How do soils develop their distinctive characteristics? Let's turn to the processes that form soils and soil layers.
Most soils have distinctive horizontal layers that differ in physical composition, chemical composition, organic content, or structure (Figure 10.12). We call these layers soil horizons. They develop through interactions between climate, living organisms, and the land surface, over time. Horizons usually develop by either selective removal or accumulation of certain ions, colloids, and chemical compounds. This removal or accumulation is normally produced by water seeping through the soil profile from the surface to deeper layers. Horizons often have different soil textures and colors.
To simplify our discussion of soil horizons, let's look at the example of soils found in moist forest climates. A soil profile, as shown in Figure 10.13, displays the horizons on a cross section through the soil. There are two types of soil horizons: organic and mineral. Organic horizons, marked with the capital letter O, lie over mineral horizons and are formed from plant and animal matter. The upper Oi horizon contains decomposing organic matter that you can easily recognize by eye, such as leaves or twigs. The lower Oa horizon contains humus, which has broken down beyond recognition.
There are four main mineral horizons. The A horizon is rich in organic matter, washed downward from the organic horizons. Next is the E horizon. Clay particles and oxides of aluminum and iron are removed from the E horizon by downward-percolating water, leaving behind pure grains of sand or coarse silt.
The B horizon receives the clay particles, aluminum, and iron oxides, as well as organic matter washed down from the A and E horizons. It's dense and tough because its natural spaces are filled with clays and oxides. Beneath the B horizon is the C horizon. It consists of the parent mineral matter of the soil. Below this regolith lies bedrock or sediments of much older age than the soil. Soil scientists limit the term soil to the A, E, and B horizons, which plant roots can readily penetrate.
There are four classes of soil-forming processes: soil enrichment, removal, translocation, and transformation. Figure 10.14 diagrams these processes and shows how they work together on the landscape.
In soil enrichment, matter—organic or inorganic—is added to the soil. Mineral enrichment of silt by river floods or as wind-blown dust is an example. Organic enrichment occurs as water carries humus from the O horizon into the A horizon below.
In removal processes, material is removed from the soil body. This occurs when erosion carries soil particles into streams and rivers. Leaching—the loss of soil compounds and minerals by solution in water flowing to lower levels—is another important removal process.
Translocation describes the movement of materials upward or downward within the soil. Fine particles—particularly clays and colloids—are translocated downward, a process called eluviation. This leaves grains of sand or coarse silt to remain, forming the E horizon. Material brought downward from the E horizon—clay particles, humus, or sesquioxides of iron and aluminum—accumulates in the B horizon, a process called illuviation.
The soil profile shown in Figure 10.13 for a cool, humid forest climate displays the effects of both soil enrichment and translocation. The topmost layer of the soil is a thin deposit of wind-blown silt and dune sand, which has enriched the soil profile. Eluviation has removed colloids and sesquioxides from the whitened E horizon and illuviation has added them to the B horizon, which displays the orange-red colors of iron sesquioxide.
The translocation of calcium carbonate is another important process. In moist climates, a large amount of surplus soil water moves downward to the groundwater zone. This water movement leaches calcium carbonate from the entire soil in a process called decalcification. Soils that have lost most of their calcium are also usually acidic, and so they are low in bases. Adding lime or pulverized limestone will not only correct the acid condition, but will also restore the missing calcium, which is used as a plant nutrient.
In dry climates, annual precipitation is not sufficient to leach the carbonate out of the soil and into the ground water below. Instead, it is carried down to the B horizon, where it is deposited as white grains, plates, or nodules, in a process called calcification.
Upward translocation can also occur in desert climates. In some low areas, a layer of ground water lies close to the surface, producing a flat, poorly drained area. As water at or near the soil surface evaporates, ground water is drawn to replace it by capillary tension, much like a cotton wick draws oil upward in an oil lamp. This ground water is often rich in dissolved salts. When the salt-rich water evaporates, the salts are deposited and build up. This process is called salinization. Large amounts of these salts are toxic to many kinds of plants. When salinization occurs in irrigated lands in a desert climate, the soil can be ruined with little hope of revival.
The last class of soil-forming processes involves the transformation of material within the soil body. An example is the conversion of minerals from primary to secondary types, which we have already described. Another example is decomposition of organic matter to produce humus, a process termed humification. Microorganisms break down the raw organic matter, producing CO2 and water and leaving behind resistant organic compounds—humus—to decay more slowly.
SOIL TEMPERATURE AND OTHER FACTORS
Soil temperature also helps to determine the chemical development of soils and the formation of horizons. Below 10°C (50°F), biological activity is slowed, and at or below the freezing point (0°C, 32°F), biological activity stops and chemical processes affecting minerals are inactive. The root growth of most plants and germination of their seeds require soil temperatures above 5°C (41°F). Plants in the warm, wet low-latitude climates may need temperatures of at least 24°C (75°F) for their seeds to germinate.
The temperature of the uppermost soil layer and the soil surface also strongly affects the rate at which organic matter is decomposed by microorganisms. In cold climates, decomposition is slow, and so organic matter accumulates to form a thick O horizon. This material becomes humus, which is carried downward to enrich the A horizon. But in warm, moist climates of low latitudes, bacteria rapidly decompose plant material, so you won't find an O horizon layer, and the entire soil profile will contain very little organic matter.
The configuration, or shape, of the ground surface also influences soil formation. Generally speaking, soil horizons are thick on gentle slopes but thin on steep slopes. This is because the soil is more rapidly removed by erosion on the steeper slopes. In addition, slopes facing away from the Sun are sheltered from direct insolation and tend to have cooler, moister soils. Slopes facing toward the Sun are exposed to direct solar rays, raising soil temperatures and increasing evapotranspiration.
Living plants and animals, as well as their nonliving organic products, have an important effect on soil. We have already noted the role that organic matter as humus plays in soil fertility. Humus holds bases, which are needed for plant growth. It also helps bind the soil into crumbs and clumps, which allows water and air to penetrate the soil freely. Plant roots, by their growth, mix and disturb the soil and provide organic material directly to upper soil horizons.
Organisms living in the soil include many species—from bacteria to burrowing mammals (Figure 10.15). Earthworms continually rework the soil not only by burrowing, but also by passing soil through their intestinal tracts. They ingest large amounts of decaying leaf matter, carry it down from the surface,
and incorporate it into the mineral soil horizons. Many forms of insect larvae perform a similar function. And moles, gophers, rabbits, badgers, prairie dogs, and other burrowing animals make larger, tube-like openings.
Human activity also influences the physical and chemical nature of the soil. Large areas of agricultural soils have been cultivated for centuries. As a result, both the structure and composition of these agricultural soils have undergone great changes. These altered soils are often recognized as distinct soil classes that are just as important as natural soils.