In last month’s article, we explained the need for installers to have basic skills to identify soil texture. The next most important soil feature to recognize and evaluate is soil color.
Color is one of the most useful properties for soil identification and appraisal. It is also fairly easy to determine. Color can be the primary indicator of zones of periodic soil saturation and of the separation distance needed to provide adequate treatment. In addition, color can help you determine if you are working in a disturbed area — a factor that could affect the type or size of the system.
On the surface
You can use the color of the surface layer to determine the organic matter content of the soil. It can indicate the effects of past vegetation or human disturbance. Some soils exhibit color directly inherited from the parent rock. They provide clues to identify and evaluate soil.
There are two primary coloring agents in soil: organic matter and iron. Most people recognize the dark surface soil as being rich in organic material. The varying shades of red, yellow and gray usually relate to the quantity and form of iron present. Red means the iron is oxidized and not hydrated with water. Yellow indicates hydration and sometimes less oxidation. Gray indicates chemical reduction due to wetness and lack of oxygen. This indicates the occurrence of periodic saturated soil conditions.
Soil horizons may contain many different colors. These colors are derived from either the native parent material or the soil-forming process. These processes may result in the formation of clay films, silt coats, organic stains, nodules, and oxides, all of different colors. One soil-forming process that needs special attention is when the soil color indicates a saturated condition.
The presence of a specific kind of mottling, now referred to by soil scientists as “redoximorphic features,” is used to estimate seasonal high-saturated soil conditions. Many state regulations require identification of these features. You can use them to identify the depth to the seasonally high water table, even during dry periods when there is no direct evidence of water. This level then sets the base to determine the elevation of the bottom of the soil treatment system. This will then determine the depth of excavation or whether you need an at-grade or mound system.
Color variables
Color is composed of three variables:
• Hue: The dominant color, such as red, yellow, green, blue or purple.
• Value: The measure of the degree of darkness or lightness of color, in relation to the total amount of light reflected, such as light red or dark red.
• Chroma: The measure of the purity or strength of color, or its departure from a neutral of the same lightness, such as dull red or bright red.
Soil color is measured by comparison with a standard color chart. The chart used to evaluate soils is the Munsell color system. The standard Munsell chart for soil color consists of about 322 color chips, systematically arranged on nine cards assembled into a loose-leaf notebook. Additional cards are available for reddish hues and for the bluish and greenish hues of gleyed soils.
The colors displayed on the individual color pages are of constant hue, designated by a symbol in the upper right corner of the chart. The color becomes lighter from the bottom of the card to the top in equal steps. Chroma increases from left to right, and grayness from right to left.
Chroma notation is indicated by the horizontal scale across the bottom of the chart. The value notation of each chip is indicated by the vertical scale in the far left column of the chart. Opposite each page of color chips is a page of color symbols and corresponding English names, so that color can be expressed both by Munsell notation and color name. So a notation might be dark brown 10YR 3/3, which is the English name, followed by hue, then value and finally chroma.
Reading color charts
To read the soil color using the Munsell charts, use the following procedure:
1. Take a ped from the horizon to be examined. Do not crush or break the soil ped.
2. Adjust the water content to moist.
3. Estimate the basic soil color (hue).
4. With the sun at your back, hold the sample behind the holes of the page. Match as closely as possible.
5. If you are not satisfied with the match, flip forward and back to see if there is a better match.
6. Record the chosen color or colors.
7. Break the ped to see if the interior color is different. If so, record those colors.
Rarely will the match be perfect, so you need to select the closest match.
Installers need to be able to interpret the meaning of the colors described. If the soil color is uniformly reddish or brownish, that indicates horizons that are well drained and consistently provide adequate oxygen to enable treatment of septic tank effluent.
As the abundance and contrast in soil mottles increase, that means the water is not draining naturally, and the system will need to be installed to provide adequate separation above this zone. Gray or olive colors indicate even longer periods of saturation.
