If you've ever had the chance to dig in a garden, field, or forest floor, you've seen first hand that the material in which ﬂowers grow looks and feels different from beach sand or potter’s clay. We call the material in a garden “dirt” or, more technically, soil. Soil consists of sediment that has been modiﬁed by physical and chemical interaction with organic material, rainwater, and organisms over time. Soil is one of our planet’s most valuable resources, for without it there could be no agriculture, forestry, ranching, or home gardening.

Video: "Introduction to Soil" (7:11)

Three key processes taking place at or just below the surface of the Earth contribute to soil formation:

First, chemical and physical weathering break up pre-existing material to produce loose debris, new minerals (such as clay), and ions in solution.

Second, rainwater slowly percolates through the debris and carries dissolved ions and clay ﬂakes downward. The region in which this downward transport takes place is called the zone of leaching, because leaching means extracting, absorbing, and removal. Farther down, new mineral crystals precipitate directly out of the water or form by reaction of the water with debris. Also, the water leaves behind its load of ﬁne clay. The region in which new minerals and clay collect is the zone of accumulation.

Third, living organisms (microbes, fungi, plants, and animals) interact with sediment by producing acids that weather grains, by absorbing nutrient atoms, and by leaving behind organic waste and remains. Plant roots and burrowing animals (insects, worms, and gophers) churn and break up the soil, and microbes metabolize minerals and organic matter and release chemicals.

Because different soil-forming processes operate at different depths, soils typically develop distinct zones, known as soil horizons, arranged in a vertical sequence called a soil proﬁle. The figure above (b) shows the idealized profile of a soil formed in a temperate forest.

The highest horizon is the O-horizon (the preﬁx stands for organic), so called because it consists almost entirely of humus (the organic component of soil, formed by the decomposition of leaves and other plant material by soil microorganisms) and contains barely any mineral matter. Below the O-horizon, we ﬁnd the A-horizon, in which humus has decayed further and has mixed with mineral grains (clay, silt, and sand). Water percolating through the A-horizon causes chemical weathering reactions to occur and produces ions in solution and new clay minerals. Downward-moving water eventually carries soluble chemicals and ﬁne clay deeper into the subsurface. The O- and A-horizons constitute dark-gray to blackish-brown topsoil, the fertile portion of soil that farmers till for planting crops. (In some places, the A-horizon grades downward into the E-horizon, a soil level that has undergone substantial leaching but has not yet mixed with organic material). Beneath the A-horizon (or the A- and E-horizons) lies the B-horizon. Ions and clay accumulate in the B-horizon, or subsoil. Note from our description that the O-, A-, and E-horizons make up the zone of leaching, whereas the B-horizon makes up the zone of accumulation. Finally, at the base of a soil proﬁle we ﬁnd the C-horizon, which consists of material derived from the substrate that’s been chemically weathered and broken apart, but has not yet undergone leaching or accumulation. This is also called the zone of weathering. The C-horizon grades downward into unweathered bedrock, or into unweathered sediment.

As farmers, foresters, and ranchers well know, the soil in one locality can differ greatly from the soil in another, in terms of composition, thickness, and texture. Indeed, crops that grow well in one type of soil may wither and die in another nearby. Such diversity exists because the make up of a soil depends on several soil-forming factors:

Climate: Large amounts of rainfall and warm temperatures accelerate chemical weathering and cause most of the soluble elements to be leached. Small amounts of rainfall and cooler temperatures result in slower rates of weathering and leaching, so soils take a long time to develop and can retain unweathered minerals and soluble components. Climate is the single most important factor in determining the rate and nature of soil development, and the degree to which soluble minerals and organic matter are distributed in the soil.
Substrate composition: Some soils form on basalt, some on granite, some on volcanic ash, and some on recently deposited quartz sand. These substrates contain different minerals and, therefore, different proportions of chemical compounds. As a result, the soils formed on these different substrates end up with different chemical compositions.
Slope steepness: A thick soil can accumulate under land that lies flat. But on a steep slope, weathered rock may wash away before it can evolve into a soil. Thus, all other factors being equal, soil thickness increases as the slope angle decreases.
Wetness: Depending on the details of local topography and on the depth below the surface at which groundwater occurs, some soil is wetter than other soil in the same region. Wet soils tend to contain more organic material than do dry soils.
Time: Because soil formation is an evolutionary process, a young soil tends to be thinner and less developed than an old soil. The rate of soil formation varies greatly with environment. A thick soil takes millennia to form.
Vegetation type: Different kinds of plants extract or add different nutrients and quantities of organic matter to a soil. Also, some plants have deeper root systems than others, which help prevent soil from washing away.

