Soil is the mixture of minerals, organic matter, and other materials that covers most of Earth’s land. Soil is an important natural resource. Nearly all living things on land depend on soil either directly or indirectly. Human life as we know it would not be possible without soil.
Soil is a storehouse of nutrients and the decayed remains of organisms (living things). This organic matter distinguishes soil from a purely mineral substance, such as sand. Most plants are rooted in soil and get nutrients from it. Animals that feed on plants depend on the soil indirectly, as do animals that feed on other animals. In addition, many kinds of animals find shelter in the soil.
Soil also has an important influence on climate. Soil contains vast amounts of carbon in the form of organic matter. In fact, there is at least twice as much carbon stored in the soil as there is in the atmosphere. Atmospheric carbon in the form of carbon dioxide is a potent greenhouse gas (see Greenhouse effect). Greenhouse gases trap heat from the sun in Earth’s atmosphere, warming the planet. Greenhouse gases thus contribute to global warming, an increase in Earth’s average surface temperature. By storing carbon, soil helps to stabilize climate. As temperatures increase, soils may release more carbon dioxide and other greenhouse gases, which could worsen global warming. For example, large areas of the Arctic are covered in frozen soil called permafrost. When permafrost thaws, it releases large amounts of carbon dioxide.
Soil includes a remarkably rich and complex community of living things. It contains a tremendous number of organisms that depend on it directly as a source of food. These organisms include microbes, fungi, and invertebrates (animals without backbones). Less than 1/20 ounce (1 gram) of soil can contain thousands of different species (kinds) of organisms. Many organisms in the soil feed on plant litter and other organic matter in a process called decomposition. Decomposition releases and recycles nutrients, helping to preserve the balance of nature (see Balance of nature).
There are many kinds of soil. Each is distinguished by characteristics including color, mineral composition, and concentration of organic matter.
Soil forms slowly, over hundreds of years. Yet soil can quickly be swept away. Human beings must carefully manage their use of soil to prevent its loss.
Composition
Soil consists of particles of minerals and organic matter, called soil particles. The spaces between soil particles are called pore spaces. In most soils, pore spaces make up about half the volume of the soil. The pore spaces are filled with water and air. Pore spaces are also occupied by plant roots, microbes, and invertebrates.
Minerals
in the soil consist mostly of sands, silts, and clays. These materials are either composed of minerals called primary minerals, or form when primary minerals break down through environmental forces. Sands and silts develop from such primary minerals as quartz and feldspars. Clays are composed of silica and alumina. As primary minerals break down, trace amounts of such elements as calcium, phosphorus, and potassium are released. These elements provide an important source of nutrients for plants and other organisms.
Organic matter
consists of animal, plant, fungal, and microbial matter in various stages of decay. The formation of organic matter is a long process. Organisms called decomposers feed on once-living matter, breaking it down into simpler components. Decomposers include certain bacteria, fungi, and microbes. Other decomposers are invertebrates, such as earthworms and insects. Ants, beetles, earthworms, and termites also help to mix the soil and create pore spaces.
Eventually, decomposers produce organic matter called humus << HYOO muhs >> . Most humus is black or dark brown. It is spongy and holds large amounts of water. Humus binds with clay, protecting the humus from further decomposition. This binding also glues soil particles together to form soil aggregates. Soil aggregates give soil its structure, promoting the formation of pore spaces. Humus makes up only a small percentage of most soils. But it can greatly increase soil’s ability to support plants. The concentration of organic matter in soil typically ranges from 1 to 10 percent.
Warm, humid environments increase the activity of decomposers, enabling them to quickly break down and recycle organic matter. As a result, soil in such areas as tropical rain forests often is poor in nutrients. In colder climates, organic matter may build up in soils. Northern Asia, Europe, and North America have large areas of soil called peat. Peat contains high concentrations of organic matter, sometimes reaching nearly 100 percent.
Water and air.
The water and dissolved substances in pore spaces is called the soil solution. The soil solution connects the pores, enabling the movement of nutrients and organisms. It provides water for plant roots and helps them to absorb nutrients. The air in pore spaces enables organisms to breathe.
