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Soil color

Soil color is the visual appearance of soil material, ranging from black to white and encompassing hues such as red, yellow, brown, and gray, which serves as a primary indicator of soil composition, environmental conditions, and formation processes.^[1] It arises from the interaction of inorganic minerals, organic matter, and water, with iron oxides often imparting dominant red or yellow tones in well-aerated soils, while reduced iron in waterlogged conditions produces grays and mottles.^[2] This color variation not only reflects soil health and drainage but also aids in agricultural assessment, archaeological analysis, and environmental classification.^[1] The primary factors influencing soil color include mineral content, organic matter accumulation, and moisture status. Iron and manganese minerals oxidize during weathering to form pigments like hematite (red) or goethite (yellow), with particle size, density, and crystal agglomeration further modulating the intensity of these colors.^[1] Organic matter, decomposing into dark humus, darkens surface horizons, particularly in fertile topsoils with high levels (typically more than 5%), correlating strongly with soil blackness or brownness.^[2]^[3] Moisture plays a critical role by altering mineral oxidation states—aerobic conditions preserve vibrant colors, whereas anaerobic saturation reduces iron to colorless forms, resulting in gray tones or mottling that signal periodic wetness.^[1] Soil color is standardized using the Munsell soil color system, which quantifies hue (color name), value (lightness), and chroma (color purity) through notations like 10YR 5/3 for a yellowish-brown.^[1] This system, based on physical color chips, ensures consistent global comparisons and is essential for soil taxonomy, where colors delineate horizons and interpret properties like drainage class.^[1] Accurate determination requires moist samples under controlled lighting to minimize variability from dryness or illumination, enhancing predictions of attributes such as organic carbon content.^[4]

Overview

Definition and Importance

Soil color refers to the apparent hue, value, and chroma of soil as observed in profiles or samples, resulting from the reflection and absorption of visible light by soil particles and constituents.^[5]^[6] This visual attribute is typically assessed in moist or dry states to account for variations in appearance.^[1] In soil science, soil color serves as a rapid, non-destructive indicator of soil genesis, fertility, drainage conditions, and potential contamination, enabling quick field assessments during surveys.^[6]^[1] It is one of the standard morphological properties in pedology, alongside texture, structure, and consistence, used to describe and classify soil horizons.^[6] For instance, dark colors often signal high organic matter content, which correlates with greater fertility, while grayish tones may indicate poor drainage due to waterlogged conditions.^[7] Color variations provide broad implications for land management, such as identifying soils suitable for agriculture versus those prone to erosion or limited by water saturation.^[1]^[7] These cues also inform ecosystem health evaluations, where shifts in color can highlight risks from erosion or pollution, aiding in sustainable practices like conservation planning.^[6]^[8]

Historical Context

The study of soil color has roots in ancient agricultural practices, where early civilizations recognized its implications for crop suitability. In ancient China, as documented in the Yugong treatise dating back approximately 2,500 years, soils were classified into categories based on observable qualities including color, reflecting an early empirical understanding of fertility and land use.^[9] Similarly, Roman agronomists in the classical period evaluated soil by color, alongside texture and smell, to assess productivity for viticulture and grain cultivation, integrating these observations into practical farming treatises.^[10] By the 19th century, scientific inquiry advanced this recognition, with Russian geologist Vasily Dokuchaev pioneering the linkage of soil color to horizon development in his 1883 work on chernozems, where he critiqued simplistic field-based color judgments while establishing color as a diagnostic tool for soil genesis and profiling.^[11] The early 20th century marked a shift toward systematic standardization, beginning with the introduction of the Munsell color system in 1905 by artist and color theorist Albert H. Munsell, which provided a precise notation framework using hue, value, and chroma to quantify soil colors objectively.^[12] This system gained traction in Europe during the 1910s, where initial systematic color charts emerged in soil surveys, facilitating more consistent descriptions amid growing pedological research.^[13] In the United States, the U.S. Department of Agriculture (USDA) began incorporating Munsell notations into soil surveys by the late 1930s, with formal adoption of the charts and standardized terms occurring in 1949 to enhance uniformity in national mapping efforts.^[13] Mid-20th-century advancements integrated soil color into broader taxonomic frameworks. The USDA's Soil Taxonomy, developed from 1951 to 1974 and first outlined in the 1960s, embedded color criteria as key diagnostics for horizon identification and soil order classification, influencing global pedology.^[14] Concurrently, the Food and Agriculture Organization (FAO) of the United Nations emphasized color in its early classification efforts during the 1950s, culminating in the 1960s UNESCO-FAO legend that used color standards to define soil units internationally, promoting harmonized mapping.^[15] Post-2000, the advent of remote sensing and digital imaging revolutionized soil color analysis, enabling large-scale mapping through satellite-derived spectral data that quantify color variations for global inventories and environmental monitoring.^[16]

