Water Survey Archived Content
 1. Soils   2. Climate   3. Other   General Information 

The most detailed Illinois GIS soil descriptions are STATSGO soil map units composed of similar soil types. Each map unit may include up to 21 individual soil types. Soil values used in this Web page are the weighted average values of the soil types contained within the STATSGO map unit; soil pH and texture are weighted averages of the surface soil layer only. The STATSGO map unit soil pH, texture, and drainage values are compared to crop requirements to obtain suitability scores for each soil characteristic.

Soil pH is a measure of acidity (hydrogen ion concentration). The pH values range from 0 to 14: 0 is most acidic, 7 is neutral, and 14 is most basic (lowest hydrogen ion concentration). Soil pH values range from ~3 to 10. Forest and bog soils of the humid east tend to be acidic and grassland and desert soils of the west tend to be basic.

Effect of pH on nutrient availability
Extremes of soil pH release substances from soils in amounts that can be toxic to plants. Acid soils may dissolve toxic amounts of metals (such as aluminum and manganese). Alkaline soils may accumulate salts and sodium carbonates in toxic concentrations that can alter soil structure, thereby making it difficult for roots to grow. Stunted root systems have trouble taking up adequate water and nutrients. Toxic metals in acid soils, subsoil nutrient depletion, and subsoil clay pans also stunt root growth.

Slightly acidic soils (pH ~6.5) are considered most favorable for overall nutrient uptake. Such soils are also optimal for nitrogen-fixing legumes and nitrogen-fixing soil bacteria. Some plants are adapted to acidic or basic soils due to natural selection of species in these conditions. Potatoes grow well in soils with pH <5.5. Blueberries and cranberries grow well in even more acidic soils (<4.5) . Sugar beets, cotton, kale, garden pea, and many grasses grow well in alkaline soil (>7.5).

Soil pH also affects the soil in other ways. For example, soil microbe activity, particularly nitrogen-fixing bacteria may be reduced in acid soil.

Agricultural soils of Illinois tend to acidify to pH values more acidic than 6.5. This acidity is managed by adding lime (carbonates of calcium and magnesium). Average soil pH values vary from mildly alkaline (7.0-7.5) to strongly acid (5.2-5.5) in extreme southern Illinois.

The Natural Resources Conservation Service has set standard soil pH classifications.
pH classifications pH values
Extremely acid < 4.5
Very strongly acid 4.5 to 5.0
Strongly acid 5.1 to 5.5
Medium acid 5.6 to 6.0
Slightly acid 6.1 to 6.5
Neutral 6.6 to 7.3
Mildly alkaline 7.4 to 7.8
Moderately alkaline 7.9 to 8.4
Strongly alkaline 8.5 to 9.0
Very strongly alkaline > 9.0
         Average pH

DRAINAGE rate refers to the rapidity and extent that water is removed from a soil by surface runoff, underground flow through the soil, and evaporative loss. Drainage also refers to soil drainage status — the frequency and duration with which soil is waterlogged. In Illinois’ climate, the drainage rate coincides with soil drainage status. If drainage is very rapid, the soil is excessively drained. If drainage is very slow, the soil suffers from excessive waterlogging and is very poorly drained.

Soil drainage extremes present the same types of problems for crops that extremes of soil pH do. Excessively drained soils do not provide most crops with adequate water and nutrients, and the structure of the soil limits root growth. Additionally, excessively drained soils tend to warm early and generally undergo marked temperature fluctuations. Water and nutrient availability are also limited in poorly drained soils because oxygen deficiency limits the ability of roots to take up adequate water and nutrients. With waterlogging, putrefaction sets in. Putrefaction occurs when partially decomposed organic matter accumulates, clogging soil pores and blocking root growth and the drainage of water through soil. Putrefaction produces toxic substances: reduced nitrogen, sulfur, metals, and organic fermentation products. Furthermore, it produces methane, a gas that attacks the atmosphere’s self-cleansing system. Additionally, poorly drained soils tend to warm up slowly in the spring and reduce the length of crop growing seasons.

Illinois’ drainage problems are principally those of poor drainage. Ditching and subsurface tiling typically improve drainage. With improved drainage, excessive organic matter is destroyed due to decomposition by oxidation rather than putrefaction. Root and soil fauna penetration of subsoil improves soil drainage and structure. The soil’s large store of organic nitrogen undergoes oxidation to water-soluble nitrate. With good drainage, nitrate may drain into groundwater and surface water at rates detrimental to water quality. The map below depicts average drainage for Illinois soils. The sandy soils in Mason and Will counties are unlike the majority of Illinois soils, which are moderately well to poorly drained.

The NRCS assigns soils to one of the following eight drainage classes:

Excessively drained
Semi-excessively drained
Very well drained
Well drained
Moderately well drained
Semi-poorly drained
Very poorly drained
Poorly drained

Texture describes the proportion of sand, silt, and clay in a soil. The relative proportion and physical properties of these separates (i.e., particles) affect drainage, water storage capacity, aeration, permeability, and other soil properties.
     Average Drainage

Sand, the largest sized particle (> 0.5 millimeters or mm), has small surface area for its mass. This, in turn, prevents sand from holding significant water or nutrients. Sand, therefore, reduces the amount of a soil’s physical and chemical activity. Sand increases the spaces between particles, letting air and water readily enter and exit the soil.

Clay, the smallest sized particle (< 0.002 mm), has the greatest surface area. Clay particles have a millionfold more surface area per mass than silt. Clay is capable of holding large amounts of water and nutrients but may prevent the release of water for plant use.

Silt is intermediary between sand and clay in size, water and nutrient retention, and chemical and physical activity. It has approximately four times the surface area of sand. Soils with large proportions of silt provide greater amounts of water for plant use than other soils.

In addition to sand, silt, and clay, soil is made up of water, air, organic matter, and other larger mineral matter. The coarse fragments of mineral matter are named by their size, shape, and composition. Examples of these coarse fragments are gravel, stones, flagstone, and chert. The amount of organic matter varies. As a general rule, the darker the soil, the higher the organic matter (and productivity).  Mucks and peat soils are examples of soils extremely high in organic matter.

Any separate alone would not be a desirable soil. Blends of these separates form soil textures, and some are ideal for plant growth. The soil texture triangle is a method of simultaneously representing the percentage of each separate in a soil. The first soil texture triangle is labeled with general texture class names. The second soil texture triangle is labeled with the soil texture class names used by the United States Department of Agriculture-Natural Resource Conservation Service.

    Soil texture triangle with general texture classes
   Soil Survey Manual -United States Department
   of Agriculture

   USDA-NRCS soil texture triangle

Predominate Soil Textures

The map above identifies the predominant (>16% area) soil textures within STATSGO map units.

Disturbed soils, such as urban areas and reclaimed stripmines, occur in Illinois. These soils are the result of removal and/or addition of soil or other materials. The maps below highlight map units that contain atypical soil types.

Muck Coarse Fragment Disturbed Soils Sandy Peat

Soil pH, drainage, and texture requirements are available for many crops and were therefore used to create suitability maps. Other soil properties with impacts on crop growth and development are soil depth, organic matter content, permeability, cation exchange capacity, salinity, and fertility. Information on the plant requirement for these soil characteristics was limited to a few crops and therefore were not included in this discussion.

 1. Soils   2. Climate   3. Other   General Information 

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Last Modified: March 15, 2021
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