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Soil Basics

The following information was excerpted from a computer-based program developed by Purdue University in conjuction with the U.S. Environmental Protection Agency. It was designed to give a brief introduction of the properties of soils as they relate to septic systems. Additional information on soils, their locations, and their limitations is available in the map sections.

Soil is not only the foundation of your dwelling, but the foundation of waste water treatment as well. Every site has unique soil characteristics that are critical in determining the size and type of system required.

All soils are composed of sand, silt, clay, and some organic matter. The relative amount of each determines the soil's texture.

Coarse soils are composed primarily of sand. They may also contain small amounts of fine gravel or rock.

Intermediate textures include significant portions of silt and may include some sand and clay.

Soils composed primarily of clays are called fine soils.

Soils are further classified into four basic structural groups. The four basic types of structure are: GRANULAR, PLATY, BLOCKY, AND PRISMATIC. The structure of a soil, as well as the texture, determine the permeability of the soil.

Underlying your site are several layers of soil composed of different minerals and amounts of organic matter. Each of the four soil structures has a specific location within these layers.

These layers make up what is called the SOIL PROFILE.

In ordinary, undisturbed soil, the four types of structures are visible in this soil profile. Granular structures predominate on the top, followed in succession by platy, blocky and finally prismatic structures.

Below the topsoil and subsoil is the substratum. It is the substratum where a limiting layer would exist. This layer of rock or dense clay can limit the effectiveness of the absorption field.

There is much more to soil than just the inorganic particles that contribute to it. Soil can be thought of as a miniature ecosystem. The soil particles are surrounded by voids which can be filled with air or water.

Soils are classified as well drained, moderately well drained, somewhat poorly drained, or poorly drained. Well drained soils are usually red or brown in color.

Poorly drained soils are usually gray in color.

Most soil layers will have a gray or mottled layer. You can determine the drainage class of your soil by the distance to this layer.

In poorly drained soils it is only 10 to 18 inches to the gray or mottled layer. Even these first 10 to 18 inches may have predominately gray color. If the first 10 to 18 inches is black, look at the six inches below the black area to determine if the soil is poorly drained.

In somewhat poorly drained soils, gray mottled colors are also found between 10 to 18 inches. But in a somewhat poorly drained soil the 10 to 18 inch zone is predominately brown with gray mottles while the very poorly drained soil is predominately gray in color.

In moderately well drained soils it is 18 to 30 inches to the gray or mottled layer.

Well drained soils have more than 30 inches of brown or black soil before the gray or mottle layer is encountered.

The distance from the trench bottoms to the limiting layer is important. It is this distance that determines how much permeable soil the effluent passes through before it reaches the bedrock, gravel, or the water table.

The water table will mound up above the limiting layer.

If the distance to the water table is very slight, water flows out of the trenches and mounds up over the limiting layer.

Where this is a problem, it may be possible to use subsurface drains to lower the water table and keep it from backing up into the trenches. Tile fields can also be constructed in mounded soil systems. See the section on Human Impacts and septic system construction for more information on these systems.

If the limiting layer is very close to the surface, and if there is enough well drained soil just below it, it may be possible to locate the trenches just below this limiting layer. Vertical drains will still be necessary (upper left corner) to keep ground water from flowing into the trenches.