What is Soil?
What is soil?. If you've ever had the chance to dig in a garden, 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 rock or 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.
How Does Soil Form?
How does soil form?. Three processes taking place at or just below the surface of the Earth contribute to soil formation. First, chemical and physical weathering produces loose debris, new minerals (such as clay), and ions in solution. Second, rainwater 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, 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 metabolise 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. Let’s look at an idealised soil proﬁle, from top to bottom, using a soil formed in a temperate forest as our example. The highest horizon is the O-horizon (the preﬁx stands for organic), so called because it consists almost entirely of humus (plant debris) 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. 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 nature of soils that develop.
- Substrate composition: Some soils form on basalt, some on granite, some on volcanic ash, and some on recently deposited quartz silt. These different substrates consist of different materials, so the soils formed on them 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.
- 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 and help prevent soil from washing away.
Soil scientists worldwide have struggled mightily to develop a rational scheme for classifying soils. Not all schemes utilize the same criteria, and even today there is not worldwide agreement on which works best. 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 based on the physical characteristics and environment of soil formation. Canadians use a different scheme focusing only on soils that develop north of the 40th parallel. The Canadian scheme works well for cooler, high-latitude climates. As we've noted, rainfall and vegetation play a key role in determining the type of soil that forms. For example, in deserts, where there is very little rainfall and sparse vegetation, an aridisol forms. (In older classiﬁcations, these were known as “pedocal” soils.) Aridisols have no O-horizon (because there is so little organic material), and the A-horizon is thin. Soluble minerals, speciﬁcally calcite, that would be washed away entirely if there were more rainfall, instead accumulate in the B-horizon. In fact, capillary action may bring calcite up from deeper down as water evaporates at the ground surface. The calcite locally cements clasts together in the B-horizon to form a rock-like mass called caliche or calcrete. In temperate environments, an alﬁsol forms this soil has an O-horizon, and because of moderate amounts of rainfall, materials leached from the A-horizon accumulate in the B-horizon. (In older classiﬁcations, these were known as “pedalfer” soils.) In a tropical climate, oxisols develop. Here, so much rainfall percolates down into the ground that all reactive minerals in the soil undergo chemical weathering, producing ions and clay that ﬂush downward. This process leaves an A-horizon that contains substantial amounts of stable iron-oxide, aluminium-oxide, and aluminium-hydroxide residues. The resulting soil tends to be brick-red and is traditionally called laterite.
|U.S. Department of Agriculture map of soil types around the world.|
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. 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. 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 a griculture, 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.