Celebrating the Next Generation

Illinois Cooperative Soil Survey

SOILS OF ILLINOIS

For many Illinoians the soil is a common place feature of nature not well understood or appreciated. It is present almost everywhere and has always been with us. Because of this, the majority of us have never taken the time to find out what soil is, how it formed, and what its' basic properties are.

The noun "soil" is derived through Old French, from the Latin "solum", which means floor or ground. In general, soil refers to the loose surface of the earth as distinguished from rock. Many people think of soil as the material which nourishes and supports growing plants. To the farmer it is the essence of survival; the medium in which his crops grow. The economic well being of a farmer is directly linked to the quality of his land. The civil engineer on the other hand looks upon soil as the material which supports foundations and roads. The soil scientist views soils as a collection of natural bodies on the earth's surface that have properties due to the integrated effect of climate and living matter acting upon parent material as conditioned by relief over periods of time.

Illinois has a wide variety of soils. They produce our food, fiber and paper; are used for our parks, wildlife preserves, lawns and play areas; support plants; and serve as foundations for our homes, highways and airports. Illinoians are very dependent on soils. Soils are one of our most basic natural resources and are indispensable to the support and growth of a strong and prosperous state.

SOIL FORMATION Many people assume that soils are all more or less alike. They are unaware that great differences in soil properties can appear side by side, within short distances. A single lot may have several different types of soil, each with its own values and drawbacks. Soils may be seasonally wet or subject to flooding. They may be shallow to bedrock. They may be too unstable to be used as a foundation for buildings or roads. Very clayey or wet soils are poorly suited to septic tank absorption fields. A high water table makes a soil poorly suited to basements or underground installations.

The soil formed at any particular site is influenced by five factors: parent material, topography, vegetation, climate, and time.

PARENT MATERIAL is material from which soil is formed. The relative proportions of sand, silt and clay particles in this material influence the texture of the soil. Texture determines the rate at which air and water move through the soil and the amount of water held in the soil. Water moves very slowly in clay soils because there is little space between the thin plate-like particles. Soil fertility is also influenced by minerals in the parent material. Loess is the most extensive soil parent material in Illinois. Soils developed primarily from loess occupy about 63 percent of the states land area. Loess is a silty wind deposit that was deposited during glacial times. The Mississippi, Illinois, Wabash, and Ohio River valleys were the major source areas for the loess. The most recent loess blanket in Illinois (Peorian loess) was deposited 12,500 to 23,000 years before present (BP). Loess blankets most of the state, except in the source areas, and ranges in depth from less than 10 inches to more than 300 inches.

Soil parent materials deposited in association with glacial ice are called glacial drift. Glacial drift includes glacial till, glacial outwash, and glacial lacustrine deposits.

Most of the glacial drift in Illinois was deposited during the Wisconsinan (11,000-60,000 years BP) and Illinoian (60,000-330,000 years BP) glacial stages.

• Glacial till was deposited directly from glacial ice through dumping, pushing, or lodging of the material.
• Glacial outwash is stratified drift "washed out" beyond the glacier itself.
• Glacial lacustrine deposits are those that were deposited in lakes following the retreat of the glacier.
• Alluvium includes sediments that have been recently deposited (since 11,000 years BP) by streams on their floodplains.

TOPOGRAPHY relates to the relative position of the soil on the landscape, it includes the presence or absence of hills and the slopes between high and low areas. Topography influences natural drainage. Gravity moves water down slopes to depressions or streams and pulls free water downward through the soil. Soils on hills tend to be dry and soils in depressions and valleys often are wet or saturated.

The amount of moisture in the soil while it is developing affects the rate of weathering and the development of soil colors. Soil colors are a reflection of the moisture status of the soil during development. Well-drained soils have uniformly brownish or yellowish brown subsoils; poorly drained soils have grayish subsoils; somewhat poorly drained soils have mottled brownish yellowish and grayish subsoils. Differences in natural soil drainage are typically associated with topography.

