Characterization and Classification of Soils of a Toposequence in a Derived Savannah Agroecological Zone of Nigeria

2.1. Site Description

The study was carried out at Eleyin Farms in Omu Aran Kwara state Nigeria. The town is situated about 16 km North-East of Otun - Ekiti in Ekiti State and about 88 kilometers south of Ilorin, capital of Kwara State, Nigeria. It is located on Latitude 8.9⁰ N and Longitude 5.6⁰ E with an altitude of 560 m. The study area has a clearly identifiable toposequence with rolling landscape with various slopes at different directions depending on where you are standing. The vegetation of the study area is transitional between the southern forests and the northern savanna zone of Nigeria. The vegetation consists of scattered thick barked trees up to 12 meters high with tall grasses of about 1.5 to 4 m. The area has a tropical climate which is influenced by the movement of the Inter Tropical Discontinuity (ITD) comprising two wind systems (the moisture laden south west monsoon from the Atlantic Ocean and the dry, cold north east trade winds from the Sahara Desert) which give rise to a rainy season from April to October and a dry season from November to March in an average year. The annual rainfall in the area ranges between 1200 - 1300 mm, and the mean annual temperature is 32°C [12]

2.2. Field Study

Three profiles designated as EP1, EP2 and EP3 were dug at the study site representing the upper slope, middle slope and lower slopes, respectively. Each profile was dug to a maximum of about 2 m except where there was an impenetrable resistance (obstruction). Profile description followed Soil Survey Manual [13] and guidelines for soil profile description [14]. For each horizon, soil samples were collected in the three profile pits.

2.3. Laboratory Analysis

A total of 15 soil samples were collected from the study site for laboratory analysis. The samples of soil collected were spread for air-drying, crushed using a mortar and pestle, and latter sieved with a 2 mm-sieve. The samples that did not pass through the sieve were discarded to give fine soil separates. The soil samples were later analyzed for some physical and chemical properties following the procedures outlined by [15].

Particle-size analysis was done using the hydrometer method [16], with sodium hexametaphosphate as the dispersing agent. From information on percent sand, silt and clay, the soil textures were determined using the USDA textural triangle. Soil organic carbon was determined by Walkley and Black sulfuric acid-dichromate digestion followed by black titration with ferrous ammonium sulfate [17]. One (1) g of air-dried soil samples were weighed in duplicate on a filter paper and transferred into a 250 ml conical flask, 10 ml 0.167 M of K₂Cr₂O₇ was added. Then 20 ml of concentrated H₂SO₄ was added and the soil and reagent were immediately swirled gently for one minute until they were thoroughly mixed. Afterward, the reagents were swirled more rapidly for one minute and then allowed to stand for 30 minutes. 100 ml of distilled water was added after 30 minutes, 4 drops of ferroin indicator were added. Titration was done with 0.5 iron (II) sulphate. The colour of the soil reagents changed from sharply green to brownish red colouration. Then the titre value was taken for each sample beginning with the blank titre value. The blank value was to standardize dichromate. The calculation was done using the following formula:

Where B = blank titre value

S = sample titre value and

N = normality of ferrous sulphate

Total nitrogen was determined using the Kjeldhal distillation method described in a study [18] One (1) gram of 2 mm sieved air dry soil sample was weighed in duplicates, and then transferred into test tubes. One macro Kjeldahl tablet was added to each individual sample in a test tube. The test tubes were then placed in a standing machine for one hour as the digester was being prepared. The test tubes were then placed on the digester and the content was digested for one hour to a temperature of 420^oC. Then the tubes were lifted from the digester and placed back on a stand to cool off for 30 minutes. Then distillation and titration were done, respectively.

