SEE-U 2000: Biosphere 2 - Module 10

## Module 10: Geology and Soils

Today we discuss the second set of our abiotic influences on ecosystem functioning—soil. This important component of the ecosystem is often thought of by ecologists as being merely the table on which the interesting activity (organismal activity) occurs. In fact, the “interesting activity” both could not occur and is usually largely determined by the characteristics of the local soil.

Soil provides a variety of requirements for plants, and hence, animal life—it is one of the most important bases of terrestrial ecology. The principal requirements that soil provides for plants are anchorage, moisture storage in that soil is like a sponge storing water, and a supply of nutrients. Soil creates habitable habitats for animals by providing space for them to live and by determining the local chemistry, soil provides the proper type of habitat.

When many people try to define soil, they realize that it is not as intuitive of a definition as it initially seems. Do we include only dead material? Only organic material? Only inorganic material? Are roots included as part of soil? Is there a minimum thickness for each layer in the soil? Must it only be material that is naturally occurring? What about transported materials such as gravel dumps? In short, what is soil?

### Definition

Many soil ecologists use a definition that was created by the Soil Science Society of America (as good of a reference as is possible) in 1973. They define soil as follows: “The unconsolidated mineral matter on the surface of the earth that has been subjected to and influenced by genetic and environmental factors such as parent material, climate (including moisture and temperature effects), macro-and microorganisms, and topography, all acting over time and producing a product—soil—that differs from the material from which it was derived by many physical, chemical, biological, and morphological properties and characteristics.” (as quoted in J. P. Kimmins, 1997, Forest Ecology: a Foundation for Sustainable Management, 2nd edition, Prentice Hall Press).

This is all well and good, but it has a number of relatively difficult terms and concepts—e.g., soil has genes? We will discuss these terms below. For the time being, we can simplify this definition with an equation: Soil = f (parent material, climate, biota, topography, time), where f is the symbol for “is a function of the interactions between”. This definition does not specify how changes in the intensity of these components will affect each other or the resultant soil. Let’s just use this as a summary of the forces influencing the local soil.

A simpler and slightly broader definition is offered by Kimmins (1997): soil is “those upper layers of the unconsolidated surface of the landscape that provide forest plants with the following necessities: water, nutrients, and a firm anchorage”. This is closer to the definition that many people would use and is the one that we’ll use today. The definition of what is soil differs between ecologists, geologists, and engineers. The above definition is that of an ecologist.

Another point is that not everything that we colloquially call soil is actually soil by the above definition. Unconsolidated materials that exist at the surface of the ground that many other disciplines may classify as soil but do not fit the ecological definition of soil are collectively referred to as non-soil. Examples of this concept would be gravel fill (both natural and unnatural), sand dumps, and other human created types of ground.

### Soil Processes

As you probably noticed in the above equation, soil is partly biological in nature, as represented by “biota”. Many animals play important roles in the soil, including insects (springtails, beetles, and ants), mites, millipedes, nematodes, annelids, mollusks, burrowing vertebrates, as well as mycorrhizae, bacteria, and plant roots. Plant roots also do much of the initial breaking of the parent material into smaller components.

Which organisms structure the soil in each ecosystem differs. In temperate terrestrial ecosystems such as the Black Rock Forest, earthworms do much of the soil turnover and processing. In tropical soils, such as in the Brazil Atlantic Forest, ants and termites do most of these functions. Desert soils are not turned over very much, but ants and burrowing mammals do most of the turnover that occurs.

Soil has been called the least renewable resource in the ecosystem. Unlike biodiversity, plant growth, water resources, and most other components, soil takes decades to be replenished if it is lost. The experience of the dust bowl years (the 1930’s and early 1940’s) in the Midwestern US and the economic devastation that it brought illustrated the gravity of this observation. Soil will renew, provided that all of the biota has not been damaged and that the climatic forces are not greatly disturbed. However, it will only do so on a time scale that is much longer than for most other ecological successional processes.

### Soil Physical Properties

Pedologists (soil biologists) categorize soils based on at least eleven properties (http://www.smartgardening.com/Soil_Characteristics_and_Testing.htm). Let’s treat them in decreasing order of importance, beginning with the three most important mechanical properties—texture, structure, and porosity.