Description and Measurement

Visual and Qualitative Methods

Visual and qualitative methods for assessing soil color involve direct field observations by soil scientists and practitioners, relying on the naked eye and standardized reference materials to describe color without instrumentation. These techniques emphasize evaluating soil samples in their natural state, breaking them to expose fresh surfaces, and comparing them to Munsell Soil Color Charts under natural daylight to approximate hue, value, and chroma.^[17] Field protocols recommend assessing color horizon by horizon during soil profile descriptions, noting depth, thickness, and boundary distinctness for each layer to capture vertical variations in color patterns.^[17] Moisture status is a critical factor, as wet or moist soils typically appear darker and more saturated than dry ones, with practitioners often moistening air-dry samples slightly to avoid glistening while evaluating the standard moist state.^[18] Qualitative descriptors provide a vocabulary for characterizing soil color patterns observed in the field, such as "mottled" for irregular color variations not linked to redox processes, "gleyed" for soils with a reduced matrix showing low chroma (≤2) and gray tones, and "variegated" for complex, intertwined color distributions like banded patterns in weathered materials.^[17] These terms help convey the spatial arrangement and intensity of colors, with moisture influencing perception—wet conditions often accentuate darker hues and reveal subtle patterns more clearly than dry states.^[18] The primary advantages of these methods lie in their simplicity, speed, and low cost, enabling immediate on-site evaluations without specialized equipment, which makes them accessible for farmers and field workers.^[17] However, they are inherently subjective, with accuracy depending on the observer's experience, lighting conditions, and familiarity with reference materials, potentially leading to inconsistencies across assessments.^[18] Training protocols outlined in resources like the USDA's Field Book for Describing and Sampling Soils emphasize hands-on practice with Munsell charts and reference samples to standardize descriptions and minimize variability.^[17] Visual methods excel at detecting redoximorphic features, such as iron oxide mottles—redder concentrations (e.g., higher chroma than the matrix) indicating oxidation—or depletions (grayer areas with chroma ≤2) signaling periodic waterlogging and reduction processes.^[17] These features are quantified qualitatively by abundance (e.g., few <2%, many ≥20%), size (e.g., fine <2 mm), and contrast (e.g., distinct), providing practical indicators of soil hydrology during field profiling.^[18]

Quantitative Systems

The Munsell Soil Color Charts offer a standardized quantitative framework for assessing soil color through three perceptual attributes: hue, value, and chroma. Hue denotes the dominant color on a cylindrical coordinate system arranged in a hue circle with 10 principal sectors (e.g., 10YR for yellow-red, 2.5Y for yellow), subdivided into 100 steps per sector for finer gradation. Value quantifies lightness, scaling from 0 (absolute black) to 10 (absolute white), while chroma measures color strength or saturation, ranging from 0 (neutral grays) to 8 or 16 depending on the hue's vividness. This system employs a notation format of hue value/chroma, such as 10YR 4/3, which describes a moderate yellow-brown soil with medium lightness and weak saturation.^[1] The 2009 revised edition expanded the charts by adding dedicated pages for 10Y and 5GY hues to better match glauconitic greens and 2.5Y/5Y for wet, low-chroma soils, enhancing accuracy for diverse mineralogical compositions. Alternative quantitative systems leverage digital colorimetry for objective analysis beyond visual matching. The CIE Lab* color space, developed by the International Commission on Illumination, provides a uniform, device-independent model where L* represents lightness (0 for black to 100 for white), a* the red-green axis (positive for red, negative for green), and b* the yellow-blue axis (positive for yellow, negative for blue). In soil science, it facilitates spectrophotometric measurements of finely ground, dry samples to correlate color parameters with properties like organic matter content, offering higher precision than Munsell for laboratory and automated applications.^[19] For broader spatial coverage, remote sensing employs RGB color models from multispectral satellite imagery (e.g., Landsat) and hyperspectral sensors (e.g., AVIRIS), which capture reflectance across visible wavelengths to derive soil color indices; these methods enable quantitative mapping of color variability at scales from field to global, integrating with machine learning for mineral identification.^[20] Standard procedures ensure reproducibility in quantitative soil color assessment. Samples are air-dried at room temperature to constant weight, sieved through a 2-mm mesh to remove coarse fragments, and gently crushed into a uniform paste or powder to eliminate texture-induced shadows. Color determination occurs by direct comparison to reference chips or digital equivalents under controlled illumination, such as diffuse daylight from a north-facing window or the D65 standard illuminant (simulating average noon sunlight with a color temperature of 6500 K), minimizing spectral distortions from artificial or directional light.^[6] Key error sources include observer variability, with inter-observer agreement typically within 1 unit for value and chroma but up to 2.5 units for hue due to perceptual differences; moisture content alters chroma and hue (e.g., increasing saturation when wet), necessitating consistent dry-state evaluation; and surface irregularities, addressed by smoothing samples. These protocols support integration of Munsell-derived color data into geographic information systems (GIS) for predictive soil mapping in initiatives like the Global Soil Partnership's post-2015 efforts to harmonize global soil databases.^[21]

Factors Determining Soil Color

Organic Matter and Humus

Organic matter in soil, derived from the decomposition of plant and animal residues, plays a pivotal role in determining soil coloration, particularly through the formation of humus, which imparts dark brown to black hues.^[1] Humus consists of complex, amorphous organic substances that absorb visible light across a broad spectrum, resulting in the characteristic darkening of soil profiles.^[22] Among these, humic acids serve as the primary pigments responsible for this effect, as they are dark-colored polymers formed during advanced stages of organic decomposition and bind strongly to soil particles.^[23] The level of organic matter content directly influences the intensity and uniformity of soil color, with higher concentrations typically producing darker tones. In topsoils with organic matter exceeding 5%, such as those in mollisols, the accumulation of humus leads to black or very dark brown colors due to the masking of underlying mineral hues.^[1] The stage of decomposition also affects shade intensity; fresh organic inputs contribute lighter browns, while fully humified materials deepen the color to near-black as polymerization progresses.^[22] For instance, Chernozem soils, characterized by high humus levels of 5-15% in the surface horizon, exhibit a jet-black appearance from this organic enrichment in grassland environments.^[24] Biological and chemical processes like melanization further enhance soil darkness, especially in regions with sufficient moisture to support microbial activity. Melanization involves the incorporation of humus into soil aggregates, forming dark coatings that reduce lightness and increase color saturation, particularly in humid to subhumid climates where decomposition rates are moderate.^[25] This process is prominent in surface horizons, where it transforms lighter parent materials into darker profiles over time through ongoing organic inputs and humification.^[26] Interactions between organic matter and mineral components promote color uniformity by creating stable complexes that distribute pigments evenly. Organic coatings, primarily from humic substances, adhere to mineral grains such as quartz and clays, effectively masking their inherent colors and imparting a consistent dark tone across the soil matrix.^[12] These coatings not only darken the soil but also protect the organic pigments from further degradation, sustaining the color in well-developed horizons.^[27]

