Remediation Weekly – Science & News Journal

www.RemediationWeekly.com
Volume 1, Number 10, August 16, 2004
Soil Formation
By Alfred R. Conklin Jr.
Dr. Alfred R. Conklin Jr. is a professor at Wilmington College in Ohio, and a Fulbright Lecturer/Researcher at Leyte State University Visca, Leyte Philippines.
Phone: 63-53-335-2600, E-mail: arconklinjr@hotmail.com

Soil is not simply broken up rock, but rather it is decomposed, weathered and ground up rock material acted upon by the soil forming factors. This can be simply observed by noting that ground up rock is gray in color, while soil has brown, black, yellow, red, and in some cases shades of gray, blue and green. If nothing else, the fact that there is a diversity of colors shows that something more than simple grinding has occurred. Soil texture, being a combination of particles of vastly differing sizes and composition, also shows that it is not simply a result of the grinding of rock.

One might start thinking about soil formation with rock deep in the earth. Here it is under tremendous pressure and high temperatures. Under these conditions, its chemical structure will arrange into the most stable form for the conditions under which it exist. When this same rock is lifted to the earth’s surface, all these conditions change, and so the rock’s structure will no longer be stable. Even without the soil forming factors, the rock will change, with the release of some components and incorporation of others, into new forms. From a human perspective, these changes may be slow. However the reactions and changes will take place.

Soil formed solely from decomposed rock is said to be forming from residuum. That is the material left over after the rock has chemically decomposed. Granite can be found in a decomposed form that looks like rock, but easily crumbles when handled. Limestone dissolves when exposed to acid, which may be in rain as carbonic acid, produced by carbon dioxide dissolved in it, or produced by plants and animals. The materials remaining are the foundation for soil development. This decomposition may be viewed as the first step in soil formation. However, it is not easily separated from the results of the action of the soil forming factors.

Physical forces also break up rock forming smaller particles, sand and silt, and releasing clay if present, from and in which soil can form. In almost all cases, physical forces come into play when rocks are transported by water, ice and gravity. Wind can also cause the physical breakdown of rock, but rock size particles are not transported by wind. It is primarily the sand and silt size particles carried by wind that impinge on rock and break it down. Both chemically decomposed and physically broken up rock constitute what is called the soil parent material.

Parent material as described above is but one of the soil forming factors, the other four being time, topography, biota and climate. Just as it is hard to impossible to separate initial decomposition of rock from the soil forming factors, it is impossible to separate the soil forming factors as to which is more important, or which happens before the others. Biota and climate are considered the most active of these factors, however all factors are functioning in concert, and eventually result in the formation of a well developed soil.

Time is a critical part of soil formation, as is topography, because parent material must stay in one place for a period of time for the other soil forming factors to function. On steep slopes, parent material may erode before significant development takes place. In low lying areas, deposition of new parent material or partially developed soil may result in many layers of partially developed material, and retard the development of a full soil profile, which would be 1.5 to 2 meters deep and have fully developed major and subordinate horizons as depicted in Figure 1.

During development in humid regions, many changes in the physical and chemical make up of parent material occur. Easily dissolved components are leached out of the forming soil by rainwater. Both surface abdication and breakup of particles caused by the loss of components takes place and the texture changes. Once in solution, components can recombine to form new inorganic compounds, which may precipitate out of solution forming new clays or other solids. Another eventuality is that these materials may be deposited on the surface of sand, silt and clay particles, significantly changing their absorbance and adsorbance characteristics.

In arid regions, there is often not enough rainfall to completely leach all salts out of soil. Indeed in some cases, rain may dissolve salts that are subsequently brought to the soil surface by evaporation. However, even in arid regions, development of soil can be observed, i.e., horizon development is evident. Sometimes this is due to long term changes in the climate, for example, from a more to a less humid or arid condition.

Biota includes plants, animals and microorganisms growing in and on soil. They can be thought of as contributing organic matter in the form of dead plants, animals, microorganism and all their excrements to the soil. This material will undergo decomposition at a slower or faster rate depending on its characteristics. Many consider this to be a one-way reaction, leading only to the production of carbon dioxide and water with the release of energy and inorganic constituents. What is often overlooked is that synthesis is also taking place, resulting in humus production as shown in Figure 2. Note that although anaerobic decomposition results in methane production, the process is basically the same for aerobic and anaerobic conditions, both of which result in the synthesis of humus.

Climate is a critical factor from numerous perspectives. The rate at which a soil forms is dependent on both temperature and rainfall. Chemical reactions take place faster at higher temperatures, and so the higher the temperature the faster the chemical changes that result in soil formation. Water, normally from rainfall, is required for both illuviation and eluviation, which result in the formation of horizons. Illuvation is the movement of material into or deposition of material removed from horizons higher in the soil profile. Eluvation is the removal of material from a soil horizon. It is thus possible to have horizons which are described as being either illuvial of eluvial. The more rainfall, the faster horizonation can take place. Also, soil biota require water, and thus, the greater the amount of water, the greater the amount of biota, which will also lead to faster soil formation.

