Soil horizons

Soil Profile and Soil Horizons

A soil profile is a vertical section through the layers that make up a soil. It extends downward from the soil surface through the plant root zone. The depth may be as shallow as a fraction of an inch where bedrock is near the surface, or it may range to several feet where there are no restricting layers. Soil depths in Iowa are commonly between 30 and 70 inches.

Soil profiles are composed of various layers called soil horizons. The horizons form roughly parallel to the land surface under the influence of weathering, leaching, and plant and microbial activity. The character of each horizon is a result of the nature of its parent material and the physical, chemical, and biological processes that have acted upon it.
The main horizons in soils and material beneath the profile are designated as:
O HORIZON: layers dominated by organic material
A HORIZON: dark-colored mineral horizons formed at the surface or below an O horizon; highest levels of biological activity
E HORIZON: a mineral layer that has lost clay without organic gain
B HORIZON: subsurface horizon (unless significant erosion has occurred), a zone of accumulation with soil structure
C HORIZON: loose underlying parent material (unstructured).
R HORIZON: hard bedrock.
These horizons may be divided into parts such as A1, A2, and A3 (subdivisions of the A horizon) when there are significant differences within a main horizon. The various horizons can be distinguished because they differ from one another in such properties as color, texture, structure, accumulations, or hardness, etc. The change from one horizon to another may be abrupt or gradual. The thickness of soil horizons ranges from a fraction of an inch to 40 inches or more.

A horizon: Color and thickness

Color:

The upper part of the soil usually contains the most plant roots, accumulates the most organic matter, and has the darkest color of any horizon in the soil. Layers having these characteristics are called A horizons. Surface horizons of tilled soils are called Ap horizons, whether the material is pure A or a mixture of the A and underlying horizons.
In addition to accumulating the largest amount of organic matter at or near the surface of the soil, the greatest amount of physical and chemical weathering of the soil also occurs in the surface horizon. Wetting and drying and freezing and thawing cycles provide for weathering of fine and very fine silt particles and for the production of clay-sized particles in the surface horizon.

The products of weathering and some organic material are subsequently slowly moved deeper in the soil profile over time with percolating water.
The color of the A horizon of a soil is a clue to its productive potential and management needs. Dark colored soils usually are fertile. The dark color indicates that abundant vegetative growth is supplying the plant remains that decay and become soil organic matter. The amount and rate of decay of plant and animal material are influenced greatly by soil temperature and aeration. Either a cool climate or excessive wetness can cause a soil to accumulate higher levels of organic matter than well-drained soils. These soils may have partially decomposed organic materials in the O and/or A horizon and may be classified as muck or peat soils.

The soil color is usually a good indicator of the organic matter content of a soil horizon up to about 4-5%. Much of the organic matter occurs as dark-colored coatings on the mineral particles. About 4-5% organic matter will make light-colored mineral particles look black when moist.  Soil colors are darker when moist compared with dry, therefore moist colors are usually used in descriptions. Contestants will use moist colors (by adding water) for evaluating soil color.

When judging soil, the soil color is described for the A or Ap horizon if an inch or more of such material is present. If an E horizon is at the soil surface, the E horizon color will be identified for the contest. Where tillage has mixed the E horizon with the A horizon, the soil color for the A horizon will be indicated. In these cases, the E horizon will be marked as not present.

Four color groups are used in soil judging: dark, moderately dark, light, and very light. Dark includes soils that are black or very dark brown when moist. Moderately dark includes soils that are very dark gray to dark brown when moist. Light colored soils are usually light brownish gray when dry but are dark gray or dark grayish brown when moist. If a Munsell color book is available, these moist soil colors can be related to values of 2, 3, 4, and 5 respectively for the dark, moderately dark, light, and very light colors.

In an increasingly disturbed landscape, some areas may have lighter materials overlying the original A horizon. In a case with significant material accumulating (more than 6 inches), the sample tubs will represent intended horizons. If under 6”, this material would likely get mixed into the A horizon, at least in agricultural settings, so will be lumped with that horizon.

Thickness:

Thickness of current A materials should be identified in the contest. Original depth of A horizon will be provided if needed to determine erosion class and native vegetation.

The A horizon is usually the most fertile part of a soil and is likely to be the most permeable for air and water movement. It usually contains a much higher concentration of plant roots than other parts of the soil. The thickness of the A horizon is therefore significant in the productivity of the soil.

The A horizon is distinguished from the B horizon based on color, texture, and structure. The A horizon tends to accumulate organic matter but loses other materials. The B horizon tends to accumulate clay or, in some soils, humus and iron or calcium. Typical characteristics are therefore a darker color and a more open structure in the A horizon versus a higher clay content and a more dense structure in the B horizon. An A horizon that has little or minimal recent disturbance will have a granular
or granular-like structure. Where excessive disturbance has occurred, the structure may be platy or have no observable soil aggregates.

Thickness of the A horizon is judged on the basis of what is present today, which could be significantly different from the original condition. An A horizon measuring more than 12 inches is considered thick. Moderately thick includes those between seven and 12 inches in thickness. Moderately thin ranges from 3-7 inches thick may include thinner A horizons with mixing from lower horizons due to tillage that are composed of more A or A and E horizon than B horizon material. An A horizon is called thin if it is less than three inches thick or if the tilled layer is more than 50% B horizon material.

