What is soil?

What is soil?

Soil scientists define soil in various ways, but all definitions illustrate the fact that soil is not as simple as many assume. Soil is the uppermost layer of material covering most of the earth’s land surface and consists of mineral particles, organic matter, microorganisms, water and air. The possible types and proportions of these components are innumerable and why so many different types of soil exist.

Soil scientists divide soil into layers they call horizons. The A horizon is the uppermost several inches and consists mostly of what we know as topsoil. It is often darkly colored and rich in organic matter, and it usually provides a favorable environment for plant growth. The next two layers, the B and C horizons, are lighter in color, lower in organic matter and relatively infertile. We call the B and C horizons the subsoil. Plant roots generally extend through the A horizon and well into the B horizon. However, the C horizon, which may be well below the surface, is comparatively inhospitable for root growth.

In landscape situations, this natural layering often is absent due to soil movement during construction. All too often, this means that no topsoil layer is present, forcing the landscape installer to modify the existing subsoil to make it more favorable to plant growth.

Aside from horizons, which describe the position of the soil layer, soil scientists also refer to soil fractions. Fractions refer to organic or inorganic (mineral) substances. Thus, most soils are composed partly of a mineral fraction and partly of an organic fraction. A few soils are almost completely organic, and others are mostly mineral.

The mineral fraction of soil, consisting of particles that ultimately originated from rock, comprises the largest percentage of most soils. The type of rock from which the mineral particles originated has some bearing on the chemistry of a soil. However, the mix of particle sizes has a greater impact on soil quality and how you must manage it. The age of the soil and how much weathering it has undergone determine particle size: Older, more weathered soils consist of smaller particles.

The smallest particles are clay. Larger (but still quite small) particles are silt, and the largest particles (that still qualify as soil) are sand. Soils rarely, if ever, consist of solely one size of particle. Thus, soils are classified according to the proportion of each particle size they contain—we commonly refer to this as soil texture.

Texture, in the broadest sense, is stated as coarse (sandy soils), medium (silts or loamy soils) or fine (clayey soils). Loamy soils are intermediate in nature and not totally dominated by the characteristics of any particular particle size, though they proportionately contain more silt than sandy or clayey soils. Thus, there is no such thing as a loam particle, only loam soils. Loamy soils generally have the best overall characteristics for plant growth.

To be even more specific, we combine these terms. For example, sandy clay has significant amounts of sand but is dominated by clay particles and clay characteristics. A sandy loam is a mix of particle sizes not totally dominated by characteristics of any particle size, but—due to a somewhat higher relative sand content—its qualities tend toward those of sand. Other terms you’ll often encounter to describe texture include light and heavy, referring to sandy and clayey soils, respectively. As we’ll see, texture, more than any other single aspect, determines the manageability of soils.

Organic matter (OM), the other soil fraction, is present in most soils, but content varies widely. Soils low in organic matter may have less than 1 percent OM content, whereas highly organic soils range far higher. Most soils contain less than 10 percent, and many—especially in arid climates—hold only 1 or 2 percent OM.

Organic matter results from decaying plant material. This decay is brought about mainly by bacteria and fungi that consume plant matter as food. The resulting residues are a rich mix of organic materials that usually have a positive effect on soil quality. As complex organic molecules break down into simpler forms, the organic matter eventually arrives at a semi-stable form we call humus—the dark colored substance we commonly associate with “rich” soil.

Humus containns a variety of carbohydrates, proteins, lignin, cellulose and other materials, but its main benefit does not lie in its nutritional content (most of which is unavailable to plants). Humus improves the physical structure and chemistry of soils so that they have better water- and nutrient-holding capacities and greater permeability. Notably, humic acid causes clay particles to aggregate into larger particles that act more like sand than clay. This improves drainage and aeration, and so is especially valuable in clay soils. Before plant material undergoes extensive decomposition—that is, before it becomes humus—it still is beneficial to soil because it improves physical structure.

As stated above, most of the nutrients in humus are unavailable to plants. Eventually, however, even humus can break down into inorganic compounds by the process of mineralization. At this point, nutrients become available to plants again, and the cycle is completed. The reverse of this process is immobilization, wherein microorganisms assimilate inorganic substances into organic compounds. Both of these processes are ongoing in soil, but the overall trend—not counting plant uptake of nutrients—is always toward mineralization.

Water is present in all soils. Texture has the greatest effect on how much water soil can hold: Finely textured soils hold more water than coarse soils. This is because of how soil particles hold onto water molecules. Water molecules “stick” to soil particle surfaces by a force called adhesion because they possess positive electrical charges that are attracted to negative electrical charges on the soil particles. Thus, a layer of water surrounds soil particles. Even soils that may seem dry have very small layers of water around each particle (though this water may be unavailable to plants). A given volume of clay soil, because of the greater number of particles present, contains a far greater surface area onto which water molecules can cling and so has excellent water retention. Sandy soils hold the least amount of water due to low soil-particle surface area.

Air is present in the pore spaces between soil particles. Because water is the other substance that can occupy significant amounts of pore space, air content is determined to a large extent by how wet soil is. The presence of air—particularly oxygen—in pore spaces is as important to most plants as water. Thus, good aeration is an important physical property of soil. Soils that hold a great deal of water are low or lacking in oxygen. That is why plants languish in saturated soils—their roots starve for oxygen.

Living organisms are prevalent in nearly all soils. Bacteria, fungi, protozoans, nematodes and larger creatures such as earthworms inhabit soils, where they live on decaying plant matter and each other. From a soil-management standpoint, the main benefit of soil organisms is their role in decomposing organic matter, which we discussed above. Warm, moist conditions favor the activity of these organisms so these types of climates favor rapid decomposition of organic matter. However, warm moist climates also favor rapid plant growth, which adds more raw material for the decay process. Thus, the cycling occurs more rapidly and on a larger scale.

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