VEGETATION influences soil development by its influence on the amount and distribution of organic matter in the soil. Organic matter influences the water-holding capacity of the soil, fertility, and root penetration. The four main types of native vegetation that influenced Illinois soils are grass or prairie and trees or forest. Although the climate of Illinois is conducive to the growth of forest about 55 percent of the state had prairie vegetation during and for some time before settlement. In central and northern Illinois, when prairie vegetation predominated, forests were largely confined to the better drained, more rolling areas bordering stream valleys. Soils formed under grass normally have a thick, dark-colored top soil high in organic matter. Soils formed under forest in Illinois have a thin light-colored topsoil with less organic matter.

CLIMATE (temperature and precipitation) affects the distribution of soluble materials in the soil. Climate also influences the amount and type of vegetation that occurs and the amount of water available to seep through the soil in some soils. In some soils water has moved clay particles from the surface layers into the subsoil. In other soils iron, aluminum and organic matter have been moved into the subsoil. The climate of Illinois during the development of our soils is difficult to characterize. The best evidence seems to indicate that there were significant fluctuations in temperature and rainfall. For some time during and after the retreat of the last glacial ice the climate in Illinois was cooler and wetter than present. A rather warm, dry period 4,000-6,000 years ago led to an expansion of grassland in the state. Since that time our climate has remained similar to that of today.

TIME is required for soils to develop. The soil-forming processes are very slow in Illinois, they began when the glaciers receded from the area and continue today. Older soils have deeper and thicker subsoils than younger soils if other soil forming factors remain constant. Each factor also affects the rate of soil-formation and the amount of influence the other factors have for example, there may be enough rain to move soluble material deeper into the soil, but there will be little movement if the parent material restricts movement of water in the soil. Another example is topography that creates a micro-climate that discourages certain plants and indirectly influences the organic matter in the soil. Each one of Illinois' over 500 soils are the product of sometimes very subtle differences among the five factors.

SOIL HORIZONS Soils are deposited in or developed into layers. These layers, called horizons can be seen where roads have been cut through hills, where streams have scoured through valleys, or in other areas where the soil is exposed.

Where soil forming factors are favorable, five or six master horizons may be in a mineral soil profile. Each master horizon is subdivided into specific layers that have a unique identity. The thickness of each layer varies with location. Under disturbed conditions, such as intensive agriculture, or where erosion is severe, not all horizons will be present. Young soils have fewer major horizons, such as a bottom land soil.

The uppermost layer generally is an organic horizon, or O horizon. It consists of fresh and decaying plant residue from such sources as leaves, needles, twigs, moss, lichens and other organic material accumulations. Below the O horizon is the A horizon.

The A horizon is mainly mineral material. It is generally darker than the lower horizons because of the varying amounts of humified organic matter. This horizon is where most root activity occurs and is usually the most productive layer of soil. It may be referred to as a surface layer in a soil survey. An A horizon that has been plowed is designated as an "Ap" horizon.

The E horizon generally is bleached or whitish in appearance. As water moves down through this horizon, soluble minerals and nutrients dissolved and some dissolved materials are washed (leached) out. The main feature of this horizon is the loss of silicate clay, iron, aluminum, humus, or some combination of these, leaving a concentration of sand and silt particles.

Below the A or E horizon is the B horizon, or subsoil. The B horizon is usually lighter colored, denser, and lower in organic matter than the A horizon. It commonly is the zone where leached materials accumulate. The B horizon is further defined by the materials that make up the accumulation, such as "t" in the form of "Bt", which identifies that clay has accumulated.Other illuvial concentrations or accumulations include iron, aluminum, humus, carbonates, gypsum, or silica.

Still deeper is the C horizon or substratum. The C horizon may consist of less clay, or other less weathered sediments. Partially disintegrated parent material and mineral particles are in this horizon. Some soils have a soft bedrock horizon that is given the designation Cr.

The lowest horizon, the R horizon is bedrock. Bedrock can be within a few inches of the surface or many feet below the surface. Where bedrock is very deep and below normal depths of observation, an R horizon is not described.

