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Soil colors attracted some attention in Russia but little in the USA prior to the present century. Two of the first three soil survey reports published by the U.S. Department of Agriculture (USDA) in 1900 did not mention soil colors. A few year later, however, colors were considered in the definition and differentiation of soil series. Efforts were also being made to identify constituents responsible for different colors. Initial steps toward soil color standards and terms in both the USA and Russia were hesitant and faltering. The Bureau of Soils, USDA, published a list of 22 names for colors in 1914 but standards were not mentioned. During the 1920s, efforts to establish color standards in the USA were initially frustrated by the state of color technology. By the end of the decade, however, a method had been developed to determine the colors of dry soil samples in the laboratory and express those in proportions of white, black, yellow, and red. Somewhat comparable efforts were underway in the former Soviet Union. Progress seems to have stalled at that point for another 10 yr. The first set of color charts for field use in the USA was published in 1941. The charts are like those of the present in many ways but are smaller and lack the Munsell notations for hue, chroma, and value. Instead, each chip is assigned a name from the ISCC-NBS system. In addition to providing the charts and names, the 1941 bulletin summarizes earlier efforts in this country to identify colors, establish standards, and assign names. Beginning in 1945, the Division of Soil Survey, Bureau of Plant Industry, Soils, and Agricultural Engineering, USDA, launched a major effort to improve standards and terminology for properties of soil horizons such as color, texture, structure, and consistence. The effort lasted about 5 yr. Early in that period, a decision was made to use constant hue charts showing chromas and values of the Munsell system with their notations. Rather than the ISCC-NBS names, folk terms were adopted for soil colors. Several years were required to reach agreement on names. The Munsell color charts and the new set of names were adopted in the American soil survey program in 1949. Worldwide use of the color charts and names was recommended by the International Society of Soil Science about 10 yr later. A few modifications have been made of some charts since then and one chart for colors of wet soils (“Gley” chart) has been added. The intermittent efforts that extended over a period of 35 yr have provided a useful system to describe soil color in the field. “When the subject of soil colors is finally elucidated, it will become possible to compile soil maps that will be equally useful to peasants and to learned agronomists” (Dokuchaiev, 1948). Together with this optimistic view in Dokuchaiev's monograph on the Russian Chernozem, published initially in 1883, are comments on the perception of soil color in the field. That perception was said to depend upon several factors. Examples of these factors are moisture conditions, the quality of light, the time of day, and lumpiness of the soil surface. These remarks are still valid.
Early understanding of soil color, early practice in describing color, the changes with time, and the efforts that eventually led to the present standards and terms are summarized in this chapter. The summary covers efforts only in the USA and the former Soviet Union because I am not acquainted with any elsewhere. The history provides insight into the frequently tangled pathway toward greater accuracy in the description of soil color.
Determination of soil color is useful to characterize and differentiate soils. The color of soil materials can be measured in the laboratory by using diffuse reflectance spectrophotometers. The spectral reflectance data given by these apparatuses are easily converted to three figures (“tristimulus values”) that define the color perceived by the human eye. In turn, tristimulus values can be converted to the Munsell notation or the parameters of other color systems. Modern, commercially available spectrophotometers not only allow a quick measurement of reflectance but usually provide color data in different systems. If care is taken in obtaining homogeneously granulated or powdered soil samples, and in preparing the white reflectance standards, high accuracy and precision are obtained. Small differences in soil color can then be used to identify and study differences in soil compositional properties. For this purpose, several “color indices” calculated from the color data can also be used.
