CHAPTER 2

Soil Basics Part I

Large Features

Soil is vastly more complex than simply ground-up rock. It contains solid inorganic and organic components in various stages of decomposition and disintegration, an aqueous solution of elements, inorganic and organic ions and molecules, and a gaseous phase containing nitrogen, oxygen, carbon dioxide, water, argon, methane, and other gases. In addition, it contains a large and varied population of macro-, meso-, and microscale animals, plants, and microorganisms. If any of these components is missing, it is not soil!

The solid portion of soil is composed of inorganic sand, silt, clay, and organic matter (OM), which interact to produce the large soil features1 (i.e., peds, profiles, pedons, and landscapes). These features, not considering rock, are discussed in this chapter. In Chapter 3, components smaller than rock, which soil scientists define as those inorganic particles smaller than 2.00 mm in diameter, are discussed. Geologic features and gravel, stones and rock, and other substances are not discussed.

Large soil components consisting of sand, silt, clay, and OM are peds, profiles, pedons, and landscapes. Peds are formed by the aggregation of sand, silt, and clay particles to form larger (secondary) soil structures that result from the action of the soil forming factors (see Figure 2.2, Figure 2.3, Figure 2.4, Figure 2.5, Figure 2.6, Figure 2.7, and Figure 2.8). Profiles develop in the loose material on the earth's surface, the regolith, and are composed of horizons of varying texture, structure, color, bulk density, and other properties. The relationship between the soil and the regolith is illustrated in Figure 2.1. Typically, horizons are, as the name implies, horizontal and are of varying thickness. A pedon is the smallest unit that can be considered “a soil” and consists of all horizons from the soil surface to the underlying geologic strata. An area consisting of similar pedons is called a polypedon.

Figure 2.1. The relationship between soil horizons, the regolith, and the underlying rock.

Figure 2.2. An example of an Alfisol; this is the Seitz soil series, the state soil of Colorado [2].

Figure 2.3. An example of a Mollisol; this is the Drummer soil series, which is the state soil of Illinois [2].

Figure 2.4. An example of an Ultisol; this is the Bama Series, which is the state soil of Alabama [2].

Figure 2.5. An example of a Spodosol; the Kalkaska soil series, the state soil of Michigan (single-grain structure in horizon C means that all the sand particles act independently of each other) [2].

Figure 2.6. An example of an Aridisol; the Casa Grande soil series, the state soil of Arizona.

Figure 2.7. An example of a Andisol; this is the Bonner soil series, found in northern Idaho, eastern Washington, and western Montana [3].

Figure 2.8. An example of a Vertisol; this is the Lualualei series, which is found in Hawaii [2].

Soil features, texture, peds, profiles, and other properties and materials go together in different ways to form different soils. Soil scientists in the United States classify soils into 12 orders, some of which are illustrated in Figure 2.2, Figure 2.3, Figure 2.4, Figure 2.5, Figure 2.6, Figure 2.7, and Figure 2.8. The orders are differentiated by their characteristics, which are a result of the action of the soil forming factors: climate, parent material, topography, biota, and time, which all interact during soil formation. Climate, moisture, and temperature determine the biota, as well as a soil's age, and whether it can develop in a certain locality. The soil parent material will be acted upon by other factors but will in turn provide the minerals needed for plant growth and other biological activity.

Consideration of the larger components of soil might seem like a strange place to start a discussion of soil chemistry, analysis, and instrumental methods. However, these larger structures can and do affect the chemistry of a soil. For instance, in many cases, the larger features control the movement of air and water in soil. Sandy textured soils will have higher infiltration and percolation rates than clayey soils. The same can be said for soils with good, strong structure, versus soils with poor or weak structure. As water moves into soil, it displaces air and, as it moves out, air replaces it. Thus, soil with poor or slow infiltration and percolation will have generally lower oxygen contents. This directly affects soil aeration and thus its oxidation–reduction status.

Infiltration and percolation rates also determine which salts have been leached out of the soil. For instance, high infiltration and percolation rates leach calcium and magnesium out of soil and they become acidic. Where calcium and magnesium are not leached out, the soils are neutral or basic. Thus, the type and amount of salts present will affect a soil's pH, which will in turn affect the solubility and availability of both natural and contaminating inorganic and organic compounds.

The species of components present will also be affected by oxidation–reduction, and pH. For example, iron is primarily in the Fe3+ (oxidized) or the Fe2+ (reduced) state depending on the oxidation–reduction potential of the soil. Speciation, which depends, in part, on the oxygen status of soil, is of environmental concern because some species are more soluble, such as Fe2+, and are thus more biologically available than others. The occurrence of a specific species is related to the chemistry occurring in a soil, which is related to its features. Thus, large features must be taken into consideration when studying soil chemistry and when developing analytical and instrumental methods.

2.1 Horizonation

The most striking large feature of soil is the occurrence of distinct horizons. For the analytical chemist, three soil horizonation conditions exist: (1) high-rainfall areas that typically have tree or tall grass vegetation and extensive horizon development, (2) low-rainfall and desert areas with sparse and desert vegetation and little horizon development, and (3) areas with rainfall between these extremes will have variable vegetation and horizonation. It is not possible to draw sharp boundaries between these areas because local conditions such as rainfall frequency, time of year, and intensity of rainfall dramatically affect the climate in transition areas. Each of these situations presents unique challenges to the analytical chemist.

2.1.1 Horizon Development under Moist Conditions

Parent material salts are dissolved when water falls on soil. These salts are leached into and eventually out of the soil. Plants grow, produce biomass, exude OM (including acids), die, and thus add OM to both the soil surface and subsurface. Silica and alumina, which are relatively immobile, slowly dissolve and are eluted into the lower areas of a developing soil. OM is decomposed, mixed by organisms, and leached into the soil. As this process continues, horizons develop in the soil parent material and, eventually, a recognizable soil profile is produced (see Figure 2.1, Figure 2.2, Figure 2.3, Figure 2.4, Figure 2.5, Figure 2.6, Figure 2.7, Figure 2.8, and Figure 2.9). The depth of the soil and the number and type of horizons that develop depends on the soil forming factors, the most active of which are climate and biota, although parent material, as mentioned earlier, is also important [1,2].

Figure 2.9. Soil structure and its most common location in a soil profile. Platy structure can be found in any horizon. Subangular and angular blocky structures can be found both higher and lower in the profile than indicated.

Soil parent material is not always derived from the underlying rock. In some cases, rock is covered by other geologic materials deposited by ice (glacial till), water (alluvium), wind (loess), gravity (colluvium), or a combination of these transporting agents. A more complete list of transporting agents and the geomorphic features they form is given in Table 2.1. Once deposited, these materials become the parent material from which soil develops.

TABLE 2.1. Soil Parent Material Transporting Agents, The Name of the Material, and the Geomorphic Features They Forma