Munsell Soil Classification

Hue, Value and Chroma

• The first scientific color chart was suggested by American artist Albert Henry Munsell in his 1907 book, "A Color Notation." The Munsell System uses hue, value and chroma to evaluate color and can be applied for many uses. Hue means the spectral color in five main groups of red, yellow, green, blue and purple, and five subgroups of red-yellow, yellow-green, green-blue, blue-purple and purple-red. Value means the degree of light from black at zero value to white at 10. Chroma is the degree of pigment saturation, or the intensity of color.

Subjectivity

• Visual subjectivity makes it difficult to compare soil colors to color samples on charts and assign a color notation. Color in soils is not easily standardized, and soil scientists have suggested pre-treatments of soil samples to narrow the variables within the samples. These include burning the soil, or ashing, to assure a completely dry sample, testing the sample both wet and dry, testing only the clay fraction separately, or removing organic matter and iron oxides. The use of an electronic spectrophotometer to compare the soil sample to the color chart can reduce subjectivity.

Notation

• The colors on the Munsell color chart can be written in a shorthand notation for reports and research that cannot include a color sample. The combination of the three color parameters -- hue, value and chroma -- are notated as H V/C, so that a soil that is reddish brown might be matched to a Munsell color identified as 5YR 5/3, meaning hue 5 yellow-red on the chart with a lightness value of 5 and a chroma of level 3. Standard colors for specific minerals are know, such as 10R 4/8 for red hematite, an iron oxide; 10YR 8/2 for white calcite, a calcium carbonate; 5Y 6/4 for jarosite, a pale yellow iron sulfate.

Worldwide Standard

• By 1937, soil science manuals included numeric charts that described traditional soil colors, such as olive gray, in terms of their percent of white, black, yellow and red in the Munsell System. Munsell's original system was improved and periodically updated by the United States Department of Agriculture, and, in 2011, is used worldwide by the Food and Agriculture Organization (FAO) of the United Nations.