Soil - 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 FORE-RUNNERS 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. (© S. Stammers/Photo Researchers. Reproduced by permission.)
L IGHT MICROGRAPH OF BLUE - GREEN ALGAE , AN EXAMPLE OF THE SIMPLEST PLANT ORGANISMS THAT WERE THE FORE - RUNNERS OF LIFE ON THE EARTH . P LANT 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 . (
© S. Stammers/Photo Researchers
. Reproduced by permission. )

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.)



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