Morphological characteristics in different land form situations
Data on Table 1.1, 1.2 and 1.3 revealed that all the soil profiles were very deep, though having varying internal drainage properties across the micro-topographic sequence. Soils on active alluvial plains were moderately well drained, sandy loam textured in surface and sandy loam to loamy sand in sub surface horizons with a regular decrease in clay fraction and subsequent increase in sand fraction with depth. The structure is fine weak sub angular blocky at surface and fine weak sub angular blocky to single grained at sub surface horizons. They have matrix colour of light yellow brown at surface with hue of 10 YR, value 6 and chroma 4 and olive brown to light gray with hue of 2.5 Y, value 4 to 6 and chroma 4. Stratified deposition of fine and coarse sand fractions exhibits fluvial nature of soils due to frequent flood hazards of Komo river. The soils have Ap-AB-2ACg horizon. They were classified as
hyperthermic Typic Udifluvents. Soils on younger alluvial plains were somewhat poorly drained, silt loam in texture throughout with yellowish brown coloured surface with hue of 10 YR, value 5 and chroma 2 and light yellowish brown to yellowish gray colour at sub surface horizons with hue of 10 YR, value 5 to 6 and chroma 2. Fluvial influence of both Abonga and Komo river has been observed in them with stratified and irregular distribution of sand and clay fractions with depth. They have also Ap-AB-2ACg horizon and were classified as
coarse loamy mixed hyperthermic Typic Fluvaquents. In older meander plains, soils were some what poorly drained, silt loam in texture throughout with yellowish brown coloured surface with hue of 10 YR, value 4 and chroma 3 and light yellowish brown to light gray colour at sub surface horizons with hue of 10 YR to 2.5 y, value 4 to 6 and chroma 2. Endo-saturation was evident at a depth below 40 cm from surface with redox depletions. They have Ap-Bw-2Bwg horizon and were classified as coarse loamy mixed hyperthermic
Typic Endoaquepts. In older alluvial plains, soils were poorly drained, loamy at surface and loamy to sandy loam at sub surface horizons, very dark gray coloured surface with hue of 2.5 Y, value 4 and chroma 1 and dark to light gray coloured sub surface with hue of 2.5 Y, value 4 to 6 and chroma 2. Dark coloured surface may represent high organic carbon content of soils. They have Ap-Bwg horizon and classified as
coarse loamy, mixed hyperthermic Humic Endoaquepts. Soils on lower piedmonts were poorly drained, silt loam textured throughout, dark gray in colour at surface with hue of 2.5Y, value 5 and chroma 2 and gray tolight gray at subsurface horizons with hue of 2.5Y, value 5 to 6 and chroma 6. Dark coloured surface is gain due to high organic carbon content which gradually decreases with depth of soils. They have Ap-Bwg horizon and classified as
coarse loamy mixed hyperthermic Typic Humaquepts (
Soil Survey Staff, 2014).
Physico-chemical characteristics of soils
The described physic-chemical characteristics of soils in Table 2 were - shows that the soils bulk density in the range of 1.35 Mg m-3 in active alluvial plains to 1.40 Mg m
-3 in younger alluvial plains. Occurrence of silt fraction of soils has a significantly positive influence on bulk density and
vice versa. In the study area, due to frequent fluvial influence of both Komo and Abonga rivers, soils have high silt deposits over sands forming alluvial fans, which may be the reason for higher bulk density of soils with higher silt and lower sand fractions, leading to internally imperfect (some what poor) drainage situations of soils in-spite of having coarse loamy texture. The effect of clay fraction has no significant impact on bulk density. Sand, silt and clay fractions of soils varied from 9.4% (older meander plains) to 87.8% (active alluvial plains), 6.5% (active alluvial plains) to 79.9% (younger alluvial plains) and 4.0% (younger alluvial plains) to 16.8% (lower piedmont), respectively. Sand and silt fractions have a very close indicating the fluvial influence of Komo and Abonga rivers. It is assumed from the study that the occurrence of silt fractions is geogenic influenced by river fluvial actions. But, origin of sand and clay involves soil forming processes from parent materials by
in situ weathering. Higher sand fractions in active alluvial plains represents slower rate of pedogenic development, whereas, higher clay fractions in lower piedmonts represent relatively rapid pedogenic developments. Different soil depth was one of the major constraints for all the soil series
(Sabareeshwari et al., 2023).
The soil pH ranged from very strongly acidic (pH 5.1) in the lower piedmont to neutral (pH 7.1) in the active alluvial plains. In terms of soil organic carbon content at the surface horizon, it varied from low (0.40%) in the active alluvial plains to high (1.92%) in the lower piedmonts. The cation exchange capacity of the soils ranged from low (5 cmol (p+) kg
-1) in the active alluvial plains to medium [8.0 cmol (p+) kg
-1] in the lower piedmonts. Furthermore, the base saturation fluctuated from 57 to 79% in the active alluvial plains.
