Coloration of Soils

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 (e.g., carbonates, sulfates, and chlorides) may also 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 instance, light gray or nearly white colors are sometimes inherited from parent material, such as marl or quartz. Parent material, such as basalt, can imprint a black color to the subsoil horizons. Some soil colors associated with soil attributes are listed in Table 14.7. 

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 

Tallle 14.7. Soil colors related to soil attributes 

Surface horizon Accumulation of organic matter, humus, low temperature, high annual precipitation amounts, high moisture, and/or litter from coniferous trees Accumulation of manganese. Parent material (e.g., basalt) 

In environments where precipitation is greater than evapotranspiration there is leaching of sequiozides, carbonates, and silicate clays. The eluviated horizon consists mainly of silica. 

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The color of soil gives an indication of its oxidation-reduction conditions and the amount of OM present. Well-aerated soils will be under oxidizing conditions iron will be in the Fe3+ state, less soluble and thus less available for chemical reaction. Under water-saturated conditions, soil will be under reducing conditions as indicated by increased yellow colorings, gleying, and mottling. Iron will be in the Fe2+ state, which is more soluble and thus more available for chemical reaction. Under these conditions, reduced species such as methane (CH4), hydrogen, (H2), and sulfides will be found. 

The titrimetric determination of soil constituents is most commonly applied to a limited number of soil analyses, namely, organic carbon, nitrogen compounds, carbonates, and chlorides. Determination of acid content by titration is generally not done because the titration curves are not amenable to typical titration analysis. Because of the color of soil and the fact that it is a suspension when stirred, it is often necessary to remove the constituent of interest before titration. In other cases, it is possible to do a direct titration using an appropriate indicator. However, even in these cases, detection of the end point is difficult. 

Being widespread in the lithosphere and insoluble in the usual range of soil pH, oxides and hydroxides of Al, Fe and Mn are common in soil clays. Red or yellow coloration of soils is apparent at Fe oxide contents of only 0.1 % or less, especially if the Fe is amorphous and coats other minerals. The most visible change occurring when soils are submerged is the conversion of the red and yellow compounds of Fe(III) to the bluish-grey compounds of Fe(II). 

Stripped Matrix. For use in all LRRs except V, W, X, and Y. A layer starting within 15 cm (6 in.) of the soil surface in which iron/manganese oxides or organic matter have been stripped from the matrix, exposing the primary base color of soil materials. The stripped areas and translocated oxides or organic matter form a faint diffnse splotchy pattern of two or more colors. The stripped zones are 10% or more of the volnme they are rounded and approximately 1-3 cm (0.5-1 in.) in diameter (Figure 3.25). 

In kaolin (clay) processing, sulfur dioxide reduces colored impurities, eg, iron compounds. In the bromine industry, sulfur dioxide is used as an antioxidant in spent brine to be reinjected underground. In agriculture, especially in California, sulfur dioxide is used to increase water penetration and the avadabiHty of soil nutrients by virtue of its abiHty to acidulate saline—alkaH soils (327). It is also usefiil for cleaning ferric and manganese oxide deposits from tile drains (328). 

The intensity and color of the poppies in Monet s painting The Poppyfield, near Giverny are influenced by the acidity or basicity of the soil. Of course artistic license could also have something to do with the intensity and color of the poppies in this painting. 

You may be surprised to learn that many metal cations act as weak acids in water solution. A 0.10 M solution of A12(S04)3 has a pH close to 3 you can change the color of hydrangeas from red to blue by adding aluminum salts to soil At first glance it is not at all obvious how a cation such as Al3+ can make a water solution addic. However, the aluminum cation in water solution is really a hydrated species, A1(H20)63+, in which six water molecules are bonded to the central Al3+ ion. This spedes can transfer a proton to a solvent water molecule to form an H30+ ion ... 

There are many naturally occurring indicators. For instance, a single compound is responsible for the colors of red poppies and blue cornflowers the pH of the sap is different in the two plants. The color of hydrangeas also depends on the acidity of their sap and can be controlled by modifying the acidity of the soil (Fig. 11.11). 

FIGURE 11.11 The color of these hydrangeas depends on the acidity of the soil in which they are growing acid soil results in blue flowers, alkaline soil results in pink flowers. 

The colors of flowering plants such as hydrangeas are highly sensitive to soil acidity. At pH > 6.5, these showy flowers are deep pink, but at pH < 5, the blossoms are vivid blue. The chemistry of these changes involves complexation of aluminum by pigments that have acidic groups, as the structures show. 

