The term “duplex” is used in Australia for soils with contrasting texture between soil horizons, although such soils are found in other parts of the world. Duplex soils are also termed “texture contrast soils”. 
With the term “duplex soil”, Northcote defined a primary profile in his Factual Key classification. 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.
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. 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.
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  they can be included in different orders such as Podosols, Sodosol, Chromosols or Kurosols.
- Hardie, Marcus A.; Richard B. Doyle; William E. Cotching; Shaun Lisson (2012). "Subsurface Lateral Flow in Texture-Contrast (Duplex) Soils and Catchments with Shallow Bedrock". Applied and Environmental Soil Science: Article ID 861358. doi:10.1155/2012/861358. Retrieved 25 October 2012.
- Chittleborough, D. J. (1992). "Formation and pedology of duplex soils". Australian Journal of Experimental Agriculture. 32 (7): 815–825. doi:10.1071/ea9920815.
- Northcote, K. H., and CSIRO. 1979. A factual key for the recognition of Australian soils / by K.H. Northcote. 4th ed. Adelaide, S. Aust: Rellim Technical Publications.
- Tennant, D, G Scholz, J Dixon, and B Purdie. 1992. Physical and chemical characteristics of duplex soils and their distribution in the south-west of Western Australia. Australian Journal of Experimental Agriculture 32 (7):827-843.
- Gardner, WK, RG Fawcett, GR Steed, JE Pratley, DM Whitfield, Hvan Rees, and Rees H Van. 1992. Crop production on duplex soils in south-eastern Australia. Australian Journal of Experimental Agriculture 32 (7):915-927
- Isbell, Raymond, and Ebooks Corporation. 2002. The Australian Soil Classification : Revised Edition. In Australian Soil and Land Survey Handbooks, 4. 153 p. Melbourne: CSIRO PUBLISHING.