If there is anything on Earth that seems simple and ordinary, it is the soil beneath our feet. Other than farmers, people hardly think of it except when tending to their lawns, and even when we do turn our attention to the soil, we tend to view it as little more than a place where grass grows and earthworms crawl. Yet the soil is a complex mixture of minerals and organic material, built up over billions of years, and without it, life on this planet would be impossible. It is home to a vast array of species that continually process it, enriching it as they do. Nor are all soils the same; in fact, there are a great variety of soil environments and a great deal of difference between the soil at the surface and that which lies further down, closer to the bedrock.
HOW IT WORKS
The Beginnings of Soil Formation
It has taken billions of years to yield the soil as we know it now. Over the course of these mind-boggling stretches of time, the chemical elements on Earth came into existence, and the uniformly rocky surface of the planet gradually gave way to deposits of softer material. This softer matter, the earliest ancestor of soil, became enriched by the presence of minerals from the rocks and, over a longer period, by decaying organic matter.
After its formation from a cloud of hot gas some 4.5 billion years ago, Earth was pelted by meteorites. These meteorites brought with them solid matter along with water, forming the basis for the oceans. There was no atmosphere as such,but by about four billion years ago, volcanic activity had ejected enough carbon dioxide and other substances into the air to form the beginnings of one. The oceans began to cool, making possible the earliest forms of life—that is, molecules of carbon-based matter that were capable of replicating themselves. (For more on these subjects, see Sun, Moon, and Earth and Geologic Time. On the relationship between carbon and life-forms, see Carbon Cycle.)
All of these conditions—Earth itself, an atmosphere, waters, and life-forms—went into the creation of soil. Soil has its origins in the rocks that now lie below Earth’s surface, from which the rain washed minerals. For rain to exist, of course, it was necessary to have water on the planet, along with some form of atmosphere into which it could evaporate. Once these conditions had been established (as they were, over hundreds of millions of years) and the rains came down to cool the formerly molten rock of Earth’s surface, a process of leaching began.
Leaching is the removal of soil particles that have become dissolved in water, but at that time, of course, there was no soil. There were only rocks and minerals, but these features of the geosphere, along with the chemical elements in the atmosphere and hydrosphere, were enough to set in motion the development of soil. While the atmosphere and hydrosphere supplied the falling rain, with its vital activity of leaching minerals from the rocks, the minerals themselves supplied additional chemical elements necessary to the formation of soil. (The chemical elements are discussed in several places, most notably Biogeo-chemical Cycles. See also Minerals and Rocks.)
Light micrograph of blue-green algae, an example of the simplest plant organisms that were the forerunners of life on the earth. Plant life was made possible by the leaching of potassium, calcium, and magnesium from rock, and, in turn, plant death lent organic matter to the ground to help form the basis for soil.
The first plants
Among the elements leached from the rock by the falling rains were potassium, calcium, and magnesium, all of which are essential for the growth of plant life. Thus, the foundation was laid for the first botanical forms, a fact that had several important consequences. First and most obviously, it helped set in motion the formation of the complex biosphere we have around us today. Not only did the simplest algae-like plants serve as forerunners for more complex varieties of plant and animal life to follow, but they also played a major role in the beginnings of an atmosphere breathable by animal life. As the plants absorbed carbon dioxide from their surroundings, there gradually evolved a process whereby the plant received carbon dioxide and, as a result of a chemical reaction, released oxygen.
In addition, plant life meant plant death, and as each plant died, it added just a bit more organic material—and with it nutrients and energy— to the ground. Notice the word ground as opposed to soil, which took a long, long time to form from the original rock and mineral material. Indeed, the processes we are describing here did not take shape over the course of centuries or millennia but over whole eons—the longest phases of geologic time, stretching for half a billion years or more (see Geologic Time). Only around the beginning of the present eon, the Phanerozoic, more than 500 million years ago, did soil as such begin to take shape.
What Is Soil?
As the soil began to form, processes of weathering, erosion, and sedimentation (see the entries Erosion and Sediment and Sedimentation) slowly added to the soil buildup. Today the soil forms a sheath over much of the solid earth; just inches deep or nonexistent in some places, it is many feet deep in others. It separates the planet’s surface from its rocky interior and brings together a number of materials that contribute to and preserve life.
