Genesis, Classification and Land Use Planning of Soils Developed From Limestone Geological Formations in Cross River State, Nigeria

Abstract

Soils developed from limestone geological formations in the humid tropics are unique in properties and differ from similar soils in areas with reduced rainfall. This research was to genetically investigate the soils developed from limestone geological formations in Cross River State regarding their morphological, physical and chemical properties and the forms of Fe and Al, elemental oxide content, and clay mineralogy in order to classify them and identify the dominant soil-forming processes. Soil mapping units were evaluated for oil palm cultivation, and indices of chemical weathering were used for pedogenetic evaluation. Google satellite imageries of the areas underlain by limestone were obtained and used as base maps. Digital elevation models (DEM) of the areas ranging from Ishibori to Agoi-Ibami and Mfamosing variously underlain by shale-limestone-sandstone (SLS), sandstone-limestone (SL), and shale-limestone with sandstone intercalation (SLSi), respectively were obtained from USGS Explorer SRTM Arc-second Global. Two elevation ranges representing mapping units in the SLS (IH1, IH2) and three elevation ranges each in SL (AI1, AI2, AI3) and SLSi (MF1, MF2, MF3) were identified. Two profile pits were sited randomly and dug in each mapping unit, while augered soils were used to confirm soil boundaries. Soil profiles were described, while core and profile samples were obtained and processed for laboratory analyses. Soil samples meant for elemental oxide content were obtained at a depth interval of 30 cm to the soil profile depth. The samples were further pulverized with an automatic mill and homogenized to 3-4 µm before analysis. Detailed soil maps were produced at 1:4000, 1:9500 and 1:15000 in SLS, SL and SLSi, respectively. The soils were 48 to 200 cm deep with epipedons ranging from 9 to 50 cm in thickness. Many to few Fe-Mn nodules and concretions were obtained in the endopedons of SLS and SL, while cracks of various widths were recorded in the endopedons of SL. Surface soil colours were in the range of dark brown (7.5YR 3/2) to black (5YR 2.5/1) in SLS, and very dark grey (7.5YR 3/1) and black (5YR 2.5/1) in SLSi. The soils were either loamy sand, sandy loam or sandy clay loam. However, more gravels were obtained in SLS. Bulk density ranged from 0.71 to 1.66 g/cm3. Saturated hydraulic conductivity had range of 0.49-243.60 cm/h in the soils and correlated with bulk density (r= -0.712). Ranges of 5.2-7.3, 5.3-7.7 and 5.1-6.7 were obtained in SLS, SL and SLSi, respectively, for soil pH (H2O). Organic C had ranges of 1.37-86.64, 1.03-22.16 and 0.69-46.34 g/kg for SLS, SL and SLSi, respectively, with comparatively higher values in the lowest elevation ranges. Calcium carbonate equivalent was 17.7-38.8, 15.8-30.0 and 6.1-45.7 g/kg in soils over SLS, SL and SLSi, respectively and correlated with clay (r= 0.387) and exchangeable Ca (r= 0.413) at p< 0.05. Dithionite extracted Fe ranged from means of 34.7 in SLS to 8.77 g/kg in SL, while dithionite Al ranged from 4.37 in SLSi to 1.53 g/kg in SL. Dithionite Fe and Al were higher than their oxalate counterpart in the studied soils. Crystalline forms of Fe and Al were highest in soils over SLS and SLSi. The highest values of immobile oxides such as TiO2, MnO2, Fe2O3 and ZrO2 had means of 1.01, 0.12, 7.18 and 0.05 %, respectively in IH1P1. Ruxton index, weathering index and molar ratio identified soils developed from SL and SLSi as the most weathered. Quartz, kaolinite, montmorillonite, vermiculite and palygorskite are the most dominant and distributed clay minerals in the studied soils. Lessivation, calcification-decalcification, gleization, lateritization and latosolization, faunal pedoturbation and mineralization were the dominant soil forming processes. IH1P1 and IH1P2 were classified as Loamy Skeletal, Mixed, Superactive, Isohyperthermic, Typic Rhodustults (Ferric Rhodic Acrisols; Ochric) and Loamy Skeletal, Mixed, Superactive, Ishyperthermic, Typic Paleustult (Ferric Acrisols; Cutanic), respectively, while IH2P1 and IH2P2 were Fluvaquentic Humaquept (Gleyic Cambisol; Humic) and Humic Endoaquept (Stagnic Gleyic Cambisol; Humic), respectively. AI1P2 qualified as Sandy, Mixed, Superactive, Isohyperthermic, Rhodic Paleudults {Ferric Acrisols (Ochric)}, while AI1P2 was Fine loamy, Mixed, Superactive, Isohyperthermic, Plinthic Paleudults xiv {Plinthic Acrisols (Differentic, Hyperdystric, Ochric)}. AI3P1 qualified as Humic Endoaquept {Dystric Gleyic Cambisols (Humic, Loamic)}, while AI3P2 met the criteria for Typic Endoaquept {Dystric Cambisols (Arenic, Ochric)}. Currently, IH1, AI1, AI2, MF1 and MF2 were S2 for oil palm cultivation; however, only MF1P2 and MF2 were S1, potentially. In the land capability classification system, IH2, AI3 and MF3 were rated as class III soils, while IH1, AI1, AI2P1, MF1P1 and MF2 were rated as class II soils. However, AI2P2 and MF1P2 met the requirements for class I soils. Soils developed from SL and SLSi are more developed than those over SLS and were more suitable for oil palm cultivation, however, the entire soils are at the transitory stage from intermediate to an advanced stage of development.