There are two exceptions to increased rainfall, resulting in increased soil formation. When soil is saturated with water, particularly for long periods of time, it is anaerobic, and anaerobic reactions tend to be slower than aerobic reactions. Thus, under saturated conditions, soil formation may be slower than under unsaturated conditions. As noted above, water moving through the soil results in the formation of horizons. Without water movement, horizonation is limited. However, there may be a large build up of organic matter and the formation of an organic soil.

Although the extent of organic soils in the world is limited, they are of great interest because they are highly productive when used for agriculture. They are also in areas that receive and accumulate material eroded from surrounding areas, which makes them important in the environmental movement of contamination. Organic matter, residence time and microbial and rhizosphere activity, illustrated in Figure 3, tend to attenuate or decompose contaminants in these eroded materials.

The result of soil formation is the development of horizons in the soil parent material. The upper horizons are typified by the loss of inorganic material, while the top most, A- horizon also is typified by the addition of organic matter. The lower B-horizons, are typified by the deposition of material, chiefly clay, although in some soils the lower horizons may have deposits of iron, aluminum and organic matter, and in limited cases, silt size particles. An illustration of the development of horizons in soil is given in Figure 1.

In the USDA, taxonomy soils are described based on their observable characteristics, starting with their diagnostic horizons. On this basis, 12 soil orders are defined. In most cases, a progression of horizon development determines a soil’s name. While in other cases, a climatic factor controlling a soil’s characteristics is used in its naming. In some cases, an overriding soil characteristic from soil parent material, vegetation or a combination of these, determine a soil’s characteristics and therefore its name.

Thus, Entisols are very young soils with very little horizon development. Inceptisols show a minimal amount of soil development. Alfisols, Mollisols, Ultisols and Oxisols have extensive horizon development (see Figure 1). On the other hand, Aridisols occur in arid regions, while Gelisols are frozen, and so only occur in northern climates. Both Andisols and Histosols derive their name from soil material. Andisols develop in volcanic ash, while Histisols are organic soils. Vertisols develop in soils having high expanding clay content, and so develop large cracks when dry. Spodosols develop in sandy soil under coniferous vegetations, and develop a subsurface horizon high in aluminum and highly decomposed organic matter. This spodic horizon may also contain appreciable iron. The twelve soil orders and some of their characteristics are given in the table below.

Each soil order can be designated as having suborders, great groups, sub-groups, families and series belonging to it. Each of these subdivisions gives additional information about the characteristics of the soil and its environment. For instance, a soil may be described as being an aquic mollisol and given the name of an Aquoll, the -oll being the formative element of all soils belonging to the order Mollisol. The prefix aquic identifies the soil as being wet or saturated with water for a significant period of time during the year. Additionally, soil phases are used in soil mapping, although they are not part of the soil taxonomy classification system.

Knowing the full name of a soil provides the most information about its characteristics. However, knowing only the order provides significant information to the researcher or person involved in a soil’s remediation. For instance, an Andisol has high infiltration rates and little clay content, although it has high phosphorus fixing capacity. Thus, many contaminants will move rapidly through an Andisol, but phosphate will not. The table gives some of the more important characteristics of the 12 soil orders. More detailed information about the soil orders is given in standard soils texts, in Soil Taxonomy published by the USDA, and in local soil surveys, which are available for most counties in the U.S.

How does soil development effect soil decontamination or remediation? There are many ways to answer this question. Organic matter, especially in the form of humus, has both a high adsorptive and cation exchange capacity. This means that soils with well developed A horizons and high in organic matter will retard the movement of pollutants, more than soils without A horizons or which are low in organic matter. Well developed soils will have well developed B horizons high in clay. Clays have high sorptive and cation exchange capacities, thus they will retard the movement of water and pollutants through soil, either slowing or preventing their movement into water supplies, and allowing more time for the isolation and remediation of contamination.

Well developed soil will have high biotic activity. It is this activity that leads to the attenuation and remediation of soil contamination. Remediation may be divided into bioremediation, which is usually understood as being carried out by microorganisms, and phytoremediation, which is carried out by plants. There is an area or volume of soil around roots having high biological activity called the rhizosphere (see Figure 3). In many cases, it is this area of high biological activity that is responsible, for remediation.

As might be expected, a fully developed soil takes a long time to produce, perhaps on the order of 1,000 or more years. This means that soil lost by erosion or by contamination can not readily be replaced. Thus, it is important to be able to determine the amount and severity of contamination of soil when it occurs, and to be able to remediate damaged soil.

Figure 1. Depiction of development of soil profiles with time. Entisols have just started to develop and have very little horizon development. The Ap represents a horizon developed as a result of plowing. Inceptisols have some horizon development as indicated by the Ap and Bw horizons. As more developemnt occures more horizons are produced as indicated in the last two profiles. B and particularly Bt horizons, are characterized by increases in clay (the t stands for ton the German word for clay). The C horizon is the parent material from which the soil is developing. Two combined capital letter designate trasnition Horizons.

Figure 2. Oxidation of organic matter (OM) in soil with the production of intermediate products (IP). The IP breakdown into carbon dioxide, water and energy (E) used by the organisms breaking down the organic matter. Inorganic components are released and humus is synthesized.

Figure 3. Rizoshpere, area of increased microbial activity around roots a tree (colors are not meant to represent true soil colors).

Table The 12 soil orders and some of their characteristics