Though its origins lie in pulverized rock and decayed organic material, soil looks and feels like neither. Whether brown, red, or black, moist or dry, sandy or claylike, it is usually fairly uniform within a given area, a fact for which the organisms living in it can be thanked. Under the surface of the soil live bacteria, fungi, worms, insects, and other creatures that continually churn through it and process its chemical contents.
A filter for water and a reservoir for air, soil provides a sort of stage on which the drama of an ecosystem (a community of mutually interdependent organisms) is played out. It receives rain and other forms of precipitation, which it filters through its layers, replenishing the groundwater supplies. This natural filtration system, sometimes augmented by a little human ingenuity, is amazingly efficient for leaching out harmful microorganisms and toxins at relatively low levels. (Thus, for instance, septic tank drainage systems process wastewater, with the help of soil, before returning it to the water table.)
By collecting rainwater, soil also gives the rain a place to go and thus helps prevent flooding. Water is not the only substance it stores; soil also collects air, which accounts for a large percentage of its volume. Thus, oxygen is made available to the roots of plants and to the large populations of organisms living underground. The creatures that live in the soil also die there, providing organic material that decays along with a vast collection of dead organisms from aboveground: trees and other plants as well as dead animals—including humans, whose decomposed bodies eventually become part of the soil as well.
Factors That Influence Soil
The processes that formed soil over the eons and that continue to contribute to the soil under our feet today are similar to those by which sedimentary rock is formed. Sedimentary rocks, such as shale and sandstone, have their origins in the deposition, compaction, and cementation of rock that has experienced weathering. Added to this is organic material derived from its ecosystem—for example, fossilized remains of animals.
Both sedimentary rock and soil are made up of sediment, which originates from the weathering, or breakdown, of rock. Weathered remains of rocks ultimately are transported by forces of erosion to what is known as a depositional environment, a location where they are sedimented. (See Sediment and Sedimentation for more about these processes.) The nature of the “parent material,” or the rock from which the soil is derived, ranks among five key factors influencing the characteristics of soil in a given environment.
The others are climate, living organisms, topography, and time.
Parent material, climate,and organisms
Minerals, such as feldspars and micas, react strongly to natural acids carried by rain and other forms of water; therefore, when these minerals are present in the rock that makes up the parent material, they break apart quite easily into small fragments. On the other hand, a mineral that is harder—for example, quartz—will break into larger pieces of clastic, or rock, sediment. Thus, the parent material itself has a great deal to do with the initial grain of the sediment that will become soil, and this in turn influences such factors as the rate at which water leaches through it.
The release of chemical compounds and elements from minerals in weathering provides plants with the nutrients they need to grow, setting in motion the first of several steps whereby living organisms take root in, and ultimately contribute to, the soil. As the plant dies, it leaves behind material to feed decomposers, such as bacteria and fungi. The latter organisms play a highly significant role in the biogeochemical cycles whereby certain life-sustaining elements are circulated through the various earth systems.
In addition, still-living plants provide food to animals, which, when they die, likewise will become one with the soil. This is achieved through the process of decomposition, aided not only by decomposers but by detritivores as well. The latter, of which earthworms are a great example, are much more complex organisms than the typically single-cell decomposers. Detritivores consume the remains of plant and animal life, which usually contains enzymes and proteins far too complex to benefit the soil in their original state. By feeding on organic remains, detritivores cycle these complex chemicals through their systems, causing them to undergo chemical reactions that result in the breakdown of their components. As a result, simple and usable nutrients are made available to the soil.
Topography and time
Then there is the matter of topography, or what one might call landscape—the configuration of Earth’s surface, including its relief or elevation. Soil at the top of a hill, for instance, is liable to experience considerable leaching and loss of nutrients. On the other hand, if soil is located in a basin area, it is likely to benefit from the vitamins and minerals lost to soils at higher elevations, which lose these nutrients through leaching and erosion.
In addition, topography influences the presence or absence of organic material, which is vital if the soil is to sustain plant life. Organic matter in mountainous areas accounts for only 1% to 6% of the soil composition, while in wet lowland regions it may constitute as much as 90% of soil content. Because erosion tends to bring soil, water, and organic material from the highlands to the lowlands, it is no wonder that lowlands are almost always more fertile than the mountains that surround them.