Mineral Composition

The mineral composition of soil significantly influences its color through the presence and oxidation state of inorganic components derived from parent materials. Iron oxides are the primary contributors to red, yellow, and brown hues in well-drained soils. Hematite (Fe₂O₃) imparts vivid red colors, particularly in hot, arid, or tropical environments where large crystals form from iron-rich parent rocks. Goethite (FeOOH) produces yellow to brown tones, dominant in temperate regions, with crystal size affecting intensity—smaller crystals (around 1-2 μm) yield brighter yellows, while larger ones result in deeper browns. Lepidocrocite, another iron oxyhydroxide (FeOOH), contributes orange-red mottles in aerated soils, often appearing as distinct patterns on cracks or aggregates.^[1]^[28] Other minerals also play key roles in soil coloration. Manganese oxides, such as todorokite (MnO₂), create black specks or coatings, adding dark contrasts in various soil types. In arid regions, carbonates like calcium carbonate (CaCO₃) and dolomite (CaMg(CO₃)₂) lend white or gray tones, reflecting high accumulation in low-rainfall environments. Kaolinite, a clay mineral prevalent in tropical highly weathered soils, contributes pale white to cream colors due to its low iron content and fine particle structure.^[1]^[29] Weathering processes alter mineral oxidation states, thereby shifting soil colors; for instance, the transformation from ferrous iron (Fe²⁺, greenish or colorless) to ferric iron (Fe³⁺, red or yellow) occurs during oxidation in oxygenated conditions. Parent rock composition further dictates initial hues—basaltic rocks, rich in mafic minerals, often yield dark soils upon weathering due to high iron and magnesium content. In Ultisols, a highly weathered soil order common in humid subtropics, red colors arise from free iron oxides like hematite and goethite, with crystalline iron comprising 0.7-2.5% of the soil mass, sufficient for vivid expression when concentrations reach 2-5%. These mineral effects can interact briefly with organic coatings to modify apparent color, though inorganic dominance prevails in mineral-rich profiles.^[1]^[30]^[31]

Hydrological and Redox Influences

Hydrological conditions significantly influence soil color through redox reactions, where the availability of water and oxygen dictates the oxidation state of elements like iron. In aerobic environments with sufficient oxygen, iron oxidizes to ferric (Fe³⁺) forms, producing red or brown hues due to iron oxide accumulations.^[32] Conversely, under anaerobic conditions from water saturation, iron reduces to ferrous (Fe²⁺) forms, resulting in gray, green, or blue colors as iron becomes more soluble and leaches away.^[33] These processes alter soil color dynamically, reflecting fluctuating oxygen levels driven by hydrology.^[34] Gleying exemplifies prolonged waterlogging, where soils develop bluish-gray or greenish-gray matrices (Munsell hues like 5BG or 5GY, value ≥4, chroma ≤1) from sustained iron reduction.^[33] Poor drainage exacerbates this by limiting oxygen diffusion, promoting bacterial reduction of iron and manganese. Mottling arises from intermittent saturation, creating patchy patterns of redox depletions—gray zones (chroma ≤2, value ≥4) depleted of iron—and redox concentrations—orange-red masses or pore linings from re-oxidized iron.^[32] Seasonal wetting and drying cycles intensify these features, as reduced iron migrates during wet periods and oxidizes during dry ones, often along root channels or cracks.^[34] Such color modifications are prevalent in wetlands and floodplains, where high water tables maintain anaerobic conditions. In histosols—organic-rich wetland soils—black colors can stem from both accumulated organics and reduced sulfur compounds, where sulfate reduces to sulfide (S²⁻) under extreme anaerobiosis, producing hydrogen sulfide odors and dark staining.^[35] These environments highlight hydrology's role in fostering redoximorphic features that persist as indicators of past or present saturation.^[33] Soil color serves as a key indicator of groundwater influence and saturation levels; for instance, dominant gray colors below 10YR 5/1 (neutral gray, chroma ≤1) within 50 cm of the surface signal frequent anaerobiosis and potential hydric conditions.^[33] Redox depletions comprising 50% or more of a horizon with value ≥5 and chroma ≤2 further confirm water table proximity, aiding assessments of drainage and wetland delineation.^[34] These visual cues provide non-invasive insights into subsurface hydrology without requiring direct measurements.^[32]

Color Categories and Examples

Dark Brown and Black Soils

Dark brown and black soils are characterized by low Munsell value and chroma, typically ranging from 10YR 2/1 (black) to 3/3 (dark brown), reflecting high concentrations of organic matter that darken the surface horizons.^[1] These colors appear uniformly in the A-horizons, often with a friable texture due to the accumulation of humus and granular structure, which enhances soil tilth and root penetration.^[1] The dark hues result primarily from the decomposition of plant residues into stable organic compounds, as seen in grassland-derived profiles where organic matter inputs exceed decomposition rates.^[1] These soils form predominantly in temperate to subhumid climates with high biomass production from grasses, leading to moderate weathering and accumulation of organic-rich topsoil under Mollisols and Vertisols orders.^[36] Mollisols, the predominant order for these colors, cover about 916 million hectares globally, mainly in midlatitude prairies and steppes, such as the US Great Plains, Russian steppes, Argentine Pampas, and Northeast China.^[36] ^[37] Formation involves bioturbation by soil fauna and stable environmental conditions that preserve humus, with parent materials like loess contributing to deep, uniform profiles.^[38] Prominent examples include Chernozem, or black earth, which features a thick mollic horizon with humus content often exceeding 4% and up to 16% in cooler regions, supporting intensive cereal production in Eurasia and North America.^[38] Andisols derived from volcanic ash also exhibit dark colors, where allophane minerals bind organic matter to produce nearly black surface layers in high-silica deposits, as in Japan and Chile.^[39] These soils imply high fertility through elevated base saturation from calcium and magnesium, neutral pH, and nutrient retention, yet they are vulnerable to erosion, with historical losses up to 50% of soil organic carbon in cultivated areas like the US Great Plains.^[37] This erosion risk stems from intensive farming on flat terrains, exacerbating degradation despite their inherent productivity.^[37]

