Redox conditions related to interflow in a soil of the Kroonstad form in the Weatherley catchment

Abstract

The Kroonstad soil form, in the Weatherley catchment, Eastern Cape Province consists of an orthic A / E / G horizon sequence, and is a typical gleyed soil of South Africa. The profile has an uncommon diffuse E / G transition. Water contents from weekly neutron water meter readings and redox response as indicated by dissolved Fe²⁺ concentration were correlated with daily rainfall data from automated weather stations. Results showed that reducing conditions were more pronounced in the E horizon than in the orthic A or G horizon as indicated by the Fe²⁺ concentration. This is contradictory to what is normally expected. Response in soluble iron concentration in the orthic A horizon was largely associated with rainfall events. Responses in the E horizon could not be linked to current rain events. A time lag was found to exist between a response in the A and the E horizon. The G horizon responded to seasonal changes. The inability to relate current rainfall events to redox conditions as indicated by soluble Fe²⁺ response in the E horizon could be attributed to water entering the E horizon laterally through interflow. This would also explain the observed time lag. This is typical of perched water tables. The lack of response in the G-horizon is an indication of a year round flow pattern controlled by seasonal changes and is typical of phreatic water tables. It is postulated that the non-abrupt E / G transition is an indication of the dominant role of the phreatic water table during pedogenesis, with upwards and lateral movement of water into the E horizon. It is further postulated that the nature of the E / G transition could serve as a valuable criterion to distinguish between the wetter and drier soils of the Kroonstad form.