What is Soil Study Guide (page 2)
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.
Soil as Recycling Compartment for the Land
Because the decomposed products of life make soil so much of what it is, without life, there is no soil. With out life, the forces of weathering that reduce rocks to particles would wash those particles away. Without life and thus soil, every sunny day would be a drought, every rain a flash flood. Without life, particles of soil would quickly wash or blow away, to aggregate in valleys of deep sediment graveyards or tumble into the sea. Much of the continents would be as bare as today's fresh volcanic fields.
We will discuss how the soil's life is essential, not only in creating soil, but in functioning as a recycling process that allows plant life to flourish. But first, we will review some of the properties of soil that make it conducive for living things. In other words, what is a healthy soil?
A healthy soil strikes a balance between water infiltration and water retention (also called water holding). If the soil is hard clay, for instance, rain water will not be able to infiltrate it and will run off the surface to be wasted. Water needs to be able to move into the soil to become stored as the reservoir of soil moisture that plants can draw upon during days or weeks when no rain occurs. Thus, infiltration is crucial. The degree of infiltration is normally determined by the amount of sand in soil, which creates large pores for water to move downward.
On the other hand, a soil can be made of too much sand. Beaches, for example, do not make a productive soil. With too much sand, the water simply runs downward as gravitational water, goes into the groundwater system, and is lost to plants. Thus, a healthy soil will retain water as capillary water stored in microscopic spaces and on the mineral grains of the soil (already discussed). Clay and silt are good at retaining water; so is the organic humus.
A healthy soil also needs to allow enough air to circulate within it, as well as between it and the atmosphere. Aeration is obviously high in sandy soils and is inhibited by clay soils. Why is air needed? First of all, the roots of plants are just like us; they need to breathe air to get oxygen for their cells. Without a fresh supply of air from the atmosphere into the soil, plants can die. This is why it can be bad to overwater house plants. It's not the water directly that can kill the plants, but rather the fact that the water is preventing their roots from getting the needed air.
Except for the kinds of bacteria that thrive in the absence of oxygen (called anaerobic bacteria), all organisms in the soil must also breathe air for their oxygen. Air moves around in the soil because a healthy soil is porous. The porosity extends right up to the surface, from the A to O horizons, and allows air to move from the atmosphere into the soil, as well as from the soil into the atmosphere.
Also, the roots of plants and organisms in the soil give off carbon dioxide gas as a waste product from their metabolisms, the same metabolisms that use oxy gen. The waste carbon dioxide enters the pores of the soil and eventually moves up and out of the soil into the atmosphere. As a result of the breathing of soil organisms, the concentration of carbon dioxide is much higher in the soil than it is in the atmosphere.
A healthy soil also has a large capacity to hold nutrients. To some extent, this property is related to water retention because the nutrients that plants use are in the form of dissolved ions in the water. But a nutrient-holding capacity is also related to the kinds of particles in the soil. Not enough nutrients can be held by the water alone, and the water can sometimes be used up or fall to minimal levels during droughts. Nutrients are also retained on soil particles, and the best kind of particle for nutrient retention is clay.
Organisms are also crucial for a healthy, natural soil. Previously, we saw that countless numbers of tiny nematode worms inhabit soils, as well as other creatures such as ants, beetles, and earthworms. Another crucial type of organism is the fungi, whose microscopic white threads decompose organic materials within the soil. When it is time for some kinds of fungi to reproduce, they make mushrooms.
The soil is an entire ecosystem. Creatures run around (millipedes, centipedes, spring tails) and creep along (slugs, snails). Some of the soil's inhabitants are single-celled protozoans, seen under a microscope. Even smaller, visible only at a microscope's highest power, are the most important inhabitants of all, the bacteria.
Soil would not be soil without its bacteria. Although all the soil creatures participate in the break down of plant debris from the O horizon of litter, the bacteria are the ones that perform the greatest part of the final step in decomposition and return the elements in the organic debris into forms available again for the plant's roots to take in for the next round of growth. Bacteria are thus key to the recycling function of soil.
We will next see how the activity of bacteria determines the amount of organic material in the soil. Bacteria, which digest and therefore break down organic matter (say, from fallen leaves), are more active at high temperatures and less active at cooler temperatures. The climate of a region, then, can affect the activity of bacteria and thus the amount of organic matter in the soil. Very cold climates tend to have thick soils with a high content of organic matter. Famous for this are the peats of northern Canada and Siberia, in which the activity of bacteria is extremely slow.
At the other end of the climate spectrum are the tropics. What are tropical soils like? You might think they would be thick, given the abundance of vegetation and growth in the tropics, but no. Tropical soils, despite the rich vegetation, tend to be thin with low amounts of organic matter, because the breakdown (decomposition) is so rapid. Bacteria in the tropics digest at a high rate and keep the organic contents of the tropical soils low. That is why the tropical soils are often not very good for agriculture, after being used just a few years following deforestation.
The main role for bacteria is to recycle elements from the dead vegetation debris of the O horizon into dissolved ions in the soil water, because these ions are then available to plants for uptake and more growth. It is an important point that the processes of physical and chemical weathering are too slow and their products too little to supply the plants with the nutrients the plants need. Plants thus depend upon recycled nutrients.
An example of the importance of recycling can be seen from numbers of that key element for plants, phosphorus. Let us consider phosphorus requirements, summed over all the plants across all the continents. That need is about 40 times higher than the flux of phosphorus supplied to the soil from the break down of minerals by weathering. That means that when a plant takes in phosphorus through its roots, 39 parts out of 40 (close to 98%) is, on average, recycled phosphorus. This recycled phosphorus is, for the most part, derived from the actions of the soil bacteria.
A final property of a healthy soil that is related to the human use of soil for agriculture is what is called workability. Can the soil be plowed easily? Does it have all the properties of an overall healthy soil that can support intensive cultivation? Can it hold water, be naturally aerated, and retain nutrients?
Traditional agricultural techniques can create a loss of soil by erosion. If the A horizon (the topsoil) is lost, productivity plummets. Plowing also creates some soil loss by exposing loose particles to the eroding effects of wind and water. Plowing can also increase the activity of bacteria and thereby reduce the organic matter of the soil. In fact, on average, cultivated soils have about 25% less organic matter than they did before they were cultivated.
Solutions to the long-term maintenance of healthy soils for human use include better cultivation techniques. One promising technique that is rapidly growing in adoption is no-till agriculture. In no-till, the entire farm soil is not plowed. Instead, only a narrow strip where the seeds will be planted is worked by machinery precisely guided down the same path year after year.
Other advances are coming in irrigation, to attempt to use the minimum amount of water at precisely the time when the water is required. The precision application of fertilizers is also under way.
Practice problems of this concept can be found at: What is Soil Practice Questions