How soil forms
Soil begins to form with weathering. Weathering is the breaking down of rocks and minerals on or near Earth’s surface. There are two kinds of weathering, physical weathering and chemical weathering. Physical weathering results from such environmental forces as precipitation, wind, and temperature changes. These forces weather rocks into smaller particles but do not change their chemical composition. Chemical weathering is caused by such processes as water dissolving minerals in rocks. It changes the chemical composition of rocks. As rocks are weathered, a soil’s parent material is formed. This parent material later breaks down further into mineral particles.
The type of soil formed is influenced by many factors. These include (1) the parent material, (2) the climate, (3) land surface features, (4) the activity of living things, and (5) time.
Parent material
is the source of the primary minerals from which soil ultimately forms. For example, granite is a common parent material for many soils. As granite weathers, it breaks down into such primary minerals as quartz and feldspars. The kind of parent material helps to determine the mineral content and texture of the soil. It also helps to determine the rate of weathering. For example, granite weathers slowly compared with a sedimentary rock, such as limestone. Sands and silts are generally created through physical weathering. Clays generally result from chemical weathering. They typically form after silica and alumina are dissolved out of primary minerals.
Influence of climate.
Climate largely determines the rate of weathering. Rocks that repeatedly freeze and thaw may quickly disintegrate. Heavy precipitation accelerates both physical weathering and the chemical reactions that dissolve minerals. The temperature and moisture of the soil greatly influence the activity of living things. Microbes, fungi, and invertebrates are most active in warm, humid environments. Soils in cool, dry regions thus tend to be shallower and less developed than those in warm, humid regions.
Influence of land surface features.
The shape of the land, also called topography, helps to determine the availability of water. For example, soil at the top of a hill is often drier than soil at the bottom. Similarly, the lay of the land influences both soil temperature and the availability of water. In the Northern Hemisphere, land on the southern side of a hill receives more sunlight than land on the northern side and as a result is usually drier. These drier conditions cause slower soil development.
Effects of living things.
Living things greatly influence soil formation. Plants hold soil in place, reducing erosion and enabling more water to soak into the ground. Plant roots can grow into tiny spaces in rocks, contributing to weathering. Both living and decaying organisms contribute to weathering by releasing acids. Animals such as ants and earthworms promote weathering by creating pore spaces. These pore spaces also enable plants to flourish. Animals can also promote weathering as they feed. Their digestive acids weather material. Moreover, their waste contains a balanced mix of nutrients for plants. Bacteria and fungi help decompose organic matter in the soil, which releases minerals and nutrients. Certain microbes convert nitrogen in the atmosphere into nitrogen that can be used by other organisms. In this way, living things create conditions that enable other organisms to flourish.
Effects of time.
Soil develops slowly. It may take hundreds of years to produce just 1 inch (2.5 centimeters) of soil. Mature soils can hold more water than less developed soils. Mature soils are generally better for growing crops.
Characteristics of soils
As soil develops, it forms layers called soil horizons. Most soils include at least three major horizons: the A, B, and C horizons. The A horizon is the uppermost of the three. It is also known as topsoil. The C horizon is the lowest of the three. It is exposed to little weathering and resembles the parent material. The A and B horizons are much more developed. Many soils also include O and E horizons. The O horizon is found above the A horizon and contains mostly organic material. The E horizon is found between the A and B horizons. It contains soil in an intermediate state of development. There also may be a number of subhorizons, representing different stages of development. Scientists describe soils by the characteristics of the soil horizons, including (1) color, (2) texture, (3) structure, and (4) chemical conditions.
Color.
Soils range in color from yellow and red to gray and white to dark brown and black. These colors result from minerals and organic matter in the soil. For example, a red color usually indicates the presence of free (unbonded) iron. A dark brown color may indicate abundant organic matter.
Texture.
The texture of soil depends upon the relative amounts of sand, silt, and clay. For example, soils classified as loam contain from 7 to 27 percent clay, from 28 to 50 percent silt, and less than 52 percent sand. Soils classified as silty clay contain more than 40 percent clay and more than 40 percent silt. The texture of soil helps to determine how quickly water drains. Sandy soils drain more quickly than soils that are rich in clay.
Structure.
Soil structure creates pore spaces and protects organic matter. Soil particles can naturally bind together to form stable aggregates called peds. Peds can range from about 1/0 inch to more than 40 inches (1 to 1,000 millimeters) in diameter. The arrangement and shape of peds determine soil structure. There are four main kinds of peds: (1) platelike, (2) prismlike, (3) granular, and (4) blocklike. Platelike peds are thin and