Figure 2. Soil color parameters.
Soil contains a record of its natural history. The length of time a soil has been in place, the impact of climate and topography, parent materials, and biological and human activities are recorded in the soil as properties of horizontal layers (horizons), which can be elucidated by a skilled evaluator. Soil drainage conditions exert a major influence on the morphology of the soil profile. An experienced evaluator can uncover important information on soil water and air movement by careful observation of soil material, and the properties and sequences of soil horizons at a site. These observations can form a reliable basis for designing appropriate wastewater systems. Of the many soil morphological properties, soil color, structure, consistence, texture and depth of plant growth are the most often used indicators of soil hydrologic conditions to determining site suitability for wastewater treatment.
Soil horizons are approximately horizontal layers that result from the interaction of soil-forming factors. Soils are identified on the basis of the morphology of their horizon sequences, and their suitability for wastewater treatment is assessed on the basis of horizon characteristics.
Soil scientists call the major horizon forms “master horizons” and classify them with alphanumeric symbols. The major horizons are:
Transitional horizons are indicated by combining the master horizon letters (such as AB, a horizon dominated by A characteristics, but transitional to B, or E/A, a mixed horizon with distinct patches of E and A horizon materials).
The major horizons are frequently subdivided to indicate specific layers having distinct features; these are commonly indicated using lower case suffixes and numeric subdivisions.
Examples of suffixes describing specific features common in Ohio soils and subordinate horizons in which they commonly occur include:
Soil color is an important indicator of soil genesis and of current and historic processes, particularly those associated with water and air storage and movement. Color is easily seen and described. However, because of the great variety of soil coloring agents, and the many processes that cause color to develop and change, exact interpretation is not always simple.
Soil color is a function of soil formation history, mineralogy of the soil constituents, biological activity, drainage conditions and aeration. The major coloring agents in soil include soil organic matter (dark), and metal oxides including iron, manganese and aluminum (red, yellow and black). The concentration and chemical form of organic matter and metal oxides are highly sensitive to drainage and aeration.
Most soil minerals are basically gray in color. Larger soil particles and aggregates (structural units of the soil made up of particles held together) are frequently coated with thin layers of very fine soil particles. Coatings of organic matter and metal oxides change the color of the underlying particles and aggregates, in a similar way to watercolor paint on a gray canvas. As drainage and aeration conditions change, processes that influence the breakdown or accumulation of organic matter, and the state and solubility of the oxide coatings also vary.
Under saturated conditions, biological activity is curtailed, and the oxidation and mineralization of organic matter, processes that involve soil microorganisms, also slow down. Dark colors, particularly in the surface layers of the soil, commonly represent accumulation of organic matter, masking gray-colored mineral material. Conversely, organic matter breaks down more quickly under well-aerated conditions, due to higher levels of biological activity.
Soils with strong, bright colors such as dominantly red materials often indicate the presence of oxide coatings, especially iron oxides. These coatings form and are stable in well-aerated conditions where plenty of oxygen is available.
Important chemical and biochemical reactions called redox reactions take place in soils under saturated conditions. As organisms remove the available oxygen, the chemical process called reduction is enhanced. During reduction, iron and other metal oxides that provide the strongest soil colors (red, yellow, black) change to less oxidized forms. The more reduced oxides are more soluble in oxygen-depleted water, and are stripped from the coatings of underlying mineral grains and aggregates. Where stripping, solution and movement of the dissolved oxides is most intense, predominantly gray soil materials remain. Soils that are dominantly gray are frequently saturated with water, and are often poorly aerated.
Some layers in the soil are only saturated seasonally. As water levels move up and down in the soil, intermittent periods of low oxygen status are associated with dissolution, movement and leaching of the oxides. Deposition (re-precipitation) of oxides can occur in dryer parts of the soil or along root channels or other pathways of air movement. Hence brighter colors tend to accumulate in some places and to disappear from others. In these intermittently saturated soils, patches or mottles of contrasting colors can be seen. Mottles are examples of redoximorphic features in the soil, and frequently indicate intermittent saturation or seasonal high water tables which may not persist throughout the year. The presence of strong redoximorphic features, particularly dull gray patches, is the most important morphological indicator of wetness in soil horizons, used by evaluators to determine limitations to wastewater treatment in the soil.
Soil colors are described in a standard way so that different individuals can share information for decision making. Color chips matching as closely as possible the variety of soil colors are printed in a standard book called the Munsell Soil Color Charts. In the Munsell system, soil color is represented in three ways: Hue, Value and Chroma, each representing a different aspect or axis of color, as illustrated in Figure 2. Alphanumeric codes provide a shorthand method for describing the soil color, and are complemented by general color names. The first numbers and letters represent the hue, followed by the value and the chroma. For example 5YR4/3 describes a soil with hue of 5YR, value of 4 and chroma of 3. This soil material would be “reddish brown.”
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| Hue | Value | Chroma |
Figure 2. Soil color parameters. | ||
Hue describes the dominant visible spectral shade (or wavelength). In the Munsell system, three dominant hues (R, red; Y, yellow; G, green) are used together with intermediate hues (for example YR for yellow-red). Soils cover a range of seven major classes of hue, ranging from red through yellow. Four equal steps are provided between each of the major hues (2.5, 5, 7.5, 10). Each of the dominant hues is represented by a letter (R for red, Y for yellow), with intermediate two-letter codes for transitions (YR for yellow-red). Each numeric code describes a single class of hue, for example, 10YR, 5R or 2.5Y. Each class of hue is allocated a single page in the Munsell book.
Value describes the relative lightness or darkness of the color. Values range from pure black to pure white. Gray is perceived as halfway between black and white. Black is assigned the value 0/, while white is 10/. A mid-gray would be 5/ with darker grays having lower values, and lighter grays having higher values. Soil color values are arranged on each page of the color book with the lightest colors at the top of the page and the darkest at the bottom.
Chroma represents the relative purity, strength and saturation of the color. The strongest expression of color (for a given hue and value) is given a high chroma code of /8. The absence of color would be described with a chroma code of /0, indicating complete dilution of the pure color by neutral gray. “Washed-out” colors have low chromas while intense saturated colors have high chromas. Chroma is arranged on each page horizontally, with the lowest (dullest) chromas on the left, and highest (most pure) chromas on the right.
The appearance of soil color varies according to lighting conditions, moisture content and treatment of the sample. For consistency, soil color should be judged under bright direct sunlight during the middle part of the day. The soil sample to be described should be moist but not wet (keep a water bottle handy to moisten dry material), and the soil should not be crushed or mixed.