Finally, time is a factor in determining the quality of soil. As with everything else that either is living or contains living things, soil goes through a progression from immaturity to a peak to old age. In the earth sciences, age often is measured not in years, which is an absolute dating method, but by the relative dating technique of judging layers, beds, or strata of earth materials. (For more about studying rock strata as well as relative dating techniques, see Stratigraphy.)
Layers in the Soil
If you dig down into the dirt of your backyard, you will see a miniature record of your regions’s geologic history over the past few million years. Actually, most homes in urban areas and suburbs today have yards made of what is called fill dirt— loose earth that has been moved into place by a backhoe or some other earthmoving mechanism. Even though the mixed quality of fill dirt makes it difficult to discern the individual strata, the soil itself tells a tale of the long ages of time that it took to shape it.
Better than a modern fill-dirt yard, of course, would be a sample taken from an older community. Here, too, however, human activities have intervened: people have dug in their yards and holes have been filled back up, for instance, thus altering the layers of soil from what they would have been in a natural state. To find a sample of soil layers that exists in a fully natural state, it might be necessary to dig in a woodland environment.
Leaves on the forest floor are an example of humus, a component of the A horizon, or top-soil.
In any case, anyone with a shovel and a piece of ground that is reasonably untouched—that is, that has not been plowed up recently—can become an amateur soil scientist. Soil scientists study soil horizons, or layers of soil that lie parallel to the surface of Earth and which have built up over time. These layers are distinguished from one another by color, consistency, and composition. A cross-section combining all or most of the horizons that lie between the surface and bedrock is called a soil profile. The most basic division of layers is between the A, B, and C horizons, which differ in depth, physical and chemical characteristics, and age.
At the top is the A horizon, or topsoil, in which humus—unincorporated, often partially decomposed plant residue—is mixed with mineral particles. Technically, humus actually constitutes something called the O horizon, the topmost layer. Examples of humus would be leaves piled on a forest floor, pine straw that covers a bare-dirt area in a yard, or grass residue that has fallen between the blades of grass on a lawn. In each case, the passage of time will make the plant materials one with the soil.
Owing to its high organic content, the soil of the A horizon may be black, or at least much darker than the soil below it. Between the A and B horizons is a noticeable layer called the E horizon, the depth of which is a function of the particulars in its environment, as discussed earlier. In rough terms, topsoil could be less than a foot (0.3 m) deep, or it could extend to a depth of 5 ft. (1.5 m) or more.
In any case, the E horizon, known also as the eluviation or leaching layer, is composed primarily of sand and silt, built up as water has leached down through the soil. The sediment of the E horizon is nutrient-poor, because its valuable mineral content has drained through it to the B horizon. (The E horizon is just one of several layers aside from the principal A, B, and C layers. We will mention only a few of these here, but soil scientists include several other horizons in their classification system.)
Subsoil, regolith, bedrock
The appearance and consistency of the soil change dramatically again as we reach the B horizon. No longer is the earth black, even in the most organically rich environments; by this point it is more likely to exhibit shades of brown, since organic material has not reached this far below the surface. Yet subsoil, which is the consistency of clay, is certainly not poor in nutrients; on the contrary, it contains abundant deposits of iron, aluminum oxides, calcium carbonate, and other minerals, leached from the layers above it.
The rock on the C horizon is called regolith, a general term for a layer of weathered material that rests atop bedrock. Neither plant roots nor any other organic material penetrate this deeply, and the deeper one goes, the more rocky the soil. At a certain depth, it makes more sense to say that there is soil among the rocks rather than rocks in the soil.
Beneath the C horizon lies the R horizon, or bedrock. As noted earlier, depths can vary. Bedrock might be only 5-10 ft. deep (1.5-3 m), or it might be half a mile deep (0.8 km) or perhaps even deeper. Whatever the depth, it is here that the solid earth truly becomes solid, and for this reason builders of skyscrapers usually dig down to the bedrock to establish foundations there.