Red and Yellow Soils

Red and yellow soils derive their distinctive warm hues primarily from the presence of iron oxide minerals, with hematite (α-Fe₂O₃) imparting red colors, often corresponding to Munsell notations around 2.5YR 4/6, and goethite (α-FeOOH) producing yellow tones, typically 10YR 6/8.^[1]^[40] These colors are most prominent in the B-horizons, where clay accumulation and iron oxide coatings enhance the vibrancy through oxidation processes. Hematite forms under well-aerated, oxidizing conditions in warmer environments, reflecting red light while absorbing yellow wavelengths, whereas goethite dominates in slightly cooler or moister settings, contributing yellowish-brown shades.^[1] These soils form through intense chemical weathering in humid tropical and subtropical climates, where high rainfall and temperatures promote the leaching of soluble bases like calcium and magnesium, leaving behind concentrated iron oxides and clays.^[41] Oxisols and Ultisols represent key soil orders exhibiting these characteristics, with Oxisols featuring deep, highly weathered profiles rich in iron and aluminum oxides, and Ultisols showing similar red or yellow subsoils but with somewhat less extreme leaching. They are widely distributed in regions such as the Amazon Basin, where red-yellow latosols (Oxisols) cover vast areas under tropical rainforests, and African savannas, dominated by Ultisols in seasonally wet environments.^[42]^[43] Representative examples include terra rossa soils in the Mediterranean region, which develop as reddish, clay-rich residuals from limestone weathering under semi-arid to subhumid conditions, exhibiting good drainage and neutral pH.^[44] Laterites, often associated with these soil orders, feature indurated red layers formed by iron oxide cementation during prolonged wet-dry cycles in the tropics.^[45] Despite their structural stability, red and yellow soils generally exhibit poor fertility due to nutrient depletion and aluminum toxicity in acidic profiles (pH often below 5.5), limiting crop options but supporting specialized plantations like rubber (Hevea brasiliensis) in Southeast Asian and African lateritic areas.^[46]^[47]

Gray, Blue, and Green Soils

Gray, blue, and green soils develop primarily under prolonged water saturation, leading to anaerobic conditions that reduce iron from its oxidized ferric (Fe³⁺) form to ferrous (Fe²⁺) iron, imparting low-chroma hues such as neutral grays (e.g., 5Y 5/1) or bluish-green tones (e.g., 5BG 4/2).^[33] These colors often appear in the soil matrix due to the dominance of reduced iron compounds, while sulfides, such as iron sulfides forming pyrite (FeS₂), contribute metallic bluish or blackish sheens in highly reducing environments.^[1] Mottled patterns are common, resulting from intermittent drainage that causes localized oxidation, creating contrasting reddish or yellowish spots within the dominant gray or blue background.^[48] Such soils are characteristic of Gleysols, which are mineral soils formed in poorly drained, waterlogged settings like marshes and floodplains, and Histosols, organic-rich soils accumulating under persistent saturation in wetlands.^[49] Prolonged anaerobiosis is essential for their formation, as oxygen depletion inhibits iron oxidation and promotes the reduction processes that yield these colors; globally, they are distributed in low-lying areas such as the Mississippi River Delta's expansive wetlands and the water-saturated tundra regions of Siberia.^[50] In these environments, seasonal flooding or high water tables maintain the reducing conditions necessary for color development.^[51] Representative examples include bog soils in northern peatlands, where blue-gray hues arise alongside methane production from methanogenic bacteria thriving in the oxygen-poor zones.^[52] Sulfidic materials in these soils, initially gray or blue due to unoxidized pyrite, can rapidly turn yellow upon exposure to air through oxidation, forming jarosite or iron (hydr)oxides that alter the color spectrum.^[53] These soils uniquely preserve high levels of organic matter because anaerobic conditions slow microbial decomposition, though the low oxygen availability limits root aeration and restricts plant growth to adapted species.^[50] Their diagnostic colors serve as key indicators of hydric conditions, signaling wetland presence and potential for waterlogging in soil surveys.^[33]

White Soils

White soils exhibit high value and low chroma in the Munsell color system, often appearing as pale white-gray tones such as 10YR 8/2, resulting from the dominance of light-colored minerals like silica, carbonates, or accumulated salts that reflect most visible light while lacking strong pigmentation.^[33]^[54] These colors arise in environments with minimal organic matter and iron oxides, which would otherwise impart darker or reddish hues, leading to a bleached or powdery appearance on dry soil surfaces.^[55] White soils primarily form in arid and semi-arid regions as Aridisols, where low precipitation limits chemical weathering and organic input, preserving light parent materials like quartz-rich sands or evaporites.^[56] They are widespread in desert landscapes, including the Sahara in Africa and the Australian outback, where eolian deposition and minimal leaching maintain their pale tones over time.^[57] These soils develop under an aridic moisture regime, with subsurface accumulations of soluble salts and carbonates enhancing their whiteness without significant horizon differentiation.^[58] Representative examples include calcic horizons in desert soils, where lime (calcium carbonate) coatings on aggregates produce a white, indurated layer, as seen in southwestern U.S. Aridisols.^[59] Sodic soils gain their white coloration from sodium salt efflorescences, forming crusts on the surface in areas with high evaporation rates, such as parts of the Great Plains.^[60] Volcanic pumice soils, derived from rhyolitic ejecta, display a porous, white matrix due to the glassy, silica-rich composition, occurring in regions like the Cascade Range.^[61] These soils often have an alkaline pH exceeding 8.0 from carbonate and salt accumulations, which restricts root growth and nutrient uptake in many plants by inducing micronutrient deficiencies.^[62] High salinity levels pose challenges for irrigation, as added water can exacerbate salt buildup, leading to reduced crop yields unless managed with leaching and amendments like gypsum.^[63]