Available P was determined by the Olsen method as described in a study [19]. The basic cations (K, Ca, Mg and Na) were extracted by leaching 5 g of soil sample with 50 mL ammonium acetate at pH 7 [20]. The exchangeable K in the extract was determined with a flame photometer, and exchangeable Na, Ca and Mg were determined using an absorption spectrophotometer. Effective CEC was the summation of NH₄OAc bases and KCl exchangeable Al and H. The base saturation was obtained by expressing Total Exchangeable Bases (TEB) as a percentage of ECEC. Soil pH was determined in 1:2 soil-water medium by weighing 10 g of 2 mm sieved soil samples into 50 ml beaker and 20 ml of water was added. The sample solution was stirred up occasionally using a glass rod. The samples were allowed to stand for 30 minutes after which the soil pH was taken using a digital electronic pH meter.

2.4. Statistical Analysis

Data collected from each experiment were expressed as means ± standard deviation of the various pedons. The data were subjected to one-way Analysis of Variance (ANOVA) to determine significant difference at 5% level of acceptance using SPSS version 21.

3.1. Morphological Properties of the Soil

EP1 profile consisted of 3 generic soil horizons with an impenetrable layer at 36 cm making it very shallow, while EP2 and EP3 consisted of 6 horizons each. The surface (A) horizons are formed as a result of deposition and accumulation of humified organic matters from plant materials. The B horizons are created by weathering of the parent materials. EP1 site has fewer horizons, this can be adduced to active processes of soil erosion taken place on the site which might have caused soil instability. EP2 and EP3 sites have relative soil stability which was evidence in soil development in the soils of these sites. The horizons have also exhibited great differences in surface soil colour patterns. The colour varied from brownish black (7.5YR 2/2) to dark brown (7.5YR 3/3) to brown (7.5YR 4/3) in EP1, EP2 and EP3 surface (A) horizons, respectively Table 2, indicating that soil colour patterns are influenced by topography through its effect on the rates of surface runoff, erosion and deposition. The profiles had also darker colour at the surface (A) as compared to the B horizon, which can be adduced to the relatively higher organic matter content in the A horizon. Generally, the value and chroma increased with soil depth and hue was redder. This can be attributed to an illuvial accumulation of Fe oxides (sesquioxides) into subsurface (B horizon) layer, which is often responsible for the apparent reddish soil coloration.

3.2. Physical Properties of the Experimental Soils

The soils of the pedons were dominated by sand (Table 3) with the highest mean percentage in EP2 (84.12) followed by EP1 (82.03) then EP3 (68.78) pedons and this may be partly attributed to parent material rich in quartz mineral, an essential component in granite, and partly to geological processes involving sorting of soil materials by biological / agricultural activities, clay migration through eluviation and illuviation, or surface erosion by runoff or their combinations [21]. Accordingly, the soil texture varied from sand (EP1 and EP2) to loamy sand in EP3, with the surface horizons overlying sandy loam, loamy sand and sandy clay loam at the subsurface, respectively. The silt distribution was irregular within soil depth irrespective of landscape positions, probably because these soils were developed in situ. It was also observed that EP1 had the highest amount of silt with an average value of 14.28% [22]. The trend of silt content in the surface horizon was EP2 < EP3 = EP1. Clay fraction was next to sand in dominance in all the pedons. Clay was higher in the subsurface than surface horizons. Its distribution within the subsoil of the EP1 and EP2 pedons was irregular, characteristic of a cambic horizon. Results show that there was an increase with depth in clay content of the soil at the lower slope and this was attributed to vertical migration of clay down the profiles and/or high rates of clay formation in subsoil horizons. The increase in clay content of the subsoil (B) horizon can be adduced to the synthesis of secondary clay and the weathering of primary minerals [23, 24]. The mean silt/clay ratio was 2.14, 0.88 and 0.92, respectively for EP1, EP2 and EP3 which was an indication that the soils of EP2, EP3 were relatively young. Silt/clay ratio of < 1.00 could mean that these soils had undergone ferralitic pedogenesis. As we move from the upper to lower slope position silt/clay ratio decreased with depth which can also be adduced to clay migration and this indicates the existence of a sheet erosion in top soil horizons.

