Introductory Soils Lab 1A Soil Morphology NRES 201 Page 1 of 13 LABORATORY 1A. SOIL MORPHOLOGY: COLOR, TEXTURE-BY-FEEL, STRUCTURE, AND CONSISTENCE 1.1 SOIL COLOR. Color is an easily determined soil characteristic. Even though soil color directly affects only the absorption of solar radiation, soil color can provide valuable information about other soil properties. The color of surface horizons is often related to their organic matter content. The color of subsurface horizons provides information about internal soil drainage. In addition, a difference in soil color is one of the properties that can be used to differentiate soil horizons. 1.1.1 Munsell color notation system. Soil color is most conveniently measured by comparison of the soil color with colors on a color chart. The collection of color charts generally used with soils is a modified version of the collection of charts appearing in the Munsell Book of Color and includes only that portion needed for soils. The Munsell notation identifies color by the use of three variables, Hue, Value and Chroma. Hue is the dominant spectral (rainbow) color; that is whether the color is yellow, red, green, or mixtures such as yellow-red. Mixtures are identified numerically according to the amount of yellow or red used to produce the mixture. 5YR is an equal mixture of red and yellow. As the number increases, the amount of the first letter color (Y, yellow) increases, and as the number decreases, the amount of the second letter color (R, red) increases. Value and Chroma are terms that refer to how the hue is modified by the addition of grey to the pure color (hue). Value is a property of the grey color that is being added to the hue. A particular grey (value) is made by mixing a pure white pigment (10) with a pure black pigment (0). If equal amounts of white and black pigments are mixed, then the value is equal to 5, if more black than white pigment is used, then the value is less than 5, if more white than black pigment is used, then the value is greater than 5. Chroma is the amount of pure hue that is mixed with a grey of a particular value to obtain the actual color. A chroma of 1 would be made by adding one unit of pure hue to a certain amount of grey, a chroma of 5 would contain 5 units of pure hue to that amount of grey. The lower the chroma the greyer the color. 1.1.2 Determination of soil color. The nomenclature for soil color consists of two complementary systems: color names and the Munsell notation of color. Color names are less precise, but convey a general concept of the color of the soil. Munsell notation is more precise and is standardized so that soil scientists in different countries will have no difficulty in communicating information about soil color. The Munsell soil color charts are set up so that each page is a separate hue, for example 10YR or 5R. The hue is given in the upper right hand corner of the chart (page). The color book is arranged with red colors in the front grading into yellow in the back. An actual chart is a collection of color chips of constant hue arranged by value and chroma. Rows of color chips areIntroductory Soils Lab 1A Soil Morphology NRES 201 Page 2 of 13 for a constant value, with chromas increasing from left to right. Columns are for a constant chroma with values decreasing from top to bottom. Soil color is determined by matching a moist soil sample with the appropriate color chip. Because a given color chart is not a collection of all possible values and chromas of a given hue, the match is often less than perfect. It is generally possible to describe one chip that is very close in color to the moist soil. The major difficulties encountered in using the soil color charts are 1) in selecting the appropriate hue card 2) determining colors that are intermediate between hues and 3) distinguishing between values and chromas when chromas are strong (high numbers). Once the appropriate color chip has been selected, the hue, value and chroma are recorded. The notation 5YR 5/4 is for a soil with a hue of 5YR, a value of 5 and a chroma of 4. The corresponding color name for 5YR 5/4 is reddish brown. Color names are given on the opposite (facing) pages of the color books. If expression of soil color more precise than whole numbers for values and chromas is desired, decimals are used, never fractions. Reproducible measurements of soil color can be obtained at two moisture contents: moist (field-capacity) and air-dry. In most soil descriptions, unless otherwise stated, colors are moist. Coatings on peds are often of a different color than ped interiors, so colors of subsoils are taken both from the coats and from the ped interiors. Surface soil colors are typically from crushed or rubbed samples. Figure 1-1. Munsell 10YR Hue page, the most common soil color.Introductory Soils Lab 1A Soil Morphology NRES 201 Page 3 of 13 Figure 1-2. Explanation of Munsell Soil Color Book page. 1.1.3 Interpretation of soil color. Soil color is due to: 1) the presence of soil organic matter (humus). Organic matter imparts a dark brown to black color to the soil. Generally the higher the organic matter content of the soil the darker the soil. 2) The oxidation status of the iron compounds in the soil. In the lower horizons, where the soil minerals are not coated with humic substances, the color of the soil minerals will predominate. In better drained and, hence, well-aerated soils Fe (III) minerals give soils a red or yellow color. In more poorly drained and, hence, poorly-aerated soils, the iron minerals are reduced and the neutral (grey) colors of Fe (II) minerals and the other soil minerals predominate. In soils with illuvial B horizons, various coatings, clay, organic matter, or carbonates, may impart a color to the ped surface. Soil organic matter. The organic matter content of soils can be estimated from the Munsell color of a soil (J. D. Alexander, Univ. of Illinois AG-1941). The most accurate estimates are obtained with medium and fine textured mineral soils. Soils with greater than 50% sand and less than 10% clay usually contain less organic matter than predicted. The performance of many herbicides is influenced by adsorption onto soil organic matter. Estimation of soil organic matter from soil color can be helpful in selecting herbicides and determining application rates. Appropriate rates maximize weed control, while minimizing crop damage and potential