Soil Color - Physical Properties

Soil color is one of the easiest physical soil properties to see. Many components in the soil influence soil color, among them organic matter, minerals and moisture, and as such, soil color can be an indicator of past environmental conditions. This section examines each of those components, and explores the importance of soil color and what it can tell us about soil conditions such as fertility and drainage. Because a word description, for example "yellowish brown," is a relative term meaning different things to different people, a standardized system is used. You will learn how to measure soil color through the use of the Munsell System, an international standard. Hue, value and chroma (three components of color) are defined and illustrated, and techniques for measuring soil color are demonstrated. Interpretation of certain soil characteristics affecting use and management of the soil through soil color is discussed in the following paragraphs.

Importance of Soil Color

(top)

Soil color is a variable property, across the landscape and with depth from the surface. Color can tell us much about the soil: the amount of organic matter present; the types of minerals and how weathered they are; the current moisture content; how long water is held in the soil (soil drainage class); and oxidation states of iron and manganese.


Mottled B-Horizon

Red and yellow colors in the subsoil indicate oxidized iron and tells us if the soil is alternately wet and dry, or if it is dry during the growing season. Gray or blue colors may indicate that soils are wet for much of the year, and any iron present is likely to be in chemically reduced form. Dark colors in the soil, especially at the surface, usually indicate higher amounts of organic matter. These soils can be high in nutrients, have favorable structure, and be easily permeable to air, water and plant roots. Light colored soils may be leached (soil constituents such as organic matter, clay, or iron move downward with percolating water faster than they are added from overlying horizons), or contain high amounts of calcium carbonates.

Patterns in soil color can be helpful in determining if a soil is poorly-drained or well-drained. Small spots or "blotches" of contrasting color (called mottles) often occur due to oxidized or reduced iron. The brightness or dullness of the matrix (the major portion of the soil) and mottles can tell us when water is in the soil, and for how long.

Measuring Soil Color using the Munsell System

(top)

Because of differences in language and experience, a simple word description of soil color is insufficient. An international standard reference system is used so that everyone knows precisely what color is being described.

The Munsell Color System has three designation components, or elements of color: hue, value and chroma. An example of a Munsell description is 10YR 5/8 (the verbal description is "yellowish brown"). The 10YR portion is the hue, or the dominant wavelength (the amount of red (R) and yellow (Y) in that color). (Imagine you are having paint mixed at the hardware store; the formula "10YR" would mean 10 parts of yellow paint to 1 part red paint, giving a yellow color with red and brown tones.)

The second portion of the Munsell description, 5 in our example, represents the color value. Value is the quantity of light reflected, or the lightness or darkness of the color. Lower numbers (toward the bottom of a page) are used for darker colors, those which are absorbing more light, and higher numbers (as you move to the top of a page) are used for lighter colors, those which reflect more light.

The final portion of the Munsell description is the chroma of the color (8 in our example 10YR 5/8). Chroma refers to the purity of the color, or degree of color saturation. Low chroma numbers are dull colors, and it may be difficult to tell when the color actually changes from one color chip to another. High chroma colors are much easier to tell apart because the colors are stronger (more saturated or intense).

To measure the color of a soil sample, use the following procedure. Place a dry sample in the palm of your hand (this may be a finely ground sample, or a soil aggregate). With your light source behind you (light shining over your shoulder), choose a page from the Munsell color book that is close to the color of your sample. Holding the color page over the sample, move the page around to view your sample through the holes in the page. Find the closest match. When you have found a close match, determine if your sample may be redder or yellower than the color chip you have chosen; if you think it may be, go one page to the front of the book (for red) or to the back of the book (for yellow) and look at the chip with the same value and chroma. Is this a better match?

Soil color is usually made for both dry and moist soil. To determine moist color, add a small amount of water to your sample. Avoid adding so much water that the soil becomes shiny; if this is the condition you will be measuring light reflected off the water rather than the soil itself. You want your moistened sample to look like a matte paint sample, not glossy paint.

Sometimes soil scientists want to know soil color on the interior of peds (structural units such as blocky or prismatic). This may help them understand water movement and behavior in the soil, or tell them if clay or organic matter has moved downward from higher in the profile. Let’s say you have a subsoil sample with a light gray matrix (around 10YR 7/2). Where the roots are, the soil color is reddish brown (around 7.5YR 5/8). The reddish brown color around the roots is from oxidized iron, and the light gray is reduced iron. This color pattern indicates that the soil is wet moist of the year, but oxygen from the plant roots diffuses outward into the soil creating thin zones of oxidized iron.

Interpreting Soil Color

(top)

Color provides important interpretive information on many of the chemical characteristics of a soil and on the drainage class and general behavior of water in soil. It may also aid a soil scientist in classifying the soil (similar to plant or animal classifications).

Soils are usually darker when moist, and become lighter as they dry. But water in the soil has a more important effect on soil color. Water influences the amount of oxygen in the soil which in turn affects several aspects of soil color. Wetter soils generally accumulate organic matter faster than drier soils. Dark colored surface horizons (values of 3 or less) are often enriched with organic matter, offering many benefits to the soil. Organic matter improves soil structure, increases water and nutrient holding capacity, and contributes nitrogen, phosphorus and sulfur to plant available nutrient pools as it decomposes.

Iron oxides minerals are one of the primary coloring agents of soil. Soils which are wet for significant parts of the growing season are usually dull (or gray) in color, and are called gley, which means that iron in the soil is in a reduced state, rather than an oxidized state. In well-drained soils, especially in warm, humid climates, oxidized iron can give the soil bright (high chroma) red and brown colors. Think of many of the soils in the Missouri Ozarks with their bright red subsoil.

Minerals other than iron can also influence the color of the soil. Manganese oxide can impart black colors, and some soils with glauconite are green. Calcite and soluble salts can give soils a whitish color.

The drainage class of the soil is important for many types of land management, including building construction, septic systems, agriculture, forestry and horticulture. Soil color patterns can inform a soil scientist about the drainage characteristics in the soil, even if the soil is not wet when it is being observed. The color relationship of the matrix and mottle chromas is the key, as illustrated below.

Drainage Class Matrix Chroma Mottle Chroma
Well-drained Bright Bright [if present]
Moderately well Bright Dull
Somewhat poorly Dull Bright
Poorly drained Dull Dull [if present]
Courtesy Dr. Randall J. Miles, department of Soil, Environmental and Atmospheric Sciences, University of MO

Summary

(top)

Soil color varies from location to location, both across the landscape and with depth. It can tell us much about minerals that may be present in the soil and the amount of organic matter. Color patterns can indicate how much water is in the soil, how long it is there, and at what time of year.

The color of the matrix and mottles are determined with a standardized system called the Munsell system. By determining the hue, value and chroma of the soil an associated verbal description can be obtained. Among other uses, Munsell colors help soil scientists to classify a soil and better understand properties important to effective management.

Activities

(top)
  1. Use the Phase 1 tutorial Munsell charts (full-screen). Obtain colors from soil samples, pictures of soil samples, or some everyday objects. What color is your skin? Your shirt?

  2. Using the provided pictures of soils samples (similar to some of the color plates in Brady and Weil (see plates 35, 37 and 38), make some general interpretations based on the soil color. Is the soil high in organic matter? What is the drainage class of the soil?