Diagnostic Horizon a soil horizon having a set of quantitatively defined properties which are used in soil classification. Criteria used are observed (colour, hardness, temperature) or values (depth, contents, volumes, densities) measured by methods independent of the observer. As the characteristics of the diagnostic horizons are the result of pedogenesis, the soil classifications based on these horizons are mainly genetic, and are achieved by morphological considerations. In the US Soil Taxonomy a number of diagnostic horizons have been distinguished. Further they have been divided into two main types. The diagnostic horizons that develop near the surface of the soil profiles have been designated as the epipedons; while those that occur in the lower zone of the soil profiles are called the endopedons or the subsurface diagnostic horizons. The eight important epipedons are anthropic, folistic, histic, melanic, mollic, ochric, plaggen and umbric epipedons.
The most widespread epipedon in the soils of Bangladesh is the ochric epipedon. This epipedon fails to meet the definition for any of the other seven epipedons, because it is too thin or too dry, has too high a colour value or chroma, contains too little organic carbon, has too high an 'n' value or melanic index or is both massive and hard when dry. The histic epipedon has very high context of organic matter (more than 30%).
The important diagnostic subsurface horizons are agric, albic, argillic, calcic, cambic, glossic, gypsic, kandic, natric, oxic, petrocalic, petrogypsic, placic, salic, sombric and spodic horizons. Based on diagnostic horizons the general soil types of Bangladesh can be grouped as the Black Terai soils (humic cambisols); Noncalcareous Brown Floodplain soils (eutric cambisols); Calcareous Brown Floodplain soils (calcaric cambisols); Noncalcareous Alluvium; Grey Floodplain soils; Calcareous Alluvium; Calcareous Dark Grey Floodplain soils (calcaric fluvisols); Peat (dystric histosols); acid sulphate soils (thionic fluvisols); Grey Piedmont soils and Grey Terrace and Valley soils (eutric gleysols); Deep Red-Brown Terrace soils and Brown Mottled Terrace soils (dystric nitosols); brown hill soils and Shallow Red-Brown Terrace soils (dystric cambisols). [Md Sultan Hussain]
See also alluvial soil.