3.3. Chemical Properties of the Experimental Soils

The soil reaction, especially for the surface horizons, was rated to be slightly acidic with mean pH values of 6.08, 6.06 and 5.84 for EP1, EP2 and EP3, respectively. The pH range of 5.5 - 7.0 as optimal for overall satisfactory availability of plant nutrients had earlier been reported [25]. The Cation Exchange Capacity (CEC) of the soils ranged from low to medium [26] i.e 4.00 to 7.03 cmol (+) kg⁻¹ of soil (Table 4) and showed vari ation with depth of the profiles. This variation indicates that the soils in the pedons vary in their clay mineral suite [27]. Higher values were generally obtained in the subsurface compared with the surface horizons. The exchange complex of the soils was dominated by Ca followed by Mg, K and Na (Table 4). Calcium (Ca) and Mg are the dominant cations in the soils and they generally increased with soil depth on the exchange complex except for EP1 and this can be adduced to the leaching of the cations. The range of Ca: Mg ratio of the soils was 1.55 - 2.62 and the K: Mg ratio range between 0.08 - 0.42 was still a good option for crop production. The trend of distribution of Ca within the horizon was in the order of EP1 < EP3 < EP2 signifying the possibility of lateral movement of the nutrient element from upper to middle slope then the lower slope. On the other hand, Mg irrespective of soil depth and landscape position was rated to be moderate. Potassium was rated low in all the horizons. Sodium was also generally rated low in all the pedons showing that the experimental soils were not sodic. Base saturation was rated medium to high and was a reflection of basic cations in the exchange complex [28]. Results (Table 4) show that organic C and total N decreased with depth. This was due to the presence of vegetative material and root activities on the surface soils as compared with the subsurface. The Organic Carbon (OC) content of the soils in the pedons ranged from 0.23 - 0.26% and was low in accordance with the rating as described in a study [26] and Soil Science correlation committee. Organic C content of less than 1.16% for tropical soils has been reported to be an indication of soil degradation and a high risk of soil erosion [29]. High intensity agricultural activities deplete soil organic matter content [30]. The total nitrogen content of the soils ranged from 0.02 to 1.11% and was also low. Although pedon EP3 has the advantage of receiving soluble nutrients from the upper slopes, its lower N content may be linked to the higher water-table which might be contributing immensely to leaching of N especially the nitrate form. The difference in organic C and total N between EP1 and EP3 could be adduced to the movement of organic material by water and clay contents variation [31]. The C: N ratio of the soils varied with depth and was within the range of 0.21 - 9.58. The ratio is not within the range to provide nitrogen in excess of microbial needs [32] indicating low microbial activities for humification and mineralization of organic residues. Available P contents of the pedons ranged from 4.93 mg kg^{− 1} in the C horizon of EP2 to 97.23 mg kg^{− 1} in the A1 horizon of EP3. It was rated medium in EP1 and EP2 to high in EP3. High level of available P in EP3 may be due to its low solubility/mobility in soil. Also, the high availability of P content in the sub soil of the three pedons could be due to deposition of P in the sub- soils [33].

Soils of the middle and lower slope positions are classified as Alfisol at Order level because of the presence of an argillic horizon. It is Ustalfs at the Suborder level because of the ustic soil moisture regime, Plinthustalfs at Great Group level and Typic Plinthustalfs at the Sub- group level. Soil of the upper slope position is classified as Inceptisol at Order level because it has cambic horizon (an altered horizon in which the parent material has been changed into soil by formation of structured - clay), Ochrepts at the suborder level, Ustochrepts at the Great Group level and Typic Ustochrept at the Sub - group level. According to the World Reference Base (WRB) EP2 and EP3 are classified as Luvisols at the Reference Soil Groups (RSGs) for having an argic horizon overlain by loamy sand. At the lower level, they are classified as Haplic Luvisols for having a texture of loamy sand. EP1 is classified as Cambisols because of a cambic horizon.