E horizon: Presence or absence

Forested soils and other intensely leached soils commonly contain an E horizon located at the surface or a few inches below the surface. An E horizon is usually
lighter in color, lower in organic matter, and contains less clay than an A or the overlying horizon, if present. In addition, an E horizon is characterized by soil aggregates
that form a layered or platy-like structure unless the horizon has been disturbed by tillage. Percolating water removes some clay and organic matter from the A horizon or soil surface downward into lower layers of the soil profile. The depletion of these materials from the surface or near the soil surface results in the formation of an E horizon. Deposition of the clay and organic coatings along pores and root channels in the B horizon help produce a contrast in the color, aggregate structure, and clay contents of the A, E, and B horizons.
Strongly developed E horizons are commonly underlain by B horizons with much higher clay contents. The higher clay content of the B horizon reduces the subsoil permeability and may cause the E horizon to stay wet for extended periods. The wet condition helps leach and acidify E horizons. The light color of the E horizon results from the natural color of sand and silt particles that have been stripped of organic matter, clay coatings, and oxidized iron compounds.
The typical gray color and acid condition of an E horizon indicate that it has relatively low soil fertility compared to thick, dark A horizons. Lime and fertilizer are likely needed to compensate for these conditions when crops are grown.

B horizon: Presence and color

The B horizon normally occurs in the subsoil position and affects water and air movement and plant growth. B horizons can be identified by texture, structure, and color. These properties may be in combination or may be alone, but of sufficient difference from other horizons to be identified as a B horizon. Some B horizons are dense enough to restrict air and water movement except when they are dry and cracked. Other soils are wet because of landscape position or contain horizons or layers with low permeability in the soil profile. Part or all of the soil may be saturated with water for extended periods of time.

Long periods of saturation are a serious restriction on the development of plant roots and therefore on the effective depth of the soil for plant growth. A soil that is too wet in the spring often limits root development enough to aggravate drought problems in the summer. A shallow root system can also produce fertility problems because the plants have too little soil from which to obtain nutrients. A favorable subsoil must retain enough water for plant growth but permit excess water to drain out so air and roots can enter and roots can grow and develop.

The color of the B horizon is a good indicator of the air and water relationships in the soil. Well aerated soils have uniform, brightly colored subsoils — commonly a shade of yellowish brown or brown. Soils that are frequently or constantly wet have dark gray, olive gray, or bluish gray colors in their subsoils. Soils with wet internal or alternating wet and dry internal conditions will have redox features. Intermediate moisture status and wet stages of internal drainage within the soil profile have redox features. Redox features consist of spots or concentrations of one or more contrasting colors on a background of another color or a near uniform gray color present in the soil matrix. Redox features indicate the presence or absence and concentration of various compounds of iron and manganese in the soil due to the degree of oxidation and hydration. Redox colors can range from a rust, dusky red, to gray or grayish green. The coloration of redox features may be expressed across a range from prominent to faint. Faint redox features are described as casts or tints in this manual. Some soils have redox features in the subsoil that are not an indication of present air-water relationships. These colors are an indication of air-water conditions that were present in the soil profile during an earlier geological period when soil development occurred in an environment that may be different from the soil environment that has dominated during the most recent climatic conditions. This is termed as relict conditions. These redox features may occur in soils occupying slope gradients greater than 2%. For the purpose of the contest these relict features will be used to determine the color of the B horizon (or C horizon if a B horizon is not present) and the internal drainage class of the soil.

Many soils have B horizons thick enough to require designation of the part to be described for the contest. If the B horizon is more than 10 inches thick, only the upper 10 inches will be described and used for the contest; if it is less than 10 inches thick, the entire B horizon will be described and used for the contest. Some soils have not had sufficient time to develop a B
horizon. The B horizon section of the scorecard will be scored for the C horizon for such soils. In these soils the 10-inch zone immediately below the A horizon will be used for the contest.

Classes will be used for designating the color of the B horizon (or buried A horizon if overlain by 24 inches or more of light-colored sediments or C horizon if there is no B horizon). Note that internal drainage and external conditions may be different, as a soil pit may contain water, as it ran in after the soil pit was dug, or a pit in a drought year still shows redox features. Soil color or redox features are a more permanent record of ‘normal’ soil conditions. These classes are:
A. Uniformly brown, dark brown, or strong brown.

B. Uniformly brown, dark brown, or strong brown. Includes olive or yellowish or reddish cast or tints or other faint redox features in the soil matrix.

C. Grayish brown or olive gray with no redox features.
D. Grayish brown or olive gray with gray or rust redox features.
E. Gray with redox features. Redox features are often rust in color, but the gray color covers more than 50% of the area.
F. Uniform gray. May have a bluish or greenish cast. Rust redox features, cast, or tints may occur around roots or small pores, but not elsewhere.
G. Black. May contain a few rust redox features. This class includes a buried A horizon if present (at a depth of 24 inches or greater and extends to the full depth of the exposed soil profile)

C horizon: Presence and color:

Presence of a C horizon within judging depth: C horizons indicate a shallow depth to development, primarily indicated by a lack of soil structure. This shallow C horizon could indicate a younger soil that has just not had enough time to change more deeply into the soil, or could indicate more recent disturbance. It is also possible to have a
C horizon above a previously developed surface. One location that may occur is on a flood plain, where more recent deposits could cover older ones. On footslopes or other disturbed landscapes, it is possible for material to have been deposited by erosion or humans on top of a previously stable soil

Additional non-contest horizons:

O horizon-An O horizon is composed of primarily organic material rather than a mineral soil with organic enrichment like an A. These might be found on the floor of an undisturbed forest, or in a peat bog/marsh/wetland area.

R horizon-These are consolidated materials, meaning they cannot be dug with hand tools. These are considered root-restrictive for plants and otherwise limit use for other purposes.