Applications in Soil Science

Role in Soil Classification

Soil color plays a pivotal role in the United States Department of Agriculture's (USDA) Soil Taxonomy system, where it is used to define diagnostic surface and subsurface horizons that form the basis for classifying soils into orders, suborders, and lower categories. For instance, the mollic epipedon, a dark, organic-rich surface horizon characteristic of Mollisols, requires a moist soil value of 3 or less and chroma of 3 or less (using Munsell notation), with a minimum thickness of 10 cm in finer textures or 18 cm otherwise, alongside organic carbon content of at least 0.6% and base saturation of 50% or more.^[64] In contrast, the ochric epipedon, found in many non-Mollisols like Entisols and Inceptisols, is identified by a moist chroma greater than 3 or value greater than 3, indicating insufficient dark coloration or organic matter to qualify as mollic.^[64] Subsurface horizons such as the argillic, which denotes clay illuviation and is key to orders like Alfisols and Ultisols, often incorporate color criteria for subgroups; rhodic subgroups require a hue of 2.5YR or redder, moist value of 5 or less, and chroma of 4 or more in at least 50% of the horizon.^[64] Redoximorphic colors, reflecting water saturation and reduction, are integral to identifying aquic conditions across multiple orders, with the 2022 edition of Keys to Soil Taxonomy specifying chroma of 2 or less (moist) in the matrix or for depletions, along with prominent redox concentrations, within 50 cm of the surface, often verified by the alpha,alpha-dipyridyl test for ferrous iron.^[64] These color thresholds guide keying decisions; for example, a red argillic horizon with hue 7.5YR or redder and chroma of 5 or more may indicate kandic properties (low-activity clay in subsurface horizons), as seen in Kandiudults, distinguishing them from standard argillic-based taxa.^[64] Color matching is performed in the field using Munsell Soil Color Charts on moist, crushed samples, ensuring consistent application in taxonomic keys. Internationally, the World Reference Base for Soil Resources (WRB), developed under the Food and Agriculture Organization (FAO) and International Union of Soil Sciences, integrates soil color through qualifiers that modify reference soil groups, emphasizing Munsell-based criteria for horizons and properties. The Rubi- qualifier, denoting reddish colors from iron oxide accumulation, applies to layers with hue redder than 7.5YR and moist chroma greater than 4.^[65] Similarly, rhodic qualifiers require hue of 2.5YR or redder, moist chroma of 5 or more, and value of 5 or less in a significant portion of the profile, as in Nitisols and Luvisols, while chromic qualifiers specify hue redder than 7.5YR with moist chroma greater than 4.^[65] For redox influences, gleyic properties—key to Gleysols and Stagnosols—involve reductimorphic colors (e.g., chroma 2 or less in hues like 10Y or G) in at least 50% of the exposed area within 25 cm-thick layers starting no deeper than 75 cm.^[65] In WRB procedures, color assessment on moist and dry samples guides horizon identification, such as the umbric horizon (value 4 or less moist, chroma 3 or less) versus mollic (similar but with higher base saturation), and supports kandic properties through red argillic-like horizons indicating low-activity clays without strict color thresholds but often featuring hues redder than 7.5YR.^[65] The FAO's legacy systems, including the 1974 Soil Map of the World, used color to name mapping units (e.g., red for ferralic soils), influencing legend creation at scales like 1:5,000,000, though WRB has superseded this for global standardization.^[66]

Indicators of Soil Properties and Health

Soil color provides a non-invasive proxy for assessing key soil properties, including organic matter content, drainage status, nutrient availability, and overall health. Dark brown or black hues in the topsoil are strongly correlated with high organic matter levels, often exceeding 4%, which enhances nutrient retention and fertility through increased cation exchange capacity (CEC).^[67] These colors arise from humus accumulation, signaling robust microbial activity and potential for higher crop yields.^[1] In contrast, red soils typically reflect well-aerated, oxidized environments due to iron oxides, indicating good drainage but lower organic matter and thus reduced inherent fertility.^[67] Gray tones, often from reduced ferrous iron, denote waterlogged, anaerobic conditions with poor drainage, limiting oxygen availability and organic decomposition.^[67] These correlations allow field practitioners to infer redox potentials briefly, as detailed in hydrological influences, without extensive lab testing.^[1] Specific color patterns serve as diagnostic health indicators for nutrient imbalances and stress factors. Yellow mottling in subsoils suggests periodic saturation that mobilizes iron oxides, creating conditions for iron deficiency in plants due to reduced availability in alkaline or wet soils.^[67] Black streaks or concretions often result from manganese oxide deposits, highlighting risks of manganese toxicity in poorly drained, acidic environments where the element becomes more soluble.^[1] Contamination events can induce abrupt color shifts; for instance, hexavalent chromium pollution yields yellow to orange staining from chromate complexes, alerting to heavy metal risks. Assessment methods leverage soil color for rapid fertility evaluations and degradation monitoring. A thick dark topsoil layer, typically greater than 20 cm, serves as a visual index for elevated CEC (often 20–30 meq/100 g or higher) and organic content, guiding soil quality benchmarks.^[68]^[69] Progressive lightening of soil color, from dark to pale brown, indicates erosion-induced loss of topsoil organics, signaling degradation and reduced productivity.^[70] In practical contexts, these indicators inform targeted management. Organic farming systems use pale or reddish soil colors to identify low organic matter, prompting compost applications to restore dark hues and nutrient-holding capacity.^[67] The U.S. Environmental Protection Agency incorporates soil color observations in Superfund site inspections to delineate contamination plumes and evaluate remediation needs, such as at chromate-impacted areas.^[71]

