenic environments in tropical landscapes (11, 36).

From a land management perspective, recognition of these pedogenic processes enables more accurate prediction of drainage behaviour, rooting conditions, erosion susceptibility, and nutrient dynamics, thereby supporting site-specific soil management and sustainable agricultural planning (27, 31, 37).

4.2 Soil fertility status and nutrient dynamics

The generally acidic soil reaction observed throughout the study area reflects prolonged leaching of exchangeable base cations under humid tropical climatic conditions. Continuous rainfall coupled with intensive weathering promotes the removal of calcium, magnesium, potassium, and sodium while increasing the relative abundance of hydrogen and aluminium ions within the soil exchange complex (27, 31). Consequently, the predominance of Ultisols is consistent with their relatively low natural fertility and moderate cation exchange capacities (38, 39).

The relatively low organic carbon and total nitrogen contents indicate rapid mineralization of organic matter under warm and humid climatic conditions. Although tropical ecosystems produce abundant biomass, elevated temperatures and microbial activity accelerate decomposition, preventing long-term accumulation of soil organic carbon unless stabilized within aggregates or protected under waterlogged conditions (27, 40). This explains the relatively higher organic carbon contents observed in poorly drained pedons.

Available phosphorus remained generally low to moderate despite adequate total phosphorus inputs from parent materials. In highly weathered tropical soils, phosphorus is commonly immobilized through adsorption onto iron and aluminium oxides, reducing its bioavailability to plants (41). This phenomenon is particularly characteristic of Acrisols and Ultisols throughout tropical Africa and Southeast Asia (20).

The relatively greater cation exchange capacities recorded in finer-textured pedons such as IKR3 and ESW3 demonstrate the beneficial influence of clay accumulation on nutrient retention. Consequently, these soils possess greater resilience against nutrient depletion than coarse-textured pedons such as GBY3. These findings underscore the importance of integrated soil fertility management through liming, organic amendments, balanced fertilizer application, and conservation agriculture to sustain crop productivity (22, 31).

4.3 Soil classification and pedogenic implications

The predominance of Ultisols under USDA Soil Taxonomy and corresponding Acrisols under the World Reference Base reflects advanced stages of tropical weathering accompanied by pronounced clay illuviation, acidic conditions, and low base saturation. These characteristics are widely recognized as typical of humid tropical landscapes developed on Precambrian Basement Complex rocks (20, 42).

The complementary use of USDA Soil Taxonomy and WRB strengthens the international relevance of the study. While USDA Soil Taxonomy emphasizes diagnostic horizons and soil moisture regimes for pedogenic interpretation, WRB facilitates harmonized international comparison using Reference Soil Groups and qualifiers (20, 42, 43). Employing both systems therefore improves scientific communication and enhances the transferability of soil information across geographical regions.

The occurrence of Aquic Hapludults, Aquic Paleudults, and Endoaquults illustrates the dominant influence of hydrological processes on soil development, whereas Typic Udipsamments reflect weak horizon differentiation associated with coarse parent materials and rapid drainage. These classifications successfully captured the morphological variability observed across the investigated pedons, confirming their suitability for describing soil diversity within humid tropical environments (25).

The concurrent application of USDA Soil Taxonomy and the WRB demonstrated that both systems effectively captured the morphological and pedogenic variability observed among the investigated pedons. Variations in diagnostic horizons, drainage conditions, profile development, and base status resulted in the identification of distinct soil taxa and Reference Soil Groups that reflected differences in weathering intensity and hydrological processes. While USDA Soil Taxonomy provides greater emphasis on pedogenic interpretation through its hierarchical structure, the WRB facilitates international comparison by harmonizing soils with similar diagnostic characteristics across different climatic regions (42, 43). The complementary use of both systems therefore strengthens the transferability of the findings and enhances their relevance for tropical soil survey, land evaluation, and sustainable land management.

4.4 Agronomic implications for sustainable land management

The land suitability assessment demonstrated that drainage condition, effective soil depth, texture, and nutrient retention constitute the principal determinants of crop suitability within the study area. The high suitability ratings assigned to pedons IKR3 and ESW3 resulted from favourable combinations of deep rooting zones, balanced texture, moderate fertility, and well-drained conditions that enhance root growth and nutrient uptake.

