Soil Evaluation for Septic System


Using soil evaluations to assess site suitability for septic systems is becoming more common in Illinois, although many terms and abbreviations used by soil classifiers are unfamiliar to those not in the field of soil science. While soil evaluation reports vary in content and format depending on the author, many common threads are shared by all of these reports. This section explains abbreviations and topics often found in soil evaluation reports, and discusses how various soil characteristics affect water flow out of septic systems into the soil.

This portion of the report describes the property location, current use, and date of investigation. Some counties require a new soil evaluation after a given amount of years have passed, so attention should be paid to the date of investigation.

Closeup of agronomy specialist testing soil sample ph value outdoors, using laboratory equipment, performing soil certification at agricultural grain field sunrise. Environment research.

Many counties utilize the Private Sewage Disposal Licensing Act and Code (State Code) issued by the Illinois Department of Public Health as their guidance for methods to conduct soil investigations, although some counties have slightly different requirements based on local ordinances. In general, three or four soil borings or backhoe pits are excavated at least 50 feet apart in the vicinity of a future septic field to a depth of at least 60 inches. Soil characteristics are described from each boring or pit, and are correlated to specifications given in the State Code to determine sewage loading rates in gallons per square foot per day. In general, the lowest loading rate observed in the upper 30 to 42 inches of the soils examined is used for septic system design and sizing.


Each soil described can be classified to the series level. A soil series is equivalent to a plant or animal species, in that a series represents a specific type of soil that can occur over a large geographic area.

One of the main advantages of a soil evaluation over a percolation test (a traditional method of testing septic field areas) is that layers within the soil that severely limit the function of septic systems can be defined. These limiting layers include bedrock, the seasonal high water table, dense soils with slow permeability, and sandy or gravelly soil with very rapid permeability. The State Code specifies that at least two feet of separation should exist between the bottom of the septic field and the limiting layer (three feet in coarser-textured soils). Installation of curtain drains, importation of fill material, or use of alternative sewage disposal systems are options that can be used where limiting layers occur at shallow depths.

Soil Descriptions

A large amount of information is included in the descriptions of soil profiles. 


Layers within the soil that differ in color, clay content, or other ways are divided into horizons. Four to seven horizons are commonly present within the top 60 inches of a soil profile. The surface, or topsoil, is generally called the A horizon. A light-colored E horizon lies near the surface of some soils as well, particularly on land that is or has been wooded. The subsoil, where clay accumulates, blocky and prismatic structure develops,and colors are variable is called the B horizon. The substratum, which consists of relatively unweathered soil material, is called the C horizon. Each of these master horizons can be subdivided if characteristics within them vary (for example, A1 and A2). Transitional horizons, such as AB or BC are also recognized. Features within each horizon can be recognized with lower case letters. Examples include Bt (clay accumulation in the B horizon), Bg (gray colors in the B horizon indicating poor internal drainage), and Ab (an A horizon that has been covered by fill or alluvial sediment). If parent materials change within the soil, it is signified with a number at the beginning of the horizon designation (examples include 2Bt and 3C).


The location of each described horizon in inches below the surface is given in this section.

Dominant Color, Munsell

Soil colors are described through use of the Munsell soil color charts. These charts consist of color chips that have been assigned names based on their hue, value, and chroma. Common colors and examples of Munsell designations that would describe these colors are given below:

  • Black 10YR 2/1 and N 2/0
  • Brown 10YR 4/3 and 7.5YR 4/4
  • Gray 2.5Y 6/2 and 5Y 5/1
  • Yellow/Red 7.5YR 6/6 and 10YR 5/8

Iron is a major coloring agent in the soils of Illinois. Brown, yellowish, or reddish colors are mainly the result of precipitated iron (essentially rust) that coats soil particles. These colors generally indicate good internal soil drainage. When a soil is frequently saturated (poorly drained), the iron is dissolved and leached away, leaving a gray color that is the base color of the soil particles. Poorly drained soils often have a uniform gray or mottled, gray/red color pattern. The black color of topsoil is created by high organic matter content, which masks the coloring effects of iron.

