Soil morphological characteristics
The morphological characteristics of soils at the four positions in the landscape are presented in Table 1. The major elevation points were 479.20, 486.00, 489.30 and 515.70 m with a slope gradient of 5.4, 8.6, 11.5 and 15.6% at points UP1, UP2, UP3 and UP4, respectively. The landscape was divided into crest, upper and middle slope. The dominant soil colour was 7.5YR and it indicates the release of Fe from the primary minerals, thereby giving the soils a reddish-brown coloration. The darker 10 YR at surface of UP4 can be attributed to deposition of materials being the lowest part of the landscape.
Generally, the surface soils were coarse textured sandy loam. while the subsurface horizons were sandy clay, sandy clay loam and clay textured. The textural change with increasing depth indicated
in situ weathering and it was evident in the presence of bits of partly weathered granitic rocks and gravels within the pedons. Mottles was observed at UP1 and it indicated poor drainage condition as a result of the lower position in the landscape. The structure was medium, coarse granular at the UP1, UP2 and UP4. The subsurface soil had well developed structure of medium to strong coarse sub angular to angular block. The consistence was no-sticky and no-plastic at the soils surface. The degree of cohesion and adhesion increases with increasing depth and indicates increase in clay content down the soil depth.
There is presence of roots down the soil depth which indicate that the soil depths are zones of rooting activities. The Fe-Mn concretions found in some parts indicate presence of plinthites and the erosion channels in some part indicates movement of materials across the landscapes. The soil depth ranges from 42 to 80 cm, restriction to soil depth are stones, gravels, rocks and hardpan.
Soil physico-chemical characteristics
The physico-chemical characteristics of soils at the four positions in the landscape are presented in Table 2. The soils were slightly to strongly acidic with soil pH values range from 5.34 to 6.11 and 5.21 to 5.95 at the surface and subsurface horizons respectively. The surface horizon had higher pH values than the subsurface horizon and higher values was recorded at the lower slope than the other slope positions. This could be attributed to phytocyling movement of bases due to intense evaporation during the dry season in the humid tropics and lateral leaching of bases from upland to the lowland (
Adegbenro, 2017). Electrical Conductivity was from 0.010 to 0.031 dsm m
-1 and 0.007 to 0.026 dsm m
-1 at the surface and subsurface horizons respectively. The values were below the critical level at 4.00 dsm m
-1 suggested by
Brady and Weil (2005) for saline soils. Therefore, the soils were rated non-saline.
The organic matter percentage and total-N were low and had similar pattern of distribution. The organic matter values were higher at the surface (1.52 to 1.79%) than the subsurface (0.26 to 1.36%) horizons. Findings is similar to reports of
Savalia and Gundalia (2010), this can be attributed to the presence of more decomposable plant materials and phytocyling. The total-N were below the critical value of 0.2% set for Nigerian soils (FFD, 2002). The available-P were high, it ranged from 15.75 to 29.10 ppm at the surface of the soils and it decreased with increasing depth.
Calcium values was from 1.81 to 4.30 cmd kg
-1 soil. This was followed by Mg having between 0.85 to 2.83 cmol kg
-1. The higher Ca content was due to the higher adsorptive power of Ca than the other cations in the soil (Amalu and Antigha 1999). The Mg values were moderate, and Na values were low. In general, the UP1 position had higher values of exchangeable cations than the other positions and could be due to lateral leaching of bases from the upland to the lowland. The effective CEC values of 5.9 to 9.26 mol kg
-1 was low and in agreement with values reported for savannah soils in Nigeria by Odunze (2016). The values were indicative of low nutrient reserved of these soils. The ECEC clay values was below the boundary of 24 cmol kg
-1 clay set by Juo (1980) for limit between high activity clay and low acidity clay at UP2, UP4 and sub soil part of UP1 and UP4, therefore, the soils were dominated by low activity clay, and nutrient loss through leaching was likely in the soils. The percentage base saturation values were very high ranging from 87.99 to 91.36%. The values indicated that the ability of the soils to hold and exchange nutrient was very high. The exchangeable Na percentage values ranged from 1.4 to 4.5%, below the critical level of 15% suggested for sodic soils by Brady and Weil (2005).
The sand content increased down the profile pit at UP1 and decreases at the other positions, this indicated that there was deposition of fine soil materials from the higher elevation to the lower UP1. There was evidence of accumulation of clay within the soil profile through the process of eluviation-illuviation because of the increase in clay content with increasing depth. The middle, upper and crest position had higher valves (13-49%) of clay than the lower slope (16-35%).
Soil classification
In the USDA soil taxonomy soil classification, the soils were classified as Alfisols at the order level because they do not have a plaggen epipedon, had an argillic and kandic horizon as well as fragipan. They were classified as Ustalfs at suborder level because they had an ustic moisture regime. At the great group and subgroup levels, UP1 was classified as Paleustalfs and Kandic Paleustalf because there was an increasing clay with increasing depth, presence mottles, hue of 7.5YR in the horizon and had an ECEC clay of less than 24 cmol kg
-1 within the kandic horizon, UP2 and UP4 were classified as Durustalfs and Typic Durustalfs because they had pan materials within 100 cm of the soil depth while UP3 was classified as Kanhaplustalf and Typic Kanhaplustalf because it was an Ustalfs with a kandic horizon.
In the FAO/UNESCO classification system, UP1 and UP3 are Lixisols at the higher category because of the presence of argic B horizon with low activity clay and base saturation above 50%. At the lower category, UP1 was Epistagnic Lixisols because it had a redoximorphic condition (mottles) within the 50 cm soil depth and UP3 was Haplic Lixisols because it had a typical expression of Lixisols. The higher and lower category classification of UP2 and UP4 were plinthosols and pisoplinthic endoduric plinthosols because they had hard pan and Fe-Mn concretions in one of the horizons.
Land suitability evaluation
The land use requirement for yam and result of suitability assessment (Tables 3 and 4). Revealed that soils at the four positions were currently and potentially marginally suitable (S3) with current and potential suitability index of 32.19 and 36.00, 37.18 and 41.56, 41.56 and 46.48 and 27.89 and 36.00 at UP1, UP2, UP3 and UP4 respectively. The soils were placed in classes lower than S1 as a result of length of dry season which is more than six months, slope, soil depth, effective CEC, organic-C and total-N.
Among the limitations to yam production in the soils of the study area, effective CEC, organic-C and total-N were fertility limitations that can be improved upon with the adoption of proper soil and crop management practices and will lead to better performance of the crops on the soils.
