Processes of Soil Formation         Next Section>>

Soil horizons form through several processes:
1) organic matter accumulates in surface horizons, 2) carbonate minerals (ground limestone or “lime”) dissolves, 3) bases such as calcium and magnesium leach out of the soil, 4) clay moves down the profile, 5) iron oxide minerals precipitate and move down the profile, 6) soil particles become weakly cemented to each other, and 7) soil structure forms. These processes work together to form soil horizons.

Accumulation of organic matter in surface A horizons makes them dark. In many poorly drained soils formed under prairie or forest, organic matter accumulated because water-logging prevented it from being oxidized or consumed by soil organisms. In better drained prairie soils, it accumulated because of the nature of the root system.

Carbonates and some bases have been leached (washed downward) by weak acid solutions from the upper part of the profile in most Indiana soils. This leaching has contributed to the development of horizons. The depth to the uppermost carbonate minerals in the soil is listed on the soil evaluation site card. For example, “Calcareous below 30 in.” on the site card means that all carbonate minerals have been leached out of the upper 30 inches of soil. After the carbonates have leached out, clay is washed from upper horizons and deposited in the subsoil as films in the pores between soil particles and on the structural surfaces along which water moves. In the soils of Indiana, leaching of carbonates and bases and translocation of clay are among the most important processes in the formation of horizons.

Iron oxide minerals are brownish or reddish. These minerals are usually very small and cover larger clear or grayish clay, silt, and sand grains like sesame seeds on a bun. Because iron oxides are on the surface of other minerals, they are responsible for the brownish and reddish color of many soil horizons.

Some soil horizons are cemented by silica, iron, or carbonates. Fragipans are mainly cemented by silica (Si). In some sandy soils, small chunks in B horizons are cemented by iron (Fe). Also, on some hillslopes, water with dissolved carbonates seeps out to the surface, and when the water evaporates, the remaining carbonate minerals cement soil particles together to form a concrete-like mass. Overall, cementation is less of a problem in Indiana than in some other states, such as in the southwestern U.S.

Soil Structure

Subsections: Significance of soil structure | Soil horizons | Recognizing soil development

Soil structure refers to the grouping of individual soil particles into clusters or aggregates, called peds. The development of structure is another indication of soil formation. Many peds have thin surface films of clay (Plate 15) or silt (Plate 13) that make the ped more distinctive. These clay and silt films are often a different color than the inside of the ped.

Soil scientists describe soil structure by noting the shape, size, and strength of development of peds. Several different shapes of soil peds are illustrated in Fig. 6. In the many Indiana subsoils, peds are subangular blocky in shape and 1/2 to 2 in. in size.

Soil Structure

Fig 6. The common types of soil structure: A-prismatic, B-columnar,
C-angular blocky, D-subangular blocky, E-platy, F-grandular
(from USDA Soil Survey Manual, 1951)

The Parr soil (Plate 5) has granular structure in the A horizon, but the individual granules are too small to see in the photograph. Platy structure is evident in the E horizon of the Miami soil (Plate 2). Such horizons often become compacted by tillage or other traffic on the soil. The upper Bt horizon of the Crosby soil (Plate 13) has fine blocky structure, but the lower Bt (Plate 15) has medium and coarse blocky structure.

Prismatic structure is illustrated in the Bt2 horizon of Miami (Plate 2). In this horizon, there are cleavage or fracture surfaces, which are emphasized by shadows, that show mainly vertical orientation. In this soil, the prisms may break into blocks.

Fragipans have a special kind of prismatic structure. The prisms are large and coated with gray silty material. Looking at a horizontal surface from above, the prism coatings form a polygonal (many-sided) pattern, like the fur of a giraffe or chicken wire (Plate 16). Looking at a vertical surface (Plate 12), the prism coatings show as vertical gray streaks in a cut surface (Bx horizon on the right side of Plate 12). They show as gray faces in a chipped surface (Bx horizon on the left side of Plate 12).

