Limnic material (WRB)

Limnic material is one of the diagnostic materials used, in the WRB system, to discriminate some soils from others.

Description

Limnic material (from Greek limnae, pool) includes both organic and mineral materials that are :

• deposited in water by precipitation or through action of aquatic organisms, such as diatoms and other algae

or :

• derived from underwater and floating aquatic plants and subsequently modified by aquatic animals.

Field identification

Limnic material occurs as subaquatic deposits (or at the surface after drainage). Four types of limnic material are distinguished :

• Coprogenous earth or sedimentary peat : dominantly organic, identifiable through many faecal pellets, Munsell colour value (moist) 4 or less, slightly viscous water suspension, non- or slightly plastic and non-sticky consistence, shrinking upon drying, difficult to rewet after drying, and cracking along horizontal planes.
• Diatomaceous earth : mainly diatoms (siliceous), identifiable by irreversible changing of the matrix colour (Munsell value 3, 4 or 5 in field moist or wet condition) as a result of the irreversibly shrinkage of the organic coatings on diatoms (use 440× microscope).
• Marl : strongly calcareous, identifiable by a Munsell colour value, moist, of 5 or more, and a reaction with 1 M HCl. The colour of marl does not usually change upon drying.
• Gyttja : small coprogenic aggregates of strongly humified organic matter and minerals of predominantly clay to silt size, 0.5 percent or more organic carbon, a Munsell colour hue of 5 Y, GY or G, strong shrinkage after drainage and an rH value of 13 or more.

See also

• The FAO reference text, (2007 version)