Soil plays an integral role in the onsite sewage treatment system. In a properly functioning system, soils filtrate and treat effluent before final dispersal. A soil evaluation is performed in the area proposed for sewage treatment. Soil color and texture impact requirements for system design.

A "perk test" is no longer used in Kalamazoo County to identify percolation rates. This procedure involved a series of holes filled with water. The holes were continuously filled as the water drained. The amount of water used over a specific period of time was documented. The calculation of the water percolation determined the "perk" of the soil.

The Environmental Health Division staff use a hand auger to expose soils at different depths (sometimes a backhoe cut is necessary). We look at color of the soil and use the field test method of rubbing the soil between the thumb and fingers to determine soil texture. The Soil Texture Guide for Field Sanitarians (pdf) is the standard used for determining soil texture.

Soil color is associated with the presence and amount of iron, manganese, and organic matter. In Michigan, iron is the primary coloring agent of soil. In aerobic (oxygen filled) soil environments, iron is usually red, yellow, or brown. In anaerobic (absence of oxygen) or water-saturated soil, iron is usually gray, green or blue. During the soil evaluation, we analyze for high groundwater signs or mottles. Mottles are different shades of colors within the soil, which define the height of seasonal high groundwater on a site. Water tables fluctuate in most soils and mottles are created with the water movement. Evidence of both dull colors (grey, green, blue) and bright colors (red, yellow, brown) in the soil indicate seasonal high groundwater. These color patterns (mottling) remain for centuries (even after artificial drainage) and do not change from season to season. Kalamazoo County requires four feet of vertical isolation between the bottom of the absorption system and signs of high groundwater.

Classes of soil texture are based on different combinations of the varying sized particles of sand, silt, and clay. The amount of each soil contained in a sample will determine its feel or texture. Clay particles are microscopic in size and, when moist, are highly plastic. The presence of silt and/or clay creates a fine texture soil, which impedes water and air movement. Sand sized particles are visible with the naked eye.

Muck and peat are not technically soil, but organic matter. They are often used interchangeably. The two are distinguished on the basis of degree of decomposition of the original organic plant remains.

A soil texture triangle is used to classify the texture class of a soil. The sides of the soil texture triangle are scaled for the percentages of sand, silt, and clay. Sand percentages are read from left to right across the triangle. Silt is read from the top to the lower right. Clay from lower left toward the top of the triangle. The intersection of the three sizes on the triangle gives the texture class. For instance, if you have a soil with 20% clay, 60% silt, and 20% sand it falls in the "silty loam" class.