Alfred R. Conklin, Jr. Ph.D. is professor of agriculture and chemistry at Wilmington College in Ohio.

al_conklin@wilmington.edu

Soil is one of the most complex components of the ecosystem. This makes it the hardest to describe, analyze, control and change. Why is it so complex? For that mater what is soil? 

An engineer friend of mine defines soil as anything he can move with a bulldozer. The soil scientist defines soil as a natural body which results from the action of soil forming factors on the loose material covering the earth’s surface. Soil forming factors are time, parent material, topography, climate and vegetation. For the engineer the soil includes some rather large rocks. But for the soil scientist it is restricted to sand silt and clay size particles. The soil scientist does considers rocks and gravel as part of soil , but these materials are not good for growing plants. From a environmental standpoint they have low physical and chemical activity thus are not as important as sand silt and clay.

When defining soil, especially with reference to environmental concerns there are the concepts of the rigolith, solum and vadose zone to consider. The rigolith is defined as the loose material over the bedrock. The solum is the volume of soil which contains the genetic soil horizons (A,B,C etc. see Figure 1). The vadose zone is the unsaturated portion of the rigolith – the loose material above the bedrock and above the ground water table.

The soil forming factors lead to the development of the genetic horizons in soil. These are significantly different from layers deposited by wind, water or ice. As indicated above there are three basic horizons in most soils. The A horizon, which is on the surface, is dark in color and high in organic matter. This is the one we try not to spill contaminants on, and try to save for farming. The B horizon is higher in clay and redder in color. This horizon often restricts water movement through soil. The C horizon comes below the B and is material which has not been acted upon by the soil forming factors.

Other horizons can exist in certain soils. An E horizon occurs just below the A in soils developed under trees. This is a horizon from which material has been leached out and it is consequently lighter in color and courser in texture than the A or B horizons. Soils receiving large amounts of organic matter may have an O surface horizon above the A. It is made up of decomposed and undecomposed plant litter.

The A and B horizons are very different from the material from which the soil is forming. The organic matter, clay minerals and physical characteristics have been changed so that they no longer have the characteristics of the parent material or of ground rock. Figure two illustrates the arrangement of soil particles. Sand, silt and clay particles are cemented together to form units called peds. The physical characteristics, particularly air and water movement, of soil are greatly affected by the stability of these units. A soil with strong, well-formed peds has higher air and water movement through it than would otherwise be the case. In most cases good ped structure will make cleanup of contaminated soils easier.

Chemistry and biology in soil are significantly different and more complex than in the laboratory. Soil is made up of solids, liquids and gases. The solids are living organisms and organic and inorganic materials – and are changing all the time. Even the inorganic component call the matrix, changes. New clays form, rock dissolves and reprecipitates as new minerals all at the same time. Organisms die and become organic matter in soil. The liquid, called the soil solution, consists of dissolved gases, organic and inorganic compounds and ions. Its composition also changes as materials dissolve and precipitate.

The gaseous phase, called soil air, consist of oxygen, nitrogen, carbon dioxide, water vapor and argon. Soil air would appear to be similar to atmospheric air. It is, except that its oxygen content is lower and its carbon dioxide content higher. Under most conditions the water vapor content of soil air is also higher than in atmospheric air. In addition it is common to find small amounts of methane and nitrogen oxides in soil air. So, even the gaseous phase of soil is not simple.

Organic matter from plants, animals and microorganisms play a large role in determining the characteristics of a soil. Humus is the material remaining after organic matter added to soil is fully decomposed. It has high affinity for water and organic compounds. It also has cation exchange capacity and helps bind soil particles together to form peds. Even small amounts of humus have a large effect on a soil’s physical and chemical properties. In addition to humus there are an immense number and verity of organic molecules in soil. It is reasonable to expect that some of these will be hydrocarbons and some will act like hydrocarbons. Finding hydrocarbons in the organic component in soil is not unusual.

Soil has another characteristic, which is very important when considering pollution. It is highly buffered. Chemist have a precise definition for this term. Here, we need only to know that buffering in soil means that it resist change. Soil in the natural state resist physical, chemical and biological change. The addition of 100 metric tons of sand to 2 hectares of soil has little effect on it texture. One metric ton of lime added to soil has a small effect on its pH. Add microorganisms to soil and they disappear very rapidly. It is also important to remember that the reverse is true. Once a soil is contaminated and thus changed, restoring it to the way it was previously is very hard to do. The longer a soil has been contaminated the harder it is to decontaminate. 

Although it contains oxygen, both oxidized and reduced compounds and environments are found in even small samples of unsaturated soil. Because of this, all soil contains both aerobic and anaerobic microorganisms. The variety of soil microorganisms seems to be limitless. Aerobic, anaerobic, phototropic, and lithotropic microorganisms abound. Bacilli, cocci, fungi, and microscopic animals as well as lager plants and animals are always present in this complex medium.

All of the characteristics of soil are important when considering how to handle a spill or contamination. Will the contamination move through soil or be adsorbed. Can the contaminant be easily washed out or will it be strongly held. Will it be broken down or remain unchanged? Is it biologically available or inert? All these and more will depend on the unique and complex nature of soil.

Figure 2. Arrangement of soil particles.

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