SOIL FEATURES

 

There are many properties or features that describe and characterize soils (fig. 4). Some of these features (such as color, texture and depth) are relatively easy to record while others require very sophisticated equipment and highly technical procedures (such as chemical data and mineralogical analysis).

 

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Figure 4. Soil properties that can influence use and management of land.

 

The Soil Profile

Due to the interactions of the five soil-forming factors, soils differ greatly. Each section of soil on a landscape has its own unique characteristics. The way a soil looks if you cut a section of it out of the ground is called a soil profile. When you learn to interpret it, the profile can tell you about the geology and climate history of the landscape over thousands of years, the archeological history of how humans used the soil, what the soil properties are used today, and the best way to use the soil. In a sense, each soil profile tells a story about the location where it was found.

 

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.

 

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Figure 5. Soil profile separated into horizons.

Where soil forming factors are favorable, five or six master horizons may be in a mineral soil profile (fig. 5). 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.

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. Some organic materials were deposited under water (fig. 6). Subdivisions of Oa, Oe, and Oi are used to identify levels of decomposition. The O horizon is dark because decomposition is producing humus.

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Figure 6. Profile on the left shows an Oi horizon at the surface; an organic horizon with little decomposition. Profile to the left shows an Oa horizon; an organic horizon that is highly decomposed.

 

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Figure 7. The soil profile to the right shows a well drained soil with an A horizon from the surface to a depth of 5 cm (2 in.). The measuring tape is in feet. The middle soil profile is a somewhat poorly drained soil with the top of the gray due to wetness occurring at 40 cm (16 in.). Tape measure is in meters. The middle soil profile has a buried A horizon starting at 1 m (40 in.). The soil profile to the left is moderately well drained with an Ap horizon from the surface to 20 cm (8 in.).

 

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 (fig. 7). It is the horizon of maximum biological activity. 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 buried beneath more recent deposits is designated as an "Ab" horizon (fig. 7). An A horizon that has been plowed or otherwise manipulated is an Ap horizon (fig. 7).

The E horizon generally is bleached or whitish in appearance (fig. 8). As water moves down through this horizon, soluble minerals and nutrients dissolve 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.

 

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Figure 8. This profile is a soil found in the Appalachian Plateau showing an E horizon from 20 to 35 cm (8 to 14 in.) and a B horizon from 35 to 55 cm (14 to 22 in.) Measuring tape is in 10 cm increments. Below 55 cm (22 in.) is a C horizon.

Below the A or E horizon is the B horizon, or subsoil (fig. 8). 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. Soil not having recognizable concentrations within B horizons but show color or structural differences from adjacent horizons is designated "Bw".

 

Still deeper is the C horizon or substratum (fig. 8). 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. C horizons described as "2C" consist of different material, usually of an older age than horizons which overlie it. 

 

The lowest horizon, the R horizon, is bedrock (fig. 9). Bedrock can be within a few centimeters of the surface or many meters below the surface. Where bedrock is very deep and below normal depths of observation, an R horizon is not described.

 

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Figure 9. This soil profile has 110 cm (44 in.) of unconsolidated soil material over hard bedrock.

 

Generally, soil horizons are found in the order presented (fig. 10). However, a soil profile may lack certain horizons or have horizons out of order due to factors that influenced that soil’s development. For example, a soil profile may lack E and B horizons if it is a young soil that has not had the time for an E and B horizon to develop. Or, a soil may have a buried A horizon if that soil has had material deposited on top of what was once the soil surface (fig. 7). This may occur on flood plains after a flooding event deposits sediments, because of erosion deposition, or because man has deposited material on top of the soil.

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Figure 10. Common horizons designations and the order in which they are most commonly found.

 

Color 

To the casual observer, color is the most noticeable soil property. Maryland soils vary in color from red, yellow and brown to gray in the subsoil (B horizon) and from black to very light gray in the topsoil (A horizon). Color is a significant indicator of several soil properties, including the organic matter content and drainage condition. The three components that have the most affect on soil color are organic compounds (usually black or dark brown), iron oxides (usually red, orange or yellow) and the color of the mineral grains (usually gray).

Black or very dark colors in the A horizon suggest relatively high organic matter contents. Most cultivated Maryland soils have organic matter in their plow layer ranging between 1 and 4 percent by weight. In some poorly drained soils, the organic matter content will reach 10 percent and higher. Generally, the darker the A horizon the higher the organic matter content. In Maryland, this generalization can be taken a step further; a deep, dark colored A horizon indicates the soil was formed under very poorly drained conditions. Organic matter enhances soil tilth (physical condition) or structure and is a natural nitrogen supplier under favorable conditions. As the organic matter content decreases, the color is determined more by the mineral components of the horizon. Pale colors indicate that the horizon has low organic matter content (fig. 11). 

 

Figure 11. The soil profile on the left has a dark surface high in organic matter, while the soil profile on the right has a pale surface low in organic matter. The measuring tapes are in meters.

Subsoil colors are not greatly influenced by organic matter. Usually, the iron compounds coating the mineral particles are largely responsible for the color of this horizon. 

Soils formed under well-drained conditions, where oxygen is readily available, have subsoils with bright colors, usually brown, red or yellow (iron oxide colors). Some grayish tones may occur in these soils, but they are associated with the weathering of rocks and not drainage. Usually, soils formed under well-drained conditions are uniform in color, however, mottles (splotches of color), may occur due to weathering of rock fragments or parent material colors, etc (fig. 12). Brown, red or yellow colors can be interpreted as indicating good natural drainage making artificial drainage unnecessary. Septic systems should work in these soils unless they contain too much clay. Also, these soils should provide good dry locations for houses with basements. 

 

Figure 12. The red and gray colors in this soil profile are inherited from its parent material.

 

When these bright colors are mixed with areas of gray (color of the mineral grains) the soil developed under conditions of imperfect drainage. The mixed pattern, called redoximorphic features, indicates that the soil is saturated with water for significant periods during the year (fig. 13). This pattern is caused when iron is reduced due to wetness and moved leaving splotches where of gray colors where the mineral grains have been stripped of iron. Artificial drainage usually is necessary for good crop production and septic systems are subject to periodic failure when installed in these soils. 

 

Figure 13. The red and gray colors in these soil profiles are due to wetness. These splotches of colors due to wetness are called redoximorphic features. The soil profiled and the left has a predominance of gray due to the loss of iron starting close to the bottom of the spade. The soil on the right has a predominance of red with gray splotches starting at 1 m (40 in.). The measuring tape for the profile on the right is in meters.

 

When gray (color of the mineral grains) predominates with only streaks and spots of brighter colors (redoximorphic features) the soil was formed under poorly drained conditions. The spots of brighter colors are where the iron has re-oxidized forming spots similar to rust. These soils are called hydric soils, soil that have a water table near the surface for significant periods of time. Artificial drainage is necessary for crop production, and these soils are poor building sites, especially where septic systems are needed. 

 

Figure 14. This soil profile is a hydric soil. The predominance of gray colors with splotches of red near the soil surface demonstrates the typical pattern of redoximorphic features found in a hydric soil.

 

When determining colors, make sure that the soil is moist. Moistened soil better illustrates color variations, making it easier to distinguish one horizon from another. Soil scientists use standard color (Munsell) charts to determine color (fig. 15); this permits uniformity and eliminates some of the human variable. According to the chart, a soil horizon described as yellowish-brown in Maryland has exactly the same color as a yellowish-brown horizon in California.

 

 

 

 

Figure 15. The Munsell soil color book is used to standardize soil color designations.