Haines TA, Komov VT, Matey VE, Jagoe CH. 1995. Perch mercury content is related to acidity and color of 26 Russian lakes. Water Air Soil Pollut 85 823-828. 

At the end of this period the solution was removed from the condenser while still hot and titrated immediately with 0.002500 N sodium thiosulfate before any appreciable oxygen could be absorbed and oxidize iodide ion to triiodide ion. The disappearance of the yellow color of triiodide ion against a white background was used for the end point. These solutions usually had a slight brown tint at the end point, which was assumed to be organic matter distilled over from the soil. Accordingly, the blank was usually titrated first and its final color was used as a standard end point color for the other three solutions run with it. 

Many gardening books will state that the acidity of the soil affects the color of certain varieties of hydrangeas. The soil pH affects the availability of aluminum in the soil and thereby indirectly affects the flower color. A low pH or acidic conditions will yield blue blooms pink blossoms will be favored by a higher pH or alkaline conditions. A purple color is the result of a more moderate pH level. Potting soils with a high level of peat moss will produce blue hydrangeas. Areas... 

Figure 9.4.1 The blue and pink colors of hydrangeas that result from growing in soil with different degrees of acidity. From Masterton and Hurley, Chemistry Principles and Reactions, 4th edition. Orlando Harcourt, 2001. Photo courtesy of Charles D. Winters. 

Fig. 3. Effect of Pb (a) and Cd (b) on substrate utilization efficiency of soil microbial communities as indicated by average well color development (AWCD) at 590 nm (from Akmal et al. 2005a).

El-Keblawy A, Ksiksi T, Al-Ammadi F (2006) Effect of polyethylene colors and thickness on the efficiency of soil solarization under the environment of UAE. In Mohamed AMO (ed) Arid Land Hydrogeology in search of a solution to a threatened resource. Taylor and Francis, London, UK, pp 177-184... 

Carbon substrate utilization (BIOLOG) Incubation of soil with substrates color development Indicates functional microbial diversity determined in nonstandard laboratory with specialized equipment produces large quantities of data complex interpretation Dick et al. (1996)... 

As a soil develops, OM decomposes to produce humus, which is black. Additionally, release of iron from minerals by weathering yields various reds and yellows. Both mechanisms yield soil coloring agents. Under oxidizing conditions, where soil is not saturated with water, the iron will be oxidized and thus in the ferric state [Fe(III)]. When the iron and OM are deposited on the surfaces of sand, silt, clay, and peds, they develop a coat that gives them a surface color. However, soil color is not only a surface characteristic but extends through the soil matrix. Under oxidizing conditions, soil has a reddish color. The chroma of this color depends to some extent on the amount of and the particular iron oxide present. 

Iron in the ferrous state is more soluble than iron in the ferric state. Indeed, in the ferric state, it is insoluble under most soil conditions. Under reducing conditions, ferrous iron may be leached out of soil, leaving it gray in color. This is the origin of the term gleying. 

Soils develop by the action of the soil forming factors on soil parent materials including material transported by different agents. The result of these soil forming factors is the formation of soil horizons, different colors, and peds. Each of these factors has a pronounced effect on a soil s chemistry. Knowledge of the soil type and profile description can provide the soil chemist, analyst, or researcher with valuable information about the characteristics of soil relevant to the development of extraction, analytical, and instrumental analytical procedures. It also is the place to start when investigating the failure of a procedure. 

Soil and soil suspensions are colored and hard to see through. Thus, it is hard to directly titrate them using colored indicators. There are typically only two cases where direct titrations of soil are carried out. The first is to determine the amount of amendment needed to bring the soil to a desired pH. The second is in the determination of soil organic matter where organic matter is oxidized with chromate and the unreacted chromate is titrated (actually called a back titration) to determine, by subtraction, the amount of dichromate reduced and thus the amount of organic matter present. 

Titration of soil pH is an old method that is not widely used today. Basically, an acid soil suspension is prepared and titrated with a standardized base, often sodium hydroxide, although various basic calcium compounds such as calcium oxide (CaO) and calcium hydroxide [Ca(OH)2] can also be used. Because of the dark color of many soils, they are often titrated using a pH meter as the indicator of the end point. A setup for the titration of soil is shown in Figure 10.1. Titration is slow in that it takes some time after the addition of titrant for some semblance of equilibrium to be reached. Once this happens, a reading can be made or simply another addition of titrant made. 

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