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Gley Color Formation - White Oak Creek, NC

Hydric soils are defined as soils that "formed under conditions of saturation, flooding, or ponding that lasted long enough during the growing season to develop anaerobic conditions in their upper part".  The presence of these soils may indicate possible locations for wetland restoration or conservation.   

Oxidation reduction reactions must occur in the upper part of the soil for it to be considered hydric. These reactions may result in the reduction and translocation of Fe oxides and might result in the development of gray and gley colors, along with brighter red or yellow mottles.  These color features are called Redoximorphic Features – and may be divided into 2 catagories – redox depletions (grays) and redox matrix (red/yellow concentrations).  Redoximorphic features are used to indicate the presence of water.  However, are not considered hydric field indicators because they cannot be used to determine how long a water table is present.

Gley colors are rare, but when they occur they are normally found in wetland soils.  Therefore, their presence in a soil profile is more significant to those who are trying to delineate wetlands.  Gley colors have hues that are blue, bluish-green, green, greenish-yellow, or purplish-blue.  Two pages in the Munsell Color Book are devoted to gley colors. Note that the colors have values between 2.5 and 8, and chromas of 1 or 2. Hues are 10Y, 5GY, 10GY,5G, 10G, 5BG, 10BG, 5B, 10B, 5PB, and N.

Gley colors appear to be formed by Fe minerals that contain Fe in a "reduced" form.  Reduced Fe has a valence state of 2+, and is written Fe(II).  Fe(II) is colorless and soluble in water.  It may form gley colors when it combines with anions such as CO₃, SO₄, and PO₄. These colors may only be visible in waterlogged, anaerobic soils. When the soils are dried the colors may change as the Fe oxidizes.

Different types of iron minerals are responsible for the color formations in soil.  The common iron minerals that affect color formation are the following: hematite, goethite, lepidocrocite, ferrihydrite, ferric hydroxide, ferrous hydoxride, “green rust”, and siderite. 

Although green rust is not a common mineral, even in small concentrations, it greatly affects iron concentrations on the soil exchange and in soil solution.  There are 2 forms of green rust that may produce gley colors in soils.  The green rust one (GR1) with planar anions e.g. Cl^- and CO₃^2- and green rust two (GR2) with three-dimensional anions in the interlayers e.g. SO₄^2-.  GRs may occur as minerals in the soil environment and may control iron solubility in water.  Green rust within the soil is a bluish-green color and when exposed to air it changes to red/yellow colors. GR1(OH-) minerals are significant control Fe2+ in soil solution.

References:

Genin, Jean-Marie R.  (1998).  Thermodynamic Equilibria in Aqueous Suspensions of Synthetic and Natural Fe (II) –FE (III) Green Rusts: Occurrences of the Mineral in Hydromorphic soils.  Environmental Science and Technology 32(8), 1058-1068.

Richardson, J.L. and M.J. Vepraskas, eds.  Wetland Soils: Genesis, Hydrology, Landscapes, and Classifications.   New York: Lewis Publishers, 2001.

https://courses.soil.ncsu.edu/ssc570

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