Introduction
We have seen how rain infiltrates the soil and the various pathways that water can take within the soil. But what exactly is soil? We all know it, have held and smelled it, so soil seems familiar. Yet soil is actually quite mysterious, not only because it is such a complex substance, but also because much of the action of the soil takes place on the microscopic and molecular levels. Soil, a unique combination of physical and biological processes, is vital to all life.
The Formation and Structure of Soil
Soil is one of the great marriages known to earth science. Two very different actors are linked to create soil. The first is physical; the partner is biological. The first comes from the rock below; the second comes from the life above. Below and above are combined in the middle—soil.
We will consider the physical partner first. The process of physical weathering was shown to break down rocks into smaller and smaller particles by the action of wind, water, and ice. Physical weathering turns the bedrock of the continental crust into broken bits and chips these bits smaller and smaller into gravel. Gravel is ground down by the processes of physical weathering still further in size, into sand, silt, and clay.
Definitions of sand, silt, and clay are somewhat arbitrary and made by agreements among soil scientists. Typically, these are the size ranges for the three types:
- Sand: 2.0-0.02 mm in diameter
- Silt: 0.02-0.002 mm in diameter
- Clay: Less than 0.002 mm in diameter
You can see that silt is roughly smaller than sand by a factor of 10, and clay is smaller than silt by a factor of 10 or more.
The physical, mineral components of soil are a mixture of particles of all these different sizes, thus a mixture of sand, silt, and clay. A good mix of all three is called a loam, a common soil combination with generally desirable properties. Loam is approximately 40% sand, 40% silt, and 20% clay. When one component is substantially more than these percentages, the soil is called by other names. For example, if the clay component is significantly higher than 20%, at the expense of silt or sand (or both), the soil could be a clay loam. Similarly, soils can be silty loams or sandy loams. If the percent of one of the components is so large that it dominates, the soil can simply be called sand, silt, or clay. The situation gets more complex with additional names, but this gives you the general idea.
There is a process known as chemical weathering. This is a physical process, too, but it works on the level of chemical changes, rather than a physical breakdown of particles. Chemical weathering involves the dissolution of minerals to make ions of elements and compounds that exist as dissolved forms in water.
We cannot always perfectly and neatly separate physical and chemical weathering sometimes, even though conceptually, they are distinct. For instance, chemical weathering alters clay particles at the same time they reach their tiny sizes from physical weathering. Nonetheless, it is important to keep in mind the distinction, because chemical weathering adds nutrientions to the soil's water, which will be important later.
The second partner in the marriage that makes soil is life. Plants live with their roots in the soil. When plants die (or leaves fall in the autumn from the trees that stay alive from year to year), a large amount of material from the photosynthesizers falls down to the top of the soil. This material is called organic, because it consists of organic molecules, molecules of complex chains of carbon atoms, with hydrogen and oxygen atoms and a host of other elements required by life. These molecules include carbohydrates such as cellulose and also proteins and lipids—all building blocks of life.
The detritus from plants, as well as dead animals and animal waste that enters the soil, like the minerals, pass through a sequence of diminishing sizes and altered compositions. Devoured by crawling and bur rowing scavengers, physically ground and chemically broken by worms, beetles, ants, and other arthropods, the decomposing organic material enters a complex web of life that includes tiny worms called nematodes, whose populations per square meter often number in the millions, each of which can gobble thousands of bacteria per minute. Together, they make an organic matrix of different-sized aggregates—sticky, spongelike, complex, and nourishing.
The result of the nonbiological processes of both kinds of weathering, plus the biological degradation of organic detritus, creates soil. Soil comes in more colors than the human skin. It also comes in layers, which themselves often have different colors. Together, the layers form the soil profile (see Figure 15.1).
The uppermost layer of soil is called the O layer, or O horizon. It consists of the detritus from plants and animals who live above the soil. In a forest, for example, the O horizon is the mat of leaves you can almost lift up with your hands, a tightly woven layer of leaves from several years. Near the base of the O horizon, the materials (think leaves or grass stalks) have decayed into a black mass called humus. Humus helps the soil be fertile for future generations of plants.
Below the O horizon is the A horizon. This also goes by the well-known name of topsoil. It is a rich mixture of the biologically decayed, organic material from the O horizon and the more physical, mineral material from the next horizon below. The topsoil is essential to the productivity of the soil. The minerals of the topsoil supply the new nutrients needed by the plants. The organic components of the topsoil provide a source of recycled nutrients for the plants. Water in the topsoil is called capillary water because it is water that is held in the pores and channels of the topsoil's materials. The capillary water can stay there for quite some time.
The next layer down is the E horizon. In the E horizon, many minerals are being leached away from the gravitational water that leaks down from the soil into the groundwater. The E horizon is lighter in color than the A horizon.
Below the E layer is the B horizon. In this layer is a deposition of minerals that were leached away by water from the E horizon.
Below B comes the C horizon, which consists of broken pieces of rock from the bedrock even farther below. The C horizon supplies the minerals for the soil at that site. The bits of rock in the C horizon are on their way, via physical and chemical weathering, to becoming the smaller bits of minerals in the upper levels of the soil. You can see now how soil is a merging of the mineral processes from below with the biological processes above.
Under the C horizon is a layer called the R horizon, for rock. It is the parent bedrock for the site, the ultimate source of the kind of minerals that the soil will contain.
Soil is typically about a meter thick, but this varies tremendously from place to place. As you can see from the descriptions of the layers, the amount of organic matter in the soil decreases with depth.
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