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Significance of Soil Structure

Soil structure also determines the amount and arrangement of empty spaces in the soil, and thus has great influence on how readily water moves through the soil and where plant roots can grow. When the soil is saturated (holds all the water it can), water moves down the cracks and pores in the soil. Roots also penetrate through these open spaces. Shape of the peds (Fig. 6) influences the path water and roots must take to get through the soil. Granular and blocky peds are about the same size in all directions. In prismatic peds, the long direction is vertical and the cracks between peds are mainly vertical, the same direction that roots and water move.

In platy structure, however, the long direction is horizontal so roots or water first move downward between peds then must move horizontally across the plates before they find another vertical space. This complex path retards movement of water and growth of roots.

Size of the peds determines the distance between the natural cracks. For example, soils such as Brookston have a prismatic structure in which many of the prism-shaped peds are about 1/2-1 inch across. Fragipan soils such as Avonburg, on the other hand, have prisms more than 4 inches across. Thus, Brookston with its smaller prisms has many more cracks through which roots can penetrate and water can flow.

Strength of development is yet another aspect of soil structure. In strongly-developed structure, the peds are quite obvious, and the soil breaks up readily into peds when dug into or picked with a knife or spatula. Often soil peds have coatings, such as clay films. In weakly-developed structure, the peds are less apparent.

Strength of structure development is very important in surface horizons. When raindrops hit a horizon with weakly developed structure, such as the gray surface layer of poorly drained soils on flat landscape positions (Clermont and Cobbsfork soils), the structure is destroyed. Individual soil particles are released and they plug up soil pores and prevent water from entering the soil. When the soil dries, it has a strong surface crust that may prevent seedlings from emerging. A soil with a strong structure, such as the dark surface layer of a prairie soil (Parr), will better withstand raindrop impact and not seal and crust so much.

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Soil Horizons

A soil horizon is a layer of soil, approximately parallel to the soil surface, with characteristics produced by soil-forming processes. Horizons produced by soil-forming processes are called genetic horizons. Besides genetic horizons, many soils have layers inherited from stratified (layered) parent material. In studying soils, all layers or horizons of a soil profile are examined regardless of how they formed.

Most soil horizons are fairly distinct, as illustrated by the soils in the color plates. Some are not so distinct, however. On very young geological deposits, such as alluvial material laid down on flood plains, recently deposited sand dunes and steep eroded hillslopes, genetic horizons may not be distinguished at all. As soil formation proceeds, horizons may be detected in their early stages only by laboratory study of the samples. Later (maybe hundreds of years), they become more clear in the field.

When describing a soil profile, capital letters O, A, E, B, C, and R are used to indicate the major kinds of horizons. Often, a lower case letter follows the capital letter and gives more information about the horizon. For example, a B horizon is a subsoil horizon in which something has accumulated. A Bt horizon is one in which clay has accumulated. Table 1 explains what the upper and lowercase letters mean. The horizon designations are illustrated in the color photographs in the center of this book. Some horizon designations have a number before the uppercase letter. It represents changes in parent materials. For example the horizons in a profile, from the top down, may be Ap, E, 2Bt, 2C. The upper two horizons formed in one kind of parent material, such as loess, and the lower two formed in a different parent material, such as till.

In soil evaluation, naming horizons is not necessary. The characteristics of the A, E, and B horizons are, however, quite helpful in determining classes of erosion, recognizing fragipans, identifying alluvium, and recognizing other soil properties.


Table 1. Common horizon designations for Indiana soils. Horizons are illustrated in the color plates. For example, the Btg horizon of the Brookston soil in Plate 4 has a dominantly gray subsoil horizon that contains more clay than the horizons above it.

O Horizon
This horizon consists of organic materials on the surface of the mineral soil such as in the Trappist soil (Plate 10). In this soil, the organic materials are predominantly decomposing leaves from hardwood trees. In bogs and marshes, the whole soil formed in organic matter, and the entire profile consists of O horizons.

