With the term “duplex soil”, Northcote defined a primary profile in his Factual Key classification.[1] He described a group of texture contrast soils where the B horizon is dominated by a texture class one and a half (or more) finer than the A horizon. In addition, the clear to sharp change between the two horizons must occur within 0.1 m.[1][2]
Texture in duplex soils is highly variable, with the top-soils ranging from coarse sand to clay loam and the subsoils from light to heavy clay.[2][3] Some duplex soils are distinguished by the presence of an A2 bleached horizon, a character also used as a diagnostic key for the distinction between these type of soils.[1]
The diagnostic properties used by Northcote for the definition of duplex soils consider only the soil texture (texture contrast and type of boundary between horizons A and B) and the colour is used for their differentiation (i.e. brown, red and yellow duplex soils). Under the Australian Soil Classification [4] they can be included in different orders such as Podosols, Sodosol, Chromosols or Kurosols.