Soil Color

Since I missed the class that you all talked about soil color in, I wanted to write my blog on soil colors and what they represent. Also, since our test is coming up and I have to take it early it would be really nice to go over it while studying.

Soil color often indicates soil moisture state and is used for determining hydric soils. Often described by using general terms, such as dark brown, yellowish brown, etc., soil colors are also described more technically by using Munsell soil color charts, which separate color into components of hue (relation to red, yellow and blue), value (lightness or darkness) and chroma (paleness or strength). The color of soil is primarly influenced by its organic matter content, which typically imparts a dark-blueish  hue to the topsoil, as well as its mineral compositions and degree of oxidation. Soils containing iron materials, have a reddish or yellowish hue indicating a well-areated condition. A grayish or greenish mottled indicated that the soil is poorly aerated and reduction of iron and other mineral elements. In some cases the aggregates in soils will have a reddish exterior (oxidized) and a grayish interior due to the locally anaerobic conditions.

Condition Dark (dark grey, brown to black) moderately dark (brown to yellow brown) light (pale brown, yellow)
organic matter high medium

low

erosion factor low medium high
aeration high medium low
Available nitrogen high medium low
fertility high medium

low

Munsell Color Chart:

Red, brown, yellow, yellowish-red, grayish-brown, and pale red are all good descriptive colors of soil, but not very exact. Just as paint stores have pages of color chips, soil scientists use a book of color chips that follow the Munsell System of Color Notation (www.munsell.com). The Munsell System allows for direct comparison of soils anywhere in the world. The system has three components: hue (a specific color), value (lightness and darkness), and chroma (color intensity) that are arranged in books of color chips. Soil is held next to the chips to find a visual match and assigned the corresponding Munsell notation. For example, a brown soil may be noted as: hue value/chroma (10YR 5/3). With a soil color book with Munsell notations, a science student or teacher can visually connect soil colors with natural environments of the area, and students can learn to read and record the color, scientifically.

I found this table at http://soils.usda.gov/education/resources/k_12/lessons/color/ and I thought it was really helpful in memorizing:

Mineral Formula Size Munsell Color
goethite FeOOH (1-2 m m) 10YR 8/6 yellow
goethite FeOOH (~0.2 m m) 7.5YR 5/6 strong brown
hematite Fe2O3 (~0.4 m m) 5R 3/6 red
hematite Fe2O3 (~0.1 m m) 10R 4/8 red
lepidocrocite FeOOH (~0.5 m m) 5YR 6/8 reddish-yellow
lepidocrocite FeOOH (~0.1 m m) 2.5YR 4/6 red
ferrihydrite Fe (OH)3 2.5YR 3/6 dark red
glauconite K(SixAl4-x)(Al,Fe,Mg)O10(OH)2 5Y 5/1 dark gray
iron sulfide FeS 10YR 2/1 black
pyrite FeS2 10YR 2/1 black (metallic)
jarosite K Fe3 (OH)6 (SO4)2 5Y 6/4 pale yellow
todorokite MnO4 10YR 2/1 black
humus 10YR 2/1 black
calcite CaCO3 10YR 8/2 white
dolomite CaMg (CO3)2 10YR 8/2 white
gypsum CaSO4√ó 2H2O 10YR 8/3 very pale brown
quartz SiO2 10YR 6/1 light gray
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One Response to Soil Color

  1. cdw526 says:

    Thanks for this. It definitely helps the studying process. And its good to know that the pretty shade of yellow we dug up last week was due to the iron content and not necessarily the horse poo.

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