The dominant particle size that is most characteristic of soils is used to characterize soil texture. This characteristic is very useful for farmers, who were the first to recognize the importance of soils in human history, when they attempt to determine the potential fertility of a land. For maximum growing utility, farmers prefer soils that had particles smaller than 2 mm in diameter. As a consequence, particles larger than 2 mm are not considered soil by most pedologists.

There are three main particle-size classes used by pedologists to characterize soil texture. Sand, the largest of these with particles 0.02-2 mm in diameter, has individual grains that grit sharply against the teeth. Silt, the next smallest class with particles of 0.002-0.02 mm in diameter, has general fine grittiness, but individual grains cannot be identified between the teeth. The smallest size class, clay, has particles of less than 0.002 mm in diameter and you cannot feel grittiness between teeth. There are 12 major types of soil textural classes that are characterized based on different combinations of these three size classes, a diagram of them can be accessed here (http://homepages.which.net/~fred.moor/soil/formed/f0107.htm).

The architecture of soils is referred to as the soil structure. Structure has two components; whether they tend to bind together (or cement) into aggregates (or peds) and the size and shape of those peds. If the soil tends not to aggregate, then it is considered structureless or massive. The tendency of soil to aggregate depends largely on the moisture level and is therefore a frequently changing property of soils.

The second component of soil structure is referred to in terms of soil horizons. The materials arriving at the surface layer, such as leaf litter and other dead organic matter, are processed within each horizon and then pass through to the layer beneath it. The horizons are abbreviated to single letters, and unfortunately the nomenclatural systems differ between continents.

The top layer is the litter layer (L), composed of litter that is slowly decomposing and has been only recently deposited, but is rapidly being broken down by biotic activity. The next layer is the F layer, a rapidly decomposing layer of litter leading to an accumulation of a layer of “raw” humus (horizon H) below it. The H layer has litter that has been broken down to particles and is usually very nutrient rich. Below the humus layer, are the eluviated layers (A horizons), layers that are very poor in metallic elements like iron and aluminum, elements which have been leached out (eluviated) of the forest floor to the B horizons below. The illuviated B horizons are layers that are rich in deposited iron, aluminum, and other minerals (illuviated) that were leached out of the A horizons. Below the B horizons lies material that are not considered soil—the C horizon consisting of unweathered parent material or compacted rock materials. The C horizon begins the bedrock layers, where biological activity is almost non-existent.

The third main soil property is porosity, the distribution of space within soils. Porosity influences the movement of water and gases, which in turn determine the activity of roots and soil organisms. Because of the inverse relationship between gas exchange and the capability for water retention, we tend to get diminished gas exchange and waterlogging of plants with increased water retention. Conversely, increasing gas exchange tends to lead to the drying out of soils.

The last eight soil properties come about as a consequence of the above three. Consistence (the physical resilience of soil or how well material stays together when pulled apart) and bulk density (the mass of soil per unit volume) influence how deeply roots can penetrate. Aeration, or the gaseous exchange in soils, primarily is determined by porosity. Soil warmth or temperature influences activity of roots and soil organisms, as well as the rates of decomposition and nutrient and water uptake.

The above seven soil properties determine the potential fertility of the soil, how well root systems are able to occupy and exploit the soil. Additionally, they influence soil stability, the degree and speed of erosion, and the degree of animal traffic that the soil can handle without breaking down.

Soil moisture, the water storage capability of soils, enables soils to serve the purpose of acting as a sponge and water repository for the plants and animals in an area. Soil moisture, in conjunction with the previous seven soil properties and topography, determines the rate at which water is lost from the soil.

Last, soil chemistry and organic matter content (the soil portion produced by living organisms) combine with soil aeration, temperature, moisture, soil chemistry, and organic matter content to determine the last soil property, soil fertility. Each of the contributing properties are not independent of each other and frequently influence each other and are also strongly influenced by animal and microbial activity

Soil is more than just a surface on which and in which most of the features that ecologists study operate. Some pedologists have asserted that the only variable important in ecology is the soil—an interesting and testable hypothesis. Try to evaluate this assertion as you ruminate on earlier components of the course and also as you learn more about ecology in the future. At a minimum, it is a controversial statement, to be certain!