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

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 (