Pedological aspects of earthenware potterymaking soils from Kweneng district, Botswana

Abstract

Ethnopedological and pedological investigations have been largely centred on agricultural land uses and associated practices. However, indigenous peasants have also relied on soil resources to eke their livelihood in other activities including non-commercial pottery making. Clayey soils, weathered sandstone and other materials are traditionally used in the Kweneng District of Botswana for earthenware pottery. This study bridges the gap in the dearth of information on the pedological characteristics of these pottery-making soils against the traditional soil knowledge. The objectives of this study are: (i) to characterize the soils using their morphological, physicochemical and mineralogical properties; (ii) to assess the degree of chemical weathering of the soils; (iii) to quantify the operating pedogenic processes and (iv) to classify the soils following USDA Soil Taxonomy and correlate it to WRB system. Soil samples were collected from two profile pits each sited in Letlhakeng, Takatokwane pan, and two excavations in Khudumolapje and Letlhakeng respectively. Standard pedological parameters, such as structure, colour, bulk density, porosity, pH, electrical conductivity, organic matter content, carbonate contents and magnetic properties were determined using routine laboratory procedures. The total elemental composition and clay mineralogy analysis were done using the x-ray fluorescence spectroscopy and x-ray diffractometry. Chemical index of alteration (CIA), chemical index of weathering (CIW) and index of compositional variability (ICV) were used to assess chemical weathering intensities. Molecular pedogenic ratios were used to quantify pedogenesis in the soils. The soil colour spanned grey to greenish at Takatokwane, brown reddish in Letlhakeng, and brownish gray in Khudumolapje. Sandy texture dominated the particle size distribution of the soils, with most having more than 60 % sand and less than 30 % clay. These soils are moderate to highly porous (20 - 40 %). The soil pH ranged from 7.0 to 9.6 with high CEC (< 20 Cmol kg-1 ) for Takatokwane and medium CEC (12-20 Cmol kg-1 ) in Letlhakeng. Element abundance followed SiO2 > Al2O3 > Fe2O3 > TiO2 in all the soil profiles. The clay mineral assemblages of the soils are smectite, sepiolite, vermiculite, kaolinite, montmorillonite and illite. Variations in the properties of these soils could be mainly attributed to the differences in topography. The degree of weathering of the soils are generally incipient as they had ICV >1 and CIA values < 60. The dominant pedogenic processes in the soils include hydrolysis, lessivage, leaching, eluviation, illuviation and gleization. Based on USDA Soil Taxonomy system, the soils qualified as Aridisols with subgroups of Natrargids and Petrocalcids at Takatokwane and Letlhakeng respectively with the WRB correlations as Gleyic Arenosols (Aridic) and Calcisols (Loamic, gleyic, aridic). Site-specific pedological studies of this kind would continue to provide insights into the best management practices needed for the sustainable use of soil resources.