Clay mineral associations in soils of the Lusikisiki area, Eastern Cape Province, and their agricultural significance

Abstract

The study deals with changes in clay mineral associations with soil development and related implications for land users in the Lusikisiki area of the Eastern Cape Province of South Africa. Fifteen profiles, encompassing all major soil forms in the region, were investigated. The pedons developed from pre-Jurassic sedimentary rocks or Jurassic dolerite under a modeled annual precipitation of −950 to 1250 mm. The least weathered horizons in soils, derived from sedimentary rocks, contained mica, illite/smectite interstratifications (I/S) and minor amounts of kaolinite, a clay mineral suite, characteristic of the geothermal history of the region. Profiles, developed from dolerite, formed from an association of plagioclase and pyroxene. The A horizons in the vast majority of the profiles were dominated by either kaolinite, quartz or gibbsite, but generally contained an association of at least two of the above minerals. In some profiles significant amounts of hydroxy-interlayered vermiculite (HIV) were also present. The weathering pathway, therefore, seems to lead from illite, I/S and smectite to HIV, kaolinite and gibbsite. Quartz is hypothesized to be of aeolian origin, possibly reflecting desert loess. Mineral parageneses were not related to topographic position and little related to parent material. Soils with greatly different degrees of pedogenesis had almost identical clay mineral suites in their uppermost horizons.