Video: "Soil and Soil Dynamics" (9:21)

Researchers have developed classification schemes for soils, using specific names for different types.

Desert soil: In deserts, where hardly any rain falls and little or no vegetation grows, an aridisol forms (also known as pedocal). Such a soil has no O-horizon because no organic matter accumulates. It has no A-horizon because hardly any leaching takes place. In fact, soluble minerals such as calcite, which would be completely leached out of soil where rainfall is heavy, tend to accumulate in the B-horizon of an aridisol. This calcite can cement grains into a rock-like mass called a caliche (also known as calcrete).

Temperate soil: In temperate environments, where moderate amounts of rain fall, an alfisol forms (also known as pedalfer). Because a large quantity of organic matter can accumulate in a temperate environment, an alfisol has a significant O-horizon. And because a relatively large amount of water percolates through an alfisol, the B-horizon accumulates only relatively insoluble minerals.
Tropical soil: In a tropical climate, where heavy rains fall, a distinct brick-red soil called an oxisol (also known as laterite) develops. The large volume of water percolating down through an oxisol leaches out most soluble minerals. As a result, the A-horizon consists mostly of insoluble residue of iron and aluminum oxide, which gives oxisol a brick-red color.

In the United States, a country that includes many climates at mid-latitudes, many soil scientists use the U.S. Comprehensive Soil Classiﬁcation System, which distinguishes among 12 orders of soil (see figure below) based on the physical characteristics and environment of soil formation. As we've noted before, rainfall and vegetation play a key role in determining the type of soil that forms.

Video: "Soil Orders" (8:16)

As we have seen, soils take time to form, so soils capable of supporting crops or forests are a natural resource worthy of protection. However, agriculture, overgrazing, and clear cutting have led to the destruction of soil. Crops rapidly remove nutrients from soil, so if they are not replaced, the soil will not contain sufﬁcient nutrients to maintain plant life. When the natural plant cover disappears, the surface of the soil becomes exposed to wind and water. Actions such as the impact of falling raindrops or the rasping of a plow break up the soil at the surface, with the result that it can wash away in water or blow away as dust. When this happens, soil erosion, the removal of soil by running water or by wind, takes place. In some localities, erosion carries away almost six tons of soil from an acre of land per year. Human activities can increase rates of soil erosion by 10 to 100 times, so that it far exceeds the rate of soil formation. The consequences of rainforest destruction have particularly profound effects on soil. In an established rain forest, lush growth provides sufﬁcient organic debris so that trees can grow. But if the forest is logged, or cleared for agriculture, the humus rapidly disappears, leaving laterite that contains few nutrients. Crop plants consume whatever nutrients remain so rapidly that the soil becomes infertile after only a year or two, useless for agriculture and unsuitable for regrowth of rainforest trees. Droughts exacerbate the situation. For example, during the 1930s a succession of droughts killed off so much vegetation in the American plains that wind stripped the land of soil and caused devastating dust storms. Large numbers of people were forced to migrate away from the Dust Bowl of Oklahoma and adjacent areas.

Video: "Surviving The Dust Bowl (American Experience)" (2:11)

In 2012, the famous videographer Ken Burns produced "The Dust Bowl", which chronicles the worst man-made ecological disaster in American history, in which the frenzied wheat boom of the "Great Plow-Up", followed by a decade-long drought during the 1930s, nearly swept away the breadbasket of the nation. You can visit the website of the series HERE. The short video below introduces the film.

Video: "Uncovering the Dust Bowl" (5:55)