A Horizon
This is the mineral horizon at the surface. It is the one in which living organisms are most active and, therefore, is marked by the dark colored humus mixed with the mineral material Under forest vegetation, the A horizon is only a few inches thick (Miami, Plate 2). But under prairie grasses (Parr, Plate 5) or in swales (Brookston, Plate 4), the A may be 10-25 or more inches thick. The A usually has the darkest color of any horizon in the soil. The Ap horizon includes the part of the soil that has been mixed by plowing or cultivation. It consists mostly of the A horizon, the A and E or, in eroded soils, the remaining A and part of the upper B horizons. Soils developed under grass vegetation usually have an AB horizon, transitional between the A and B horizons.

E Horizon
Forest soils commonly contain E horizons a few inches below the surface. An E is lighter in color, lower in organic matter and less fertile than an A. Clay, aluminum, iron, and some nutrients have been washed or leached from the E horizon. The E is apparent in the Miami soil (Plate 2) and the Hosmer soil (Plate 12). In Crosby soil (Plate 3), part of the E remains undisturbed below the plowed (Ap) horizon, especially on the right side of the profile.

B Horizon
This is the mineral horizon below an A and/or E horizon and is commonly called the subsoil. It also has distinctive characteristics caused by: 1) accumulation of clay, iron, aluminum, or a combination of these; 2) prismatic or blocky structure; 3) reddish or brownish colors; 4) weak cementation, resulting in brittle material, or 5) some combination of the factors.

Together, A, E, and B horizons are called the solum. If a soil lacks B and E horizons, as in recent alluvial materials, the A horizon alone is the solum.

C Horizon
The material that cannot be designated as A, E, or B horizon because it lacks soil development is the C horizon, often called the substratum. The C horizon may be the weathered rock material immediately below the solum, or it may be material that was moved by ice, water, or wind.

In most soils, the C horizon is like the material from which the overlying horizons formed. If this is the case, we can say that the C is the parent material of the A, E, and B horizons. Sometimes the C horizon material is different from the material in which the A, E, and B horizons formed.

R Horizon
This horizon is the consolidated (hard) bedrock, such as limestone, sandstone, or shale. The depth to bedrock in Indiana varies from a few inches to several hundred feet.

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Recognizing Soil Development

In soil evaluation, contestants must learn to recognize soil development because very weak soil development is one of the criteria for identifying alluvium and local overwash. Soil development is recognized by profile differences in texture, structure, color, or, some combination of these properties.

As pointed out in the section on soil formation processes, clay is formed during soil development and tends to move down in the profile. Thus in a developed soil, there is usually more clay in the B horizon (subsoil) than there is in the A or E horizon or in the C horizon. Also, this B horizon usually has blocky or prismatic soil structure development.

Soil particles are grouped into peds that can be recognized in a chipped (not cut) portion of the profile; and they can be removed by gently probing into the soil with a knife or spatula. These ped surfaces are often covered with clay films or clay skins. To learn to recognize soil structure, you probably will have to work with an experienced person such as a soil evaluation coach or a soil scientist.

In well and moderately well drained soils, the B horizons are usually more red or brown than the A, E, or C horizons. This is caused by accumulation of iron released from soil minerals to form red or brown iron oxide minerals. In somewhat poorly and poorly drained soils, almost all the profile has either gray colors due to reduction and removal of iron, or mottled colors due to migration of iron over short distances within a horizon to cause depletion in the gray areas and concentration in the brownish or reddish spots.

Soil development is reflected by profile differences in texture, color, structure, or cementation, or by changes in several of these properties. In some sandy soils, profile differences are in color only. Therefore, careful observation and experience are needed to judge soil development. It is often helpful to carry some material from one site to another during practice sessions so that materials can be compared directly.

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Chapter 1 Sections

Factors of Soil Formation

Processes of Soil Formation

References, Further Information

Purdue University
Purdue Agronomy