In contrast, hydromorphic pedons such as ESW2 exhibited severe drainage limitations that substantially reduced suitability, particularly for maize. Waterlogging limits oxygen diffusion into the rhizosphere, suppresses root respiration, reduces nutrient uptake efficiency, and increases denitrification losses (20, 27, 31). Consequently, drainage improvement may be more effective than fertilizer application alone in enhancing productivity on these soils.

Cassava exhibited greater tolerance to moderate soil acidity and temporary wetness than maize, illustrating the importance of crop-specific land evaluation in tropical agriculture. Matching crop requirements with soil characteristics therefore remains fundamental to sustainable intensification, precision agriculture, and climate-resilient land-use planning (3, 4, 22).

4.5 Implications for tropical soil survey and sustainable land management

Beyond the immediate study area, the findings contribute to broader efforts aimed at improving tropical soil information for sustainable land management. Integrating classical soil survey with multivariate statistical analysis and internationally recognized classification systems provides a robust framework for identifying benchmark soils and predicting agricultural potential. Such approaches complement emerging digital soil mapping initiatives and support evidence-based decision-making for agricultural development (3, 4, 44).

The study further demonstrates that field-based morphological observations remain indispensable despite increasing reliance on remote sensing and digital technologies. Soil morphology records pedogenic processes that cannot always be inferred from remotely sensed data alone; consequently, integrating traditional pedology with modern analytical techniques offers the most comprehensive understanding of tropical soil variability (4, 40).

4.6 Study limitations, sources of uncertainty, and future research

This study was based on nine representative benchmark pedons selected to capture the dominant soil-landscape units across Southern Ekiti State. While this approach is consistent with conventional soil survey methodology and provides a reliable basis for soil characterization and classification (43), it may not fully represent the fine-scale spatial variability of soils within the study area. In addition, field observations were conducted during a single sampling campaign; therefore, seasonal variations in soil moisture conditions, biological activity, and the expression of redoximorphic features were not assessed. Although laboratory analyses followed standardized procedures and quality-control protocols, minor analytical uncertainties and the inherent subjectivity associated with field morphological descriptions are unavoidable in pedological investigations (45–47).

Furthermore, the FAO land evaluation framework applied in this study primarily considers soil properties under assumed management conditions and does not explicitly account for socio-economic factors, climate variability, irrigation, or farmer management practices that may influence crop performance (5, 13). Future research should integrate digital soil mapping, proximal and remote sensing technologies, detailed mineralogical and micromorphological analyses, and machine-learning approaches to improve the spatial prediction of soil properties and land suitability. Long-term monitoring of soil quality and crop responses under different management systems would also strengthen sustainable land-use planning in humid tropical environments (5, 12, 36).

5 Conclusion

This study demonstrated that soil profile morphology is a reliable indicator of the geo-environmental processes controlling soil genesis, classification, and agronomic potential across the tropical landscapes of Southern Ekiti State, Nigeria. The observed variations in soil colour, texture, structure, drainage conditions, horizon development, and redoximorphic features reflected the combined influence of parent material, topography, and prolonged tropical weathering. These pedogenic processes were further corroborated by the physicochemical characteristics of the soils, resulting in the identification of distinct soil groups classified as Ultisols and Entisols under the USDA Soil Taxonomy and their corresponding Reference Soil Groups under the World Reference Base (WRB). The integration of morphological observations with laboratory analyses, multivariate statistics, and land suitability evaluation provided a comprehensive understanding of soil variability and demonstrated the effectiveness of combining classical pedology with modern analytical approaches.

The land suitability assessment further revealed that drainage condition, effective soil depth, texture, and nutrient retention are the principal factors influencing crop performance within the study area, with pedons IKR3 and ESW3 exhibiting the greatest agricultural potential. These findings underscore the importance of site-specific soil management strategies, including drainage improvement, integrated soil fertility management, and crop–soil matching, to enhance sustainable agricultural production in humid tropical environments. Overall, this study provides valuable baseline information for soil resource assessment, land-use planning, and sustainable land management in Southern Ekiti State and similar tropical landscapes. Future studies integrating mineralogical analyses, digital soil mapping, and long-term field monitoring are recommended to further improve the spatial prediction of soil properties and support climate-resilient agricultural planning.

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Footnotes

Edited by: Layla M. San-Emeterio, Swedish University of Agricultural Sciences, Sweden

Reviewed by: Nur Ulina Warnisyah Sebayang, Universitas Sumatera Utara, Indonesia

Jamal Doski, Duhok Polytechnic University, Iraq