Bandyopadhyay et al., (2017) suggests the highly acidic nature of soils in the lower piedmont, attributed to an abundance of pH-dependent charges at the surface resulting from elevated organic carbon levels and similar research findings were reported in soils of Nagaland.
Adhikary et al., (2021) found clear support for the soil chemical properties like pH and EC shows to vary significant with the application of blending of saline and sweet water in pitcher pot for pitcher irrigation with different type of tillage. On the other hand, active alluvial plains comprise soils with higher pH due to higher base saturation percentage carried in high silt fractions (
Birk land, 1984) deposited by rivers. low clay content (4.6-8.1%) in soils may be attributed to low CEC and low sum of cations in these soils
(Gerrad et al., 1992, Queiroz et al., 2022, Akpan et al., 2023). Tamiru et al., (2022) share similar result for soil chemical and physical properties like soil organic matter, soil pH, cation exchange capacity, bulk density and soil moisture shows significant difference between conserved and non-conserved lands.
Hydromorphic consideration
Hydromorphic characterization and index (HI) of soils were presented in Table 3. The Hydrologic Index (HI) ranged from 20.6 in soils of older alluvial plains to 4.5 in soils of active alluvial plains, following the order: Humic Endoaquepts (11.8) > Typic Humaquepts (9.1) > Typic Endoaquepts (6.4) > Typic Fluvaquents (6.1) > Typic Udifluvents (5.2). Higher HI values in lower piedmont and older alluvial plains may be attributed to higher degree of hydromorphosis occurring at major proportions of soil depth (P1 82.4 and P2 87.1). It is justified with the fact that hydromorphic soils if better understood by evaluation these parameters as supported by
(Chaplot et al., 2000) This has been indicated by dark gray colour of the surface horizons as well as occurrence of iron (Fe
2+) and manganese (Mn
2+) in reduction phases at a depth below 15 to 40 cm along with redoximorphic depletions at a depth below 40 to 70c min soils of lower piedmonts and older alluvial plains. Sub surface matrix colour of 2.5 Y hue with chroma of 2 is indicative of reduction phases of iron and manganese with depletions, which supports the formation of soils of
Aquepts sub orders in these two afore said land forms. On the other hands, soils of younger alluvial plains and active alluvial plains were much less hydromorphic compared to that in lower piedmonts and older alluvial plains. The proximity of the river banks of Komo and Abonga rivers can be attributed to the stratification of sand and silt deposition, leading to soil conditions ranging from imperfect to moderately well-drained. Some sparse and faint redoximorphic features were observed at depths ranging from 33 to 46 cm, although depletions of iron and manganese are scarce. Frequent flooding hazards may have a causative effect disturbed hydromorphosis in these soils. These soils were there for have all most no profile development and were placed under
Fluvents and
Aquents sub orders. Following these characterization parameters, similar results also reported by
Gangopadhyay et al., (2015) in soils of Assam.
Pedological considerations
The assessed pedogenic indices
viz., sand/ silt and have inverse relationship with pedogenesis, whereas, CEC/ clay and silt/ clay have positive influence on the same (
Schatzael and Anderson, 2015). It was apparent from this study that soils on lower piedmonts, older alluvial plain sand older plains have higher value of CEC/ clay and silt/ clay compared to that on younger and active alluvial plains. Hence, pedogenic processes are more rapid in these landforms. On the contrary, higher sand/ silt ratio in active in active alluvial plains leads to formation of lithologically discontinued gleyed layers below 60 cm from surface. Lithological discontinuity (LD) appears to decline the rate of pedogenesis in soils on active alluvial plains
(Bandyopadhyay et al., 2017). Clay, CEC and SOC were found to have synergistic relationship with HI (Fig 2a, 2b, 2c.)
vis-à-vis pedonesis. Thus, the process of hydromorphosis and humification were simultaneous in P1 and P2, which was responsible for the formations of Typic Humaquepts and Humic Endoaquepts in lower piedmonts and older alluvial plains, respectively. Occurrence of humic hydromorphic soils is an indicative of relative elderliness of land forms (
Breemen and Buurman, 1998). On the contrary, non humic hydromorphic soils in active alluvial plains and younger alluvial plains exhibits their juvenile nature. The older meander plain represents an intermediate stage of weathering, which stands in between older alluvial and younger alluvial plains. Their formation may be attributed to shifting of river courses in the geological past in the estuary region at Andem region Gabon. Same results found from
(Adewumi et al., 2020).
