Mineral weathering, pedogenesis and characterization of expansive soils from Serowe, Central Botswana

Abstract

Expansive soils tend to be fairly high in natural fertility and their unique morphology makes them quite interesting. Soils found in Serowe, Central Botswana, commonly exhibit variable degrees of periodic shrinking and swelling which often leave visible cracks and sometimes undulating micro-topography (gilgai) on the landscape. The use of expansive soils is often constrained due to severe management problems often associated with their physico-chemical properties. Therefore, basic site-specific pedological research is pivotal to understanding some of the unresolved edaphological and environmental aspects of expansive soils. The overarching aim of this study is to undertake a pedological investigation of the expansive soils from Serowe, which hitherto have been poorly studied. The main objective of this study is to advance our understanding on the pedology of the soils from Serowe. Specific objectives include to: (i) characterize the soils using macromorphological, physical, chemical, mechanical and mineralogical properties; (ii) evaluate mineral weathering intensities of the soils; (iii) identify the pedogenic processes responsible for their formation; and (iv) provide an updated high resolution classification of the soils in accordance to FAO-WRB system and USDA Soil Taxonomy. Five representative soil profiles (SP1, SP2, SP3, SP4 and SP5) were randomly sited on flat to undulating landscapes and samples were collected from each genetic horizon for laboratory analysis. Selected pedological parameters including particle size distribution, pH (H2O and KCl), bulk density, porosity, organic matter, calcium carbonates contents, electrical conductivity, cation exchange capacity, coefficient of linear extensibility (COLE) and Atterberg limits were determined using routine laboratory procedures. X-ray fluorescence spectrophotometer and x-ray diffractometer were used for total elemental composition and clay mineral analysis respectively. Geochemical ratios including CIA, CIW, and ICV were used to assess mineral weathering; mass-balance plots for elemental redistribution and pedogenic ratios for quantitative pedogenesis. In-situ observations showed that all the soils have vertic properties and were generally very deep (>150 cm). By the virtue of their colour, the soils qualified as dark and chromic. With the exception of SP3, all other soil profiles had sandy clay loam texture with sand content ranging from 60.2 to 85.0 %, silt 0.2 to 15.0 % and clay content 20.8 to 33.8 %. SP3 and SP5 had consistently subangular blocky structure while SP1, SP2 and SP4 had subangular blocky structure on the top horizons (<25 cm) and angular blocky structure in the subsurface horizons. The bulk density values ranged from 1.14 to 1.67g/cm3 with medium to high porosity (20 – 40 %). pH (H2O) values ranged from 6.9 to 8.0. The electrical conductivity values ranged from 0.07 to 0.40 dS/m. CEC values for all profiles were medium to high (˃ 12 Cmol/kg). The COLE rod values were high to very high (0.4 - 0.8). Elemental dominance for profiles SP1, SP2, SP3 and SP4 was: SiO2 ˃ Al2O3 ˃ Fe2O3 ˃ CaO ˃ MgO, whereas SP5 had SiO2 ˃ Al2O3 ˃ CaO ˃ Fe2O3 ˃ MgO. The soils were young and immature with ICV ˃ 1 with incipient to moderate weathering (50 – 80 %) except four horizons; SP1 (A and Ak), SP2 (Bt) and SP3 (Bsstb) which showed intense weathering (˃ 80%). The soils had mixed mineralogy consisting of smectite, nontronite, montmorillonite, vermiculite, illite, mica, sepiolite, palygorskite, chlorite, chrysotite and kaolinite. Local environmental factors including parent material and topography exerted the most influence on the pedoclimate which led to the pedogenesis of these soils via humification, clay-humic complexation, profile homogenization by vertic pedoturbation, calcification, hydrolysis, and salinization. In line with WRB system, SP1, SP2 and SP4 were Grumic, gleyic Vertisols (Calcuric, mesotrophic) whereas SP3 and SP5 are Vertic, gleyic Acrisols (Abruptic) and Vertic, gleyic Alisols respectively. The Soil Taxonomy correlation were Glayey, smectitic, hyperthermic haplusterts for SP1, SP2 and SP4, and were fine-silty, mixed, hyperthemic haplargirds for SP3 andSP5. Application of the recommended practices would go a long way in ensuring sustainable management of the soil resources given its huge agronomic potentials.