We prepared sets of <2 mm soil samples, distributed them to soil scientists, and asked them to determine the dry and moist Munsell color of each soil. We observed that soil scientists agreed on the same color chip for a single color component (hue, value, or chroma) 71% of the time, and there was an average of 52% agreement for all three color components. The standard deviation (SD) varied from 0.45 (value-moist) to 0.68 (chroma-moist) with an average SD of 0.57. Regression equations were computed that compared the mean soil color with the nearest color chip noted by an individual soil scientist, and the coefficient of simple determination (r2 ) ranged from 0.49 (chroma-moist) to 0.79 (value-moist and dry). When “in-between” colors were estimated the r2 improved and ranged from 0.70 (chroma-moist) to 0.95 (value-moist). A detailed evaluation was made of data from a commercial tristimulus colorimeter, and results were compared to colors described by soil scientists. The r2 ranged from 0.88 (chroma-moist) to 0.96 (value-dry); however the slopes and intercepts were different. Commercial colorimeters have great potential as tools for measuring soil colors, but field colors by soil scientists are not identical to instrumental data. They differ because the sensor, light source, and angle of light refraction are different for each color measurement method. The inherent complexity of color identification by humans vs. instrumental measurements is also a contributing factor.
Iron oxides are useful field indicators of pedogenic environments for three reasons: (i) they include several minerals, (ii) these minerals have different colors, and (iii) the type of mineral formed is influenced by the environment. Therefore, recognizing the Fe-oxide mineral in the field by its color has a potential to yield information about pedogenesis. Hematite-containing soils (usually with associated goethite) have mostly hues between 5YR and 10R, whereas goethite-containing soils with no hematite have hues between 7.5YR and 2.5Y. Orange colors with a hue of 7.5YR and a value of ≥6 are often due to lepidocrocite. Ferrihydrite can be distinguished from goethite by its more reddish hue (5-7.5YR) and from lepidocrocite by its lower value (≤6). These mineral-specific colors, however, also vary somewhat with concentration, crystal size, degree of cementation, and possibly isomorphous substitution. Poorly crystalline goethite, lepidocrocite, and ferrihydrite may have lower values than better crystalline specimens, and cementation also leads to lower values. Small hematite crystals are bright red (2.5YR-10R), whereas the color of larger crystals or crystal aggregates may reach into the red purple (RP) range. This chapter reviews the relationships between Fe oxides and soil color and also briefly considers the occurrence and pedogenetic implications of these minerals.
Quantitative relationships between soil color and organic matter content are only poorly understood, but they are of considerable practical importance in mapping and classifying soils, interpreting soil properties, and in designing sensors for agricultural equipment. We studied the color-organic matter relationships for Ap horizons from Indiana and Illinois soils to test the hypothesis that Munsell value and organic matter content are more closely related for soils occurring together in soil landscapes than for soils over a wide geographic region. Two sample sets were collected. Sample set 1 consisted of 105 Ap horizons from throughout Indiana, while set 2 consisted of 10 to 15 Ap horizons from each of 16 landscapes in Indiana and Illinois. Organic matter content was determined by dry combustion, and Munsell colors of both moist and dry samples were calculated from reflectance spectra. The relationship between Munsell value and organic matter content: (i) was poor for Indiana soils statewide (sample set 1), (ii) was predictable (r2 > 0.9) within soil landscapes if soil textures did not vary widely, (iii) was linear within landscapes with silty and loamy textured soils but was curvilinear within landscapes with sandy-textured soils, (iv) was similar among landscapes having the same soil textures and parent materials, and (v) was not predictable if soil texture varied widely (sands vs. silts and loams) within the landscape. In a separate study, we measured the colors of various organic and inorganic fractions from the Ap horizons of four Indiana soils. The purified humic acid content was about 15 times the purified fulvic acid content for all four soils. The black humic acid, which masked the yellowish brown color of the fulvic acid, was responsible for the dark color of the soil organic matter.