Practical Uses in Agriculture and Environmental Assessment

In agriculture, soil color serves as a practical indicator for site selection, particularly in assessing drainage suitability for crops. Gray or mottled colors in subsoils often signal prolonged saturation and poor drainage, prompting farmers to avoid such sites for crops requiring well-aerated conditions, such as root vegetables or grains, to prevent root rot and yield losses.^[72] Conversely, darker brown hues indicate higher organic matter content and better moisture retention, making these soils preferable for water-intensive crops like rice or alfalfa.^[73] This visual assessment allows rapid field decisions without extensive testing, enhancing efficiency in land preparation.^[12] Soil color also informs fertilizer and amendment strategies, especially in precision agriculture where darker tones correlate with elevated organic matter, reducing the need for nitrogen supplements. By mapping color variations across fields, growers adjust application rates to match soil fertility gradients, minimizing over-fertilization and nutrient runoff. For instance, statistical models linking Munsell color values to soil attributes have enabled variable-rate fertilizer prescriptions, optimizing inputs based on organic carbon estimates derived from hue and chroma.^[74] In variable-rate systems, soil color and texture data integrate with GPS-guided equipment to tailor seeding and liming.^[75] In environmental assessment, remote sensing leverages satellite imagery to map soil color for erosion risk evaluation. Landsat sensors capture visible bands to quantify color changes over time, identifying exposed bare soils prone to erosion through spectral indices that detect shifts in redness or brightness indicative of sediment loss.^[16] This approach has been applied since the 1990s to monitor degradation in arid regions, where lighter, desaturated colors signal topsoil removal and guide conservation priorities.^[76] For wetland delineation, soil color criteria are central to field protocols, with low-chroma grays (e.g., 2 or less) or prominent mottles in the upper 30 cm confirming hydric conditions under U.S. Army Corps of Engineers guidelines.^[33] Tools like the Munsell Color Chart facilitate on-site verification, ensuring accurate boundary mapping for regulatory compliance and habitat protection.^[77] Case studies highlight these applications in practice. Restoration projects in global drylands use amendments to improve soil health, with a 2023 meta-analysis showing substantial gains such as 72% increase in soil organic carbon and 76% in nitrogen where restoration practices were applied.^[78] Mobile tools like the SoilWeb app further support these efforts by querying USDA soil surveys to display profile sketches with color notations, enabling users to assess site suitability and plan interventions based on local data.^[79]

