Hydrological classification of orthic A horizons in Weatherley, South Africa

Abstract

Orthic A horizons carry little interpretive, especially hydrological, value. This paper aims to elucidate the hydrological interpretation of orthic A horizons. Measured water contents in the orthic A horizons of 28 profiles in the Weatherley catchment of South Africa were used to classify the topsoils into wetness classes. The very transitory wetness class had no Fe mottles; overlaid red apedal B, yellow-brown apedal B, or neocutanic B horizons; and had >650 mm below the orthic A horizon to an impervious layer. The transitory wetness class had few Fe mottles and had <650 mm to an impervious layer. The common wetness class had common Fe mottles; had <550 mm to an impervious layer; and overlaid an E or soft plinthic B horizon with many Fe mottles. The persistent wetness class had common or many Fe mottles; overlaid a G horizon or an E horizon with many Fe mottles. Duration of water saturation in the orthic A horizons generally increased with the occurrence of the following subsoil horizons: neocutanic B < yellowbrown apedal B < yellow E < grey E < soft plinthic B < G. Depth to the impervious layer generally decreased in the same order.

Keywords: topsoil, Weatherley, wetland, wetness class

South African Journal of Plant and Soil 2012, 29(2): 101–107