Redoximorphic Features (Mottles)

In areas where the water table fluctuates, soil horizons often contain many different colors. This variable color pattern indicates how high the seasonal high water table (SHWT) reaches in a soil. This is important since the SHWT is considered a limiting layer. These contrasting soil colors traditionally are referred to as mottles, or more recently as redoximorphic (redox) features. The term “redox” comes from a combination of the terms “reduction” and “oxidation”, that are natural chemical/biological processes affecting iron and other minerals in the soil.


Coatings of clay or organic matter are often deposited in the B horizons by water percolating downward. The color of these coatings is determined, preceded by a description or code defining the abundance and contrast of the coatings that is similar to the code used for redox features.


The structure of a soil is a description of the shapes soil assumes in different parts of its profile over time. Structure is formed largely by cycles of wetting/drying and freezing/thawing, the soil’s chemical composition, and the aggregating effect of some soil microbes. Well-structured soils have large amounts of interconnected pores that accelerate water and air movement. Weakly structured soils have less continuous pore space, which slows water and air movement. Structure is ranked on a scale of 0 to 3 (structureless, weak, moderate, strong). Structure size is also determined, and classified as fine (f), medium (m), and coarse (c). Blocky, prismatic, granular, and single grain structures are generally favorable for septic systems in soils with low to moderate clay contents.


Texture is a group of terms that describe the amount of sand, silt and clay present in soils. These terms include sand (s), loamy sand (ls), sandy loam (sl), sandy clay loam (scl), loam (l), clay loam (cl), silt loam (sil), silty clay loam (sicl), silty clay (sic), and clay (c). In general, as silt and clay content increases in a soil, the permeability decreases. Silt loam and silty clay loam textures are very common in Illinois, having formed in loess parent material. When clay content in soils exceeds 35% (heavy cl, heavy sicl, sic, or c textures), the soils are generally poorly suited for conventional septic systems because of slow permeability.


Consistence is a measure of how easily soil can be crushed between the thumb and forefinger. Classes of consistence include very friable (vfr), friable (fr), firm (fi), very firm (vfi), and extremely firm (xfi). In general, as soil consistence increases in firmness, permeability decreases due to a diminishing volume of pore space within the soil.

Drainage Class

Drainage class describes the relative wetness of a soil prior to modification by drain tile or other means. This designation is not precisely defined, but is broken into seven classes: very poorly, poorly, somewhat poorly, moderately well, well, somewhat excessively, and excessively. The main factors considered when determining drainage class are soil color patterns, texture, and landscape position. Bright soil colors combined with high and/or sloping landscape position generally indicate a drainage class of moderately well or better. Somewhat excessively and excessively drained soils usually combine these features with high contents of sand and/or gravel. Somewhat poorly and poorly drained soils are common on low flats and floodplains, and are often tile drained to enhance agricultural production. These soils feature gray colors near the surface and often have thick, black surface layers, although some have thin or light-colored surfaces that indicate they were formed under forest vegetation. Very poorly drained soils generally lie in enclosed depressions and are frequently ponded. Peat and muck soils very high in organic matter are usually very poorly drained.


The slope of a soil is measured with a clinometer, and expresses how many feet the ground surface falls over a distance of 100 feet as a percentage. A two percent slope indicates a ground surface that falls two feet in 100 feet. Slopes often change rapidly over a short distance. Aspect is the direction that the measured slope is facing as one looks downhill.

Soil Group and Loading Rate

The Soil Group is determined by looking at a soil horizon’s texture, structure, consistence, and parent material, and using that to assign a sewage loading rate in gallons per square foot per day. In general, the lowest loading rate observed in the upper 30 to 42 inches of the soils examined is used for septic system design and sizing.

Perc Rate

Approximate percolation rates of each horizon can be correlated to loading rates. This information can help you correlate perc rates that may be more familiar to you with soils information.

If you have further questions or would like to learn more details about soils and their interpretations, please feel free to contact any member of the Illinois Soil Classifiers Association.

Information supplied by the Illinois Soil Classifiers Association.