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Calibration methods for measuring the color of moist soils with ...
As Vasily Dokuchaev wrote in 1883, some researchers judged Chernozems (Mollisols) only by the soil color under field condition. However, Dokuchaev (1883) ...<|separator|>
[10]
Soil Color - an overview | ScienceDirect Topics
The Munsell System of Color Notation, a color order system invented by Albert H. Munsell in 1905, is the most-used soil color system prior to the development ...
[11]
Soil Color Standards and Terms for Field Use—History of Their ...
Jan 1, 1993 · Soil colors attracted some attention in Russia but little in the USA prior to the present century. Two of the first three soil survey ...Missing: ancient modern
[12]
[PDF] Soil Taxonomy Soil Properties - Transportation Research Board
Soil Taxonomy is a comprehensive soil classification developed from 1951 to 1974, considering all soil properties, and is a six-category system.
[13]
(PDF) Classification Systems: FAO - ResearchGate
Jun 26, 2021 · The development and evolution of the FAO soil classification system since its origin in 1960 until its replacement by the World Reference Base for Soil ...
[14]
Remote sensing of the Earth's soil color in space and time
Dec 15, 2023 · We developed an innovative system that leverages 35 years of satellite imagery in conjunction with in-situ soil spectral measurements.
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[PDF] Field Book for Describing and Sampling Soils, Version 4.0
The first USDA guide for soil horizon identification and description was released in 1937 (Bureau of Chemistry and Soils, 1937). Dr. Roy Simonson and others ...
[16]
[PDF] Soil Survey Field and Laboratory Methods Manual
Soil Survey Field and Laboratory Methods. Manual. Soil Survey Investigations Report No. 51, Version 2.0. R. Burt and Soil. Survey Staff (ed.). U.S. Department ...
[17]
Full article: Soil color analysis for statistically estimating total carbon ...
Jun 17, 2014 · The L* value ranges from 0 (black) to 100 (white), whereas positive a* and b* values indicate the degree of redness and yellowness, respectively ...
[18]
Soil Color and Mineralogy Mapping Using Proximal and Remote ...
Hyperspectral sensors probably are the future of remote sensing. New covariates for soil predictions may be produced by mining data from a single sensor or ...
[19]
[PDF] SOIL ORGANIC CARBON MAPPING Cookbook
The Global Soil Partnership (GSP) aims to promote sustainable soil management at all levels and in all land uses through normative tools that rely on evidence-.
[20]
The influence of organic matter on soil color using the regression ...
In general, darker soils are more humus than light ones. Organic matter influences the soil color, due to the formation of organic-mineral complexes. The ...
[21]
The Value of Humic Substances in the Carbon Lifecycle of Crops
These humic substances, which contribute to the brown or black color of surface soils, can be divided into three major categories: humin, humic acids (HAs), and ...
[22]
Chronic drought decreased organic carbon content in topsoil greater ...
In addition, our mesic grassland sites are characterized by chernozem soil with a high percentage of humus (5–15 %), available calcium ions bound to soil ...
[23]
Processes involved in soil formation | PPTX - Slideshare
Melanization • Melanization isthe development of dark, humus-rich coatings on ped faces and mineral grains, rendering the horizon a dark brown or black. • This ...
[24]
Global black soil distribution - Hep Journals
The distinct black color of these soils is due to the accumulation of organic matter from the decaying roots of grasses, a process termed melanization.<|separator|>
[25]
Soil color – Introduction to Soil Science
Accumulation of organic material turns the soil darker, as is commonly found in the surface layers. This may be several feet in some prairie-derived soils as ...
[26]
Relations Between Iron Oxides, Soil Color, and Soil Formation
Jan 1, 1993 · 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 ...
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Kaolinite - an overview | ScienceDirect Topics
Pure kaolinite is white while some isomorphic substitutions, especially in the Al3 + sites, attribute grayish or yellowish color. Contaminants such as iron and ...Missing: pale | Show results with:pale
[28]
How do parent materials influence soil characteristics? - TutorChase
Soils derived from basalt are often dark-coloured due to the high iron content, while those from sandstone or granite are usually light-coloured.
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(PDF) Properties of iron oxides in red Oxisols and red Ultisols as ...
Oct 15, 2020 · Hematite is the dominant iron oxide in all soils developed on limestone irrespective of rainfall, whereas for other parent materials the ...
[30]
Redoximorphic Features for Identifying Aquic Conditions
Aug 12, 2015 · Redoximorphic features include Fe nodules and mottles that form in the soil by the reduction, translocation, and oxidation of Fe or Mn oxides.
[31]
[PDF] Field Indicators of Hydric Soils in the United States
Ferrous iron in the soil can oxidize rapidly, resulting in the development of colors with higher chroma or redder hue. Soils that are saturated at the time of ...
[32]
Morphological Features of Soil Wetness | NC State Extension
Redox depletions show where reduction has occurred, while redox concentrations show where oxidation occurs.Missing: processes | Show results with:processes
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None
### Summary of Hydric Soils Content
[34]
[PDF] Overview of Mollisols in the world: Distribution, land use and ...
Mollisols, also known as Black or Prairie Soils, cover 916 million ha, 7% of the world's ice-free land, and are found in North America, Eurasia, and South ...
[35]
[PDF] Global status of black soils - FAO Knowledge Repository
Black soils have a growing awareness of importance, but face threats like soil carbon loss, erosion, and nutrient imbalance. They offer multiple benefits.
[36]
Chernozem - an overview | ScienceDirect Topics
Chernozem is a highly fertile soil from prairie-steppe grasslands, with high organic matter, bioturbation, and a silty to loamy texture.
[37]
Volcanic Ash Soils - an overview | ScienceDirect Topics
In Andisols developed from low-silica, high-iron and -magnesium ash, the iron oxide masks the black color of organic matter to produce a dark-brown soil (Figure ...
[38]
Significance of color | Determining natural soil drainage
Iron oxide minerals color the soil brown or red. Goethite (FeOOH) is a common iron oxide mineral that colors the soil brown or yellow (hues of 7.5YR to 5Y).
[39]
5.10 - Oxisols | Soil Genesis and Development, Lesson 5
They are composed of the most highly weathered tropical and subtropical soils, and are formed in hot, humid climates that receive a lot of rainfall. Oxisols are ...Missing: yellow | Show results with:yellow
[40]
[PDF] Soil Indices for Comparative Analysis of Agrarian Systems
Bright red and yellow soils in the tropics may suggest high levels of iron ... savannas by ultisols, and tropical moist forests by oxisols. The areal ...
[41]
[PDF] LAURA ANNE GERMAN The Dynamics of Terra Preta
... soil horizon that is acidic, nutrient-poor and high in exchangeable aluminum. The predominant soils on the Amazonian terra firme are Red-Yellow Latosols (by.
[42]
[PDF] the mystery of - Western Kentucky University
The mystery is how terra rossa, a red soil, forms. It's not from simple dissolution or glaciers, and the source of its iron is unknown.
[43]
[PDF] Genesis and Hardening of Laterite in Soils - AgEcon Search
It is nearly void of bases and primary sili cates, but it may contain large amounts of quu,rtz and kaolinite. It is either hard or capable of hardening on ...
[44]
Oklahoma Soil Fertility Handbook
... toxic elements and nutrient elements. Toxic elements like aluminum (Al) and manganese (Mn) are the major causes for crop failure in acid soils. These ...
[45]
[PDF] Environmental Consequences of Rubber Plantations in Kerala