Life Beneath the Surface
The ground beneath our feet—that is, the topmost layer, the A horizon—is full of living things. In fact, there are more creatures below Earth’s surface than there are above it. The term creatures in this context includes microorganisms, of which there might be several billion in a sample as small as an acorn. These include decomposers, such as bacteria and fungi, which feed on organic matter, turning fresh leaves and other material into humus. In addition, both bacteria and algae convert nitrogen into forms usable by plants in the surrounding environment (see Nitrogen Cycle).
We cannot see bacteria, of course, but almost anyone who has ever dug in the dirt has discovered another type of organism: worms. These slimy creatures might at first seem disgusting, but without them our world could not exist as it does. As they burrow through soils, earthworms mix organic and mineral material, which they make available to plants around them. They also may draw leaves deep into their middens, or burrows, thus furnishing the soil with nutrients from the surface. In addition, earthworms provide the extraordinarily valuable service of aerating the soil, or supplying it with air: by churning up the soil continuously, they expose it to oxygen from the surface and allow air to make its way down below as well.
Nor are these visible, relatively large worms the only ones at work in the soil. Colorless worms called nematodes, which are only slightly larger than microorganisms, also live in the soil, performing the vital function of processing organic material by feeding on dead plants. Some, however, are parasites that live off the roots of such crops as corn or cotton.
Ants and larger creatures
Likewise there are “bad” and “good” ants. The former build giant, teeming mounds and hills that rise up like sores on the surface of the ground, and some species have the capacity to sting, causing welts on human victims. But a great number of ant species perform a positive function for the environment: like earthworms, they aerate soil and help bring oxygen and organic material from the surface while circulating soils from below.
In some areas, much larger creatures call the soil home. Among these creatures are moles, who live off earthworms and other morsels to be found beneath the surface, including grubs (insect larvae) and the roots of plants. As with ants and earthworms, by burrowing under the ground, they help loosen the soil, making it more porous and thus receptive both to moisture and air. Other large burrowing creatures include mice, ground squirrels, and prairie dogs. They typically live in dry areas, where they perform the valuable function of aerating sandy, gravelly soil.
Soils and Environments
In discussing our imaginary journey through the depths of the soil, it has been necessary to use vague terms concerning depths: “less than a foot,” for instance. The reason is that no solid figures can be given for the depth of the soil in any particular area, unless those figures are obtained by a soil scientist who has studied and measured the soil.
Depth is just one of the ways that the soil may vary from one place to another. Earlier we mentioned five factors that affect the character of the soil: parent material, climate, living organisms, topography, and time. These factors determine all sorts of things about the soil—most of all, its ability to support varied life-forms. Collectively, these five factors constitute the environment in which a soil sample exists.
A desert environment might be one of immature soil, defined as a sample that has only A and C horizons, with no B horizon between them. On the other hand, the soil in rainforests suffers from just the opposite condition: it has gone beyond maturity and reached old age, when plant growth and water percolation have removed most of its nutrients.
Whether in the desert or in the rainforest, soils near the equator tend to be the “oldest,” and this helps explain why few equatorial regions are noted for their agricultural productivity, even though they enjoy otherwise favorable weather for growing crops. Soils there have been leached of nutrients and contain high levels of iron oxides that give them a reddish color. Moreover, red soil is never good for growing crops: the ancient Egyptians referred to the deserts beyond their realm as “the red land,” while their own fertile Nile valley was “the black land” rainforests. If soil is so poor at the equator, why do equatorial regions such as the Congo or the Amazon River valley in Brazil support the dense, lush rain-forest ecosystems for which they are noted? The answer is that the abundance of organic material at the surface of the soil continually replenishes its nutrient content. The rapid rate of decay common in warm, moist regions further supports the process of renewing minerals in the ground.
The burrowing prairie dog helps aerate sandy, gravelly soil in dry areas.
This also explains why the clearing of tropical rainforests, an issue that environmentalists called to the world’s attention in the 1990s, is a serious problem. When the heavy jungle canopy of tall trees is removed, the heat of the sun and the pounding intensity of monsoon rains fall directly on ground that the canopy would normally protect. With the clearing of trees and other vegetation, the animal life that these plants support also disappears, thus removing organisms whose waste products and bodies would have decayed eventually and enriched the soil. Pounded by heat and water and without vegetation to resupply it, the soil in an exposed rainforest becomes hard and dry.
