Gleyic colour pattern (WRB)
Gleyic colour pattern is one of the diagnostic properties used, in the WRB system, to discriminate some soils from others.
Contents
Description
The term "Gleyic colour pattern" (from Russian gley, mucky soil mass) refers to soil materials saturated with groundwater (or were saturated in the past, if now drained) for a period that allows reducing conditions to occur (this may range from a few days in the tropics to a few weeks in other areas).
Criteria
A gleyic colour pattern shows one or both of the following :
- 90 percent or more (exposed area) reductimorphic colours, which comprise neutral white to black (Munsell hue N1/ to N8/) or bluish to greenish (Munsell hue 2.5 Y, 5 Y, 5 G, 5 B)
or :
- 5 percent or more (exposed area) mottles of oximorphic colours, which comprise any colour, excluding reductimorphic colours.
Field identification
A gleyic colour pattern results from a redox gradient between groundwater and capillary fringe causing an uneven distribution of iron and manganese (hydr)oxides. In the lower part of the soil and/or inside the peds, the oxides are either transformed into insoluble Fe/Mn(II) compounds or they are translocated; both processes lead to the absence of colours with a hue redder than 2.5 Y. Translocated Fe and Mn compounds can be concentrated in the oxidized form (Fe[III], Mn[IV]) on ped surfaces or in biopores (rusty root channels), and towards the surface even in the matrix. Manganese concentrations can be recognized by strong effervescence using a 10-percent H2O2 solution.
Reductimorphic colours reflect permanently wet conditions. In loamy and clayey material, blue-green colours dominate owing to Fe (II, III) hydroxy salts (green rust). If the material is rich in sulphur (S), blackish colours prevail owing to colloidal iron sulphides such as greigite or mackinawite (easily recognized by smell after applying 1 M HCl). In calcareous material, whitish colours are dominant owing to calcite and/or siderite. Sands are usually light grey to white in colour and often also impoverished in Fe and Mn. Bluish-green and black colours are unstable and often oxidize to a reddish brown within a few hours of exposure to air.
The upper part of a reductimorphic layer may show up to 10 percent rusty colours, mainly around channels of burrowing animals or plant roots.
Oximorphic colours reflect alternating reducing and oxidizing conditions, as is the case in the capillary fringe and in the surface horizons of soils with fluctuating groundwater levels. Specific colours indicate ferrihydrite (reddish brown), goethite (bright yellowish brown), lepidocrocite (orange), and jarosite (pale yellow). In loamy and clayey soils, the iron oxides/hydroxides are concentrated on aggregate surfaces and the walls of larger pores (e.g. old root channels).
RSG in which gleyic colour pattern can be observed
See also
- The FAO reference text, (2007 version)
