Subsoil color reflects more strongly in most soils the imprint of physico-chemical processes. In particular, the redox status of Fe and to a lesser extent Mn, strongly influence the wide variation found in subsoil color. Soil color can provide information about subsoil drainage and the soil moisture conditions of a soils. The colors of the Fe oxides and hydroxides were described in chapter 7.2 . In well aerated soils, Fe³⁺ is present which give soil a yellow or redish color. In more poorly drained soils (anaerobic conditions) iron compounds are reduced and the neutral gray colors of Fe²⁺ or bluish-green colors of iron sulfides, iron carbonates, or iron phosphates are visible. A black color in the subsoil can be related to an accumulation of manganese.

In arid and semi-arid environments, the influence of soluble salts (carbonates, sulfates, chlorides etc.) may impart a strong influence on soil color. For example, in arid or subhumid regions, surface soils may be white due to evaporation of water and soluble salts.

Colors associated with minerals inherited from parent materials may also influence color in horizons that have not been extensively weathered. For examle, light gray or nearly white colors is sometimes inherited from parent material, such as marl or quartz. Parent material, such as basalt, can imprint a black color to the subsoil horizons. 

Table 9.4.1. Soil colors accociated with soil attributes.

Soil color is usually registered by comparison of a standard color chart (Munsell Book of Colors). The Munsell notation distinguishs three characteristics of the color: hue, value, and chroma.

Hue: It is the dominant spectral color, i.e., wheather the hue is pure color such as yellow, red, green, or a mixture of pure colors.

Value: It describes the degree of lightness or brightness of the hue reflected in the property of the gray color that is being added to the hue.

Chroma: It is the amount of a particular hue added to a gray or the relative purity of the hue.

Figure 9.4.1. Munsell soil color chart.

The soil colors are given in the order: hue, value, and chroma. For example, 2.5YR 4/2 describes the hue 2.5YR, dark-grayish brown with a value 4 and a chroma of 2. It should be stressed that soil color is dependent on soil moisture, hence if soil color is recorded also the soil moisture conditions have to be described (e.g. soil color dry, soil color wet). In the upper midwest and other humid areas, colors are conventionally recorded moist. This convention may differ in other climatic regimes.

Many soils have a dominant soil color. Other soils, where soil forming factors vary seasonally (e.g. wet in winter, dry in summer) tend to exhibit a mixture of two or more colors. When several colors are present the term mottling or redoximorphic features (RMF) is used. In such a case, several soil colors have to be recorded, where the dominant color is first, following by a description of the abundance, size, and contrast of the other colors in the mottled pattern. Mottling/RMFs are described by three characteristics: contrast, abundance, and size of area of each color.

Redoximorphic features are a color pattern in a soil due to loss (depletion) or gain (concentration) of pigment compared to the matrix color. It is formed by oxidation / reduction of Fe and/or Mn coupled with their removal and translocation or a soil matrix color controlled by the presence of Fe²⁺. RMFs are described separately from other mottles or concentrations! Based on the Field Book for Describing and Sampling Soils (Schoeneberger et al., 1998) RMFs are described in terms of kind, color & contrast, qantity, size, shape, location, composition & hardness, and boundary. RMFs occur in the soil matrix, on or beneath the surface of peds, and as filled pores, linings of pores, or beneath the surface of pores.

Mottles are areas of color that differ from the matrix color. These colors are commonly lithochromic or lithomorphic attributes retained from the geologic source rather than from pedogenesis. Mottles exclude RMFs and ped & void surface features (e.g. clay films). Based on the Field Book for Describing and Sampling Soils (Schoeneberger et al., 1998) mottles are described in terms of quantity, size, color & contrast, moisture state, and shape. Example: Few, medium, distinct, reddish yellow moist (7.5YR 7/8), irregular mottles.

However, a variety of other features in a horizon may have colors different from the matrix, such as infillings of animal burrows (krotovinas), clay coatings (argillans) and precipitates of calcium carbonate. In all instances where specific soil features are described, the shape and spatial relationships of the feature (i.e., where is it located, on a ped face, in the matrix...) to adjacent features should be described in addition to its color, abundance, size and contrast.

Table 9.4.2. RMFs/mottles in soils are described in term of abundance, size, and contrast.

 

Contrast	Abbreviation	Visibility
faint	f	difficult to see, heu and chroma of matrix and mottles closely related
distinst	d	readily seen, matrix and mottles vary 1 - 2 hues and several units in chroma and value
prominent	p	conspicious, matrix and mottles vary several units in hue, value, and chroma

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