[1] [2] With the term "duplex soil", Northcote defined a primary profile in his Factual Key classification.
In addition, the clear to sharp change between the two horizons must occur within 0.1 m.[3][4] 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.
[4][5] 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.
[3] 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 [6] they can be included in different orders such as Podosols, Sodosol, Chromosols or Kurosols.