In deserts the soil typically comes from sandstone or shale parent material, and the lack of abundant rainfall, vegetation, or animal life gives the soil little in the way of organic sustenance. For this reason, the A horizon level is very thin and composed of light-colored earth. Then, of course, there are desert areas made up of sand dunes, where conditions are much worse, but even the best that deserts have to offer is not very good for sustaining abundant plant life.
A horizon: Topsoil, the uppermost of the three major soil horizons.
Aerate: To make air available to soil.
Atmosphere: In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%).
B horizon: Subsoil, beneath topsoil and above regolith.
Bedrock: The solid rock that lies below the C horizon, the deepest layer of soil.
Biogeochemical cycles: The changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.
Biosphere: A combination of all living things on Earth—plants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.
C horizon: Regolith, which lies between subsoil and bedrock and constitutes the bottommost of the soil horizons.
Decomposers: Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.
Decomposition reaction: A chemical reaction in which a compound is broken down into simpler compounds or into its constituent elements. In the earth system, this often is achieved through the help of detritivores and decomposers.
Detritivores: Organisms that feed on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers, but unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots.
Ecosystem: A community of interdependent organisms along with the inorganic components of their environment.
Erosion: The movement of soil and rock due to forces produced by water, wind, glaciers, gravity, and other influences. In most cases, a fluid medium, such as air or water, is involved.
Fill dirt: Loose earth that has been moved into place by a backhoe or some other earthmoving machine, usually as part of a large construction project.
Geosphere: The upper part of Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.
Humus: Unincorporated, often partially decomposed plant residue that lies at the top of soil and eventually will decay fully to become part of it.
Hydrosphere: The entirety of Earth’s water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.
Leaching: The removal of soil materials that are in solution, or dissolved in water.
Organic: At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide.
Parent material: Mineral fragments removed from rocks by means of weathering. Along with organic deposits, these form the basis for soil.
Regolith: A general term describing a layer of weathered material that rests atop bedrock.
Sediment: Material deposited at or near Earth’s surface from a number of sources, most notably preexisting rock. There are three types of sediment: rocks, or clastic sediment; mineral deposits, or chemical sediment; and organic sediment, composed primarily of organic material.
Sedimentary rock: One of the three major types of rock, along with igneous and metamorphic rock. Sedimentary rock typically has its basis in the deposition, compaction, and cementation of rock that has experienced weathering, though it also may be formed as a result of chemical precipitation. Organic sediment also may be a part of sedimentary rock.
Sedimentation: The process of erosion, transport, and deposition undergone by sediment.
Soil horizons: Layers of soil, parallel to the surface of Earth, that have built up over time. They are distinguished from one another by color, consistency, and composition.
Soil profile: A cross-section combining all or most of the soil horizons that lie between Earth’s surface and the bedrock below it.
Topography: The configuration of Earth’s surface, including its relief as well as the position of physical features.
Weathering: The breakdown of rocks and minerals at or near the surface of Earth due to physical, chemical, or biological processes.
Only those species that can endure a limited water supply—for example, the varieties of cactus that grow in the American Southwest—are able to survive. But lack of water is not the only problem.
Desert subsoils often contain heavy deposits of salts, and when rain or irrigation adds water to the topsoil, these salts rise. Thus, watering desert top-soil can make it a worse environment for growth.
In striking contrast to the barren soil of the deserts and the potentially barren soil of the rainforest is the rich earth that lies beneath some of the world’s most fertile crop-producing regions. On the plains of the midwestern United States, Canada, and Russia, the soil is black—always a good sign for growth. Below this rich topsoil is a thick subsoil that helps hold in moisture and nutrients.
The richest variety of soil on Earth is alluvial soil, a youngish sediment of sand, silt, and clay transported by rivers. Large flowing bodies of water, such as the Nile or Mississippi, pull soil along with them as they flow, and with it they bring nutrients from the regions through which they have passed. These nutrients are deposited by the river in the alluvial soil at its delta, the place where it enters a larger body of water—the Mediterranean Sea and the Gulf of Mexico, respectively. Hence the delta regions of both rivers are extremely fertile.