The colors of acid sulfate soils reflect various stages in the process of sulfuricization. Sulfidic mineral materials associated with potential acid sulfate conditions typically have chroma ≤ 1 and values ≤ 4. Hues (when present) range from 10YR through 2.5Y and 5GY, to 5G and 5B. Black (N2) materials often contain Fe monosulfides occurring as very fine, x-ray amorphous particles. Monosulfides evolve H2S when exposed to HCl. Pyrite, the most common Fe sulfide mineral, is also black when powdered, but does not pigment soil materials as much as the monosulfides because of coarser grain size and does not evolve H2S with HCl. Sulfidic organic materials commonly have higher chromas (normally 2 or more) and browner hues than sulfidic mineral materials. The oxidation of sulfides may produce a variety of relatively soluble sulfate minerals, including melanterite, rozenite, and copiapite, that have different colors but that are transient phases in the formation of jarosite and insoluble Fe oxides. The latter may include ferrihydrite, goethite, hematite, lepidocrocite, and an akaganéite-like mineral (schwertmannite) that is especially abundant in acid drainage waters. All these minerals possess distinctive colors and may occur in active or postactive acid sulfate soils. In particular, jarosite and Fe oxides are often found as bright mottles in channels and on ped faces in sulfuric horizons with low chroma (≤ 2) colors (reflecting less-oxidized conditions) in ped interiors. Jarosite may have lower chromas than was specified for the sulfuric horizon in Soil Taxonomy. Revised sulfuric horizon criteria in the 1992 Keys to Soil Taxonomy do not specify the color of jarosite and permit jarosite to be absent from sulfuric horizons in some cases. In postactive acid sulfate soils, jarosite is commonly lost by dissolution from the surficial and upper B horizons. In tropical areas, the Fe released by jarosite decomposition appears to form hematite. Other sulfate minerals that are white or colorless, such as gypsum and barite, may also appear in some postactive acid sulfate soils. The identification of acid sulfate soils by color or mineralogy at various development stages is an obvious aid to understanding their genesis. The recognition of potential acid sulfate conditions is also important for preventing undesirable environmental situations that may result when sulfidic materials are drained or inadvertently exposed at the earth's surface.
Color in landscapes has been used for decades as a basic tool for assessing soil drainage. Parent material largely governs the types of minerals and the textural distribution of soils and sediments present in a landscape. Minerals color the soil and often reflect the influence of texture and soil drainage, aeration, and configuration of the water table. Hydrologic conditions (notably recharge, flowthrough, and discharge) influence leaching, reduction-oxidation, and the accumulation of precipitates of Fe and Ca. The water table position and fluctuation are also affected. Geomorphic setting includes climatic factors and general water circulation in open or closed drainage systems. We present a generalized soil mottle sequence that reflects the morphological impact of alternating reducing and oxidizing conditions (redoximorphic features) proceding from dry to very wet landscape positions: (i) unmottled peds with high chroma; (ii) low chroma colors on ped edges with little Fe removal; (iii) distinct albans or gray areas that represent Fe removal from low chroma areas on ped edges or from around root channels with a kneaded color of 3 chroma; (iv) thick albans resulting from Fe removal around all macropores such as roots and ped faces, abundant Fe-Mn concretions in ped interiors and a kneaded color of 2 chroma or less; and (v) zones of Fe accumulation (reddish colors or furrows) around ped faces or root channels or gley coloration in ped interiors.
Red beds are sediments and sedimentary rocks with hues ranging from 2.5YR to 5R. Reddish pigmentation of these deposits is present in both surface exposures and subsurface well cores. Strata containing red beds commonly exhibit color mottling and may include layers that are not red. The fine-grained hematite (< 2 μm) that pigments red beds can form in the weathering zone and below the zone of soil formation. Many pre-Quaternary red beds contain buried and lithified paleosols that can be identified by micro- and macro-morphologic features such as: specific types of carbonate nodules and pisolitic concretions, smooth curving slickensided surfaces, and fossil roots. Below the zone of soil formation, hematitic pigment forms by diagenetic processes including: oxidation of Fe following the alteration of ferromagnesian minerals and volcanic ash, and oxidation of Fe sulfides. Unresolved problems in the genesis of red beds include the relative importance of inherited pigment, and of time and temperature in hematite formation.