soil color

The Munsell System

  • Hue

    • The pure colors have the number 5 is “pure”

      • 5 indicates the primary color is pure

      • 5R is pure red

    • move to the left of the primary hue, the number increases

      • 10RP indicates it is mostly red, and slightly purple

    • A perfect 50/50 mix has a 5 with two letters

      • ex: 5RP is 50% red 50% purple

    • Moving one more chip to the left:

      • 10P means it is mostly purple and slightly red

  • Value

    • Value is the lightness of a color

    • ranges from 0 (very dark, black) to 10 (very light, white)

    • Relates to how much light is reflected/absorbed

      • White reflects light

      • black absorbs light

  • Chroma

    • Chroma is the concentration of purity of the color

    • The higher the concentration, the higher the chroma number.

    • ranges betweem 0-12

    • If color was a wagon wheel:

      • Hue would be the individual spokes

      • Value would be position on the axel

      • chroma is the distance from the axel.

Recording Soil color

  • Soil must be moist

    • not soaked to the point of glistening (matte finish)

  • sometimes soil colors are reported as dry color

    • unless a soil profile description explicitly says soil colors were recorded as dry color, assume that soil colors are moist colors.

  • Always use sunlight

  • no sunglasses or tinted lenses

  • use fresh face or ped

  • don’t crush or rub the soil

  • Determine the matrix (dominant color) and all subordinate colors (mottles)

  • HVAC (hue value&chroma)

How is Soil Color Used?

  • Diagnostic Epipedons

    • the more OM, the darker the color

    • mollic and umbric epipedons have lots of OM

    • specific values and chromas are required for the epipedons

  • Separating soil series

    • soils of the same taxonomic family can be separated by soil colors

  • Determining depth to seasonal high water table

    • agricultural drainage

    • wetland delineation

    • oxidation-reduction reactions cause gray colored “mottles” to form below the water table

Oxidation: produces electrons when two things are present

  • e- produced when OM is eaten

Reduction: reactions with e- and H+

Oxidation and Reduction occur together

What happens in waterlogged soils?

  • OM continues to be decomposed

  • no O2 present

  • Instead, “alternative electron acceptors” are used

    • NO3-. MnO2, Fe2O3, SO42- and CO2

Iron Reduction Reaction

Formation of Redoximorphic Features

  • Mottles form where there is:

    • OM

    • active microbes

    • Saturated conditions

    • Fe present

  • Water table:

    • Depth at which there is free-standing water

    • depth varies by season

  • Seasonally high water table

    • marked by depth of mottles with chroma 2 or less

Drainage Class and Saturated Conditions

  • Developed by USDA to identify areas of farmland needing drainage.

  • Saturated conditions in the rooting zone negatively impacts plants.

  • Saturated conditions near the surface can reduce soil strength causing problems when machinery is being used

    • Construction

    • Planting/harvesting

Wetland Delineation