Due to intense chemical weathering soils in Kerala are deep to very deep. Lateritic soil is dominant upto 300m. From 300 to 600m altitudinal rise is abrupt ...
[46]
Wetland Soil - an overview | ScienceDirect Topics
Sometimes, soil takes on hues of green or blue that indicates complete reduction of Fe3+ in the soil matrix. In wetlands that dry down periodically, reduced Fe ...
[47]
Gleysol - an overview | ScienceDirect Topics
Waterlogging causes the development of gleyic features, which include soil colors reflecting reducing conditions such as a bluish to greenish soil matrix (Fig.Missing: blue | Show results with:blue
[48]
[PDF] Illustrated Guide to Soil Taxonomy
Soil moisture state can also influence the hue, value, or chroma observed for the soil (value is particularly susceptible to change in soil moisture state) ...
[49]
Wetland characterization and classification for sustainable ...
Gleysols are the most typical mineral wetland soils. Their formation is conditioned by waterlogging at shallow depth for some or all of the year.Missing: gray | Show results with:gray
[50]
High rates of anaerobic methane oxidation in freshwater wetlands ...
Jun 30, 2015 · The anaerobic oxidation of methane serves as a major methane sink in marine environments and effectively limits fluxes of this greenhouse gas ...
[51]
Soil-color changes by sulfuricization induced from a pyritic surface ...
Virtually achromatic colors in the preactive stage because of materials enriched in pyrite (sulfidic materials) turned yellowish during the active oxidation ...
[52]
The Art of Soil Color
Mar 7, 2012 · If there is a lot of oxidized iron, those colors will be very bright. If there is very little iron, or none at all, a soil could appear grey.Missing: ferrous | Show results with:ferrous
[53]
5.5 - Aridisols | Soil Genesis and Development, Lesson 5
Aridisols are commonly light in color, and low in organic matter content. Lime and salt accumulations are common in the subsurface horizons.Missing: white | Show results with:white
[54]
Aridisols - Natural Resources Conservation Service
Aridisols are soils that are too dry for the growth of mesophytic plants. They are common in the deserts of the world. The central concept of Aridisols is that ...Missing: white | Show results with:white
[55]
The contribution of aeolian material to an Aridisol from southern ...
For example, given that the Umm Sham formation is white and dominated by quartz grains, the blackish colour and the dominance of Fe and Mn in the desert ...
[56]
Aridisols | University of Idaho
Aridisols are CaCO3-containing soils of arid regions that exhibit subsurface horizon development. They are characterized by being dry most of the year and ...Missing: white color formation
[57]
[PDF] Calcic soils and calcretes in the southwestern United States
The major process in the formation of pedogenic calcrete and calic soils is the leaching of calcium carbonate from upper soil horizons by downward percolating ...
[58]
[PDF] TECHNICAL NOTE - Natural Resources Conservation Service - USDA
Saline soils are often referred to as “white alkali” because of the white salt crust that forms on the soil surface. Saline soils are characterized by the ...
[59]
Horticultural and Agricultural Pumice
Our white pumice can also be color-dyed if necessary. click to view photos. perennial crops (like this vineyard) grown in pumice-amended soil seeds coated ...
[60]
Managing Salt-affected Soils for Crop Production
Saline-sodic soils must be treated for sodicity first. These soils first require calcium additions to correct sodium, followed by leaching to remove salts. When ...
[61]
Reclaiming Slick-Spots and Salty Soils | Oklahoma State University
The salts are light colored, and when accumulation has continued for several weeks, they form a very thin white film on the soil surface. During hot, dry ...
[62]
[PDF] Keys to Soil Taxonomy 12th edition
Page 1. United States. Department of Agriculture. Keys to. Soil Taxonomy. Twelfth Edition, 2014 ... color, national origin, age, disability, sex, gender ...
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[PDF] IUSS Working Group WRB. 2022. World Reference Base for Soil
Dec 18, 2022 · International soil classification system for naming soils and creating legends for soil maps. 4th edition. International Union of Soil Sciences.
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[PDF] World reference base for soil resources 2014
The WRB is a soil classification system for naming soils and creating soil map legends. It is hoped that this publication will contribute to the understanding ...
[65]
FAO/UNESCO Soil Map of the World | FAO SOILS PORTAL
FAO and Unesco decided in 1961 to prepare a Soil Map of the World at 1:5 000 000 scale. The project was completed over a span of twenty years.FAO Soil Legacy Maps · Harmonized world soil · Global Soil Organic Carbon...
[66]
Soils, Plant Nutrition and Nutrient Management - MU Extension
Color is an indicator of organic matter content, drainage and aeration. Black High in organic matter (4 percent or more). Brown Good organic matter content and ...Missing: factors | Show results with:factors
[67]
Fundamentals of Soil Cation Exchange Capacity (CEC)
Since a soil's CEC comes from the clay and organic matter present, it can be estimated from soil texture and color. Table 1 lists some soil groups based on ...Missing: indices topsoil
[68]
Soil organic matter in cropping systems
Histosols make up only about 1% of soils worldwide and most soils have a much lower content of soil organic matter. Soils of the Upper Midwest. Soils in the ...
[69]
[PDF] Identification of Severely Eroded Soils from Remote Sensing Data ...
several categories of eroded soils. Considering the gradual change of color according to the level of soil degradation, it should be theoretically possible ...
[70]
Guide To Soil And Description For Hazardous Waste Site ...
This guide is intended to assist field personnel who must identify, describe, and interpret the site and soil characteristics of hazardous waste sites
[71]
Soil Judging in Virginia | VCE Publications
Dec 5, 2022 · Thus, the occurrence of gray subsoil mottles or dominant color indicates that the soil is saturated for a significant period of time (or has a ...<|separator|>
[72]
Decoding Soil Color: Origins, Influences, and Methods of Analysis
Soil color serves as a crucial tool for reconstructing ancient landscapes and detecting soils modified by historical agricultural practices [1].
[73]
Statistical relationships between soil colour and soil attributes in ...
Soil colour has become one of the most innovative indicators used to adjust amendment and fertilizer rates in precision agriculture.
[74]
[PDF] Precision Farming Tools: Variable-Rate Application - VTechWorks
Crop producers and consul- tants have crafted strategies for varying inputs based on (1) soil type, (2) soil color and texture, (3) topog- raphy (high ...<|separator|>
[75]
Remote Sensing of Soil Degradation: Introduction - Lobell - ACSESS
Jan 1, 2010 · In the 21st century, mapping and monitoring the occurrence of soil degradation will be an important component of successful land management.
[76]
Field Notes: Decoding Soil with the Munsell Color Chart
Mar 26, 2025 · Subtle variations in soil color can indicate changes in moisture levels, organic content, or chemical composition—factors that influence ...
[77]
a case study of soil spectroscopy | Precision Agriculture
Mar 12, 2022 · This study shows that substantial financial gains can be obtained from sensor-based precise application of P and K fertiliser, with savings of up to £121 ha −1 ...
[78]
Soil restoration increases soil health across global drylands: A meta ...
Jun 22, 2023 · Darker colours represent conditions where soil restoration has a greater effect size, and lighter colours represent conditions where soil ...
[79]
A Smartphone App Provides New Way to Access Soil Survey ...
Feb 3, 2012 · SoilWeb can retrieve a graphic summary of soil types in response to a user inquiry in the form of soil profile sketches. Each profile sketch ...Missing: queries | Show results with:queries