SOIL CLASSIFICATION Soils may be grouped in a variety of ways, depending upon the characteristics on which the groupings are based and on the uses to be made of the groups. In soil classification, soils are grouped on the basis of properties at various levels or categories. The lowest and most detailed level is the individual kind of soil or soil series that is given a place name, such as Drummer, Tama, and Hickory. The soil order is the highest and most generalized level.

There are 12 orders or major soil groups in the soil classification system; they are believed to include all of the soils of the world. The soil orders are separated from one another on the basis of several critical horizons that give a key to the main soil-forming processes; the absence of these horizons indicates a lack of development. Soils that have undergone similar development and have similar kinds of horizons tend to be grouped in the same order. For example, most (but not all) soils with thick dark-colored A horizons are included in the Mollisol order. Soils that lack distinctive horizons, such as the light-colored, recently deposited alluvial soils, are placed in the Entisol order.

Between the soil order and soil series, there are four other categories of soils; suborders or subdivisions of the orders, great groups, subgroups, and soil families. The soil family is the category immediately above the soil series.

Six of the 12 soil orders are important in Illinois: the Mollisols, Alfisols, Entisols, Inceptisols, Ultisols, and Histosols. The Mollisols and the Alfisols are by far the most extensive in the state. Mollisols in Illinois are the dark-colored soils formed under grass. The thick, dark surface layer of the Mollisols was formed by the decomposition of grass roots.

• Mollisols occupy about 45 percent of the state's land area and are most extensive in central and northern Illinois.
• Alfisols are generally the light-colored soils that formed under forest. The Alfisols predominate in southern Illinois. They occupy about 45 percent of the state.
• Entisols include most of the light-colored, recently deposited alluvial soils in southern and western Illinois. These soils have not been in place long enough to develop recognizable horizons. They occupy about 7 percent of the state.
• Inceptisols include soils that have weakly developed horizons. Inceptisols occupy about 2 percent of the state.
• Histosols include the organic soils. The mucks and peats. They occur in bogs and marshes mostly in western Illinois. They occupy less than 1 percent of the state.
• Ultisols are the "old soils" located in southern Illinois. They occupy less than 1 percent of the state.

THE TOP TEN

More than 600 soil series are recognized in Illinois.
The ten most extensive soils in Illinois are-

• Drummer - This poorly drained "prairie soil" formed in 40 to 60 inches of loess over Wisconsinan drift. It occupies nearly 1.6 million acres. It has a high management level productivity index (PI) of 150. Drummer is the proposed state soil of Illinois.
• Tama - This well and moderately-well drained "prairie soil" formed in more than 60 inches of loess. It occupies about 1.4 million acres and has a PI of 150.
• Hickory - This well and moderately-well drained "timber soil" formed in less than 20 inches of loess over Illinoian drift. It occupies 1.3 million acres in the state. It has a PI of 80.
• Fayette - This well drained "timber soil" formed in more than 60 inches of loess. There are about 925,000 acres mapped in Illinois. It has a PI of 155.

• Sable - This poorly drained "prairie soil" formed in more than 60 inches of loess. There are about 925,000 acres mapped in Illinois. It has a PI of 155.
• Flanagan - This somewhat poorly drained "prairie soil" formed in 40 to 60 inches of loess over Wisconsinan drift. It occupies about 840,000 acres. It has a PI of 160.
• Bluford - This somewhat poorly drained "timber soil" formed in 30 to 55 inches of loess over Illinoian drift. There are about 780,000 acres mapped in Illinois. It has a PI of 110.
• Cisne - This poorly drained "gray prairie soil" formed in 30 to 55 inches of loess over Illinoian drift. About 730,000 acres occur in south central Illinois. It has a PI of 115.
• Ipava - This somewhat poorly drained "prairie soil" formed in more than 60 inches of loess. It occupies about 720,000 acres. It has a PI of 160.
• Muscatine - This somewhat poorly drained prairie soil formed in more than 60 inches of loess. There are about 690,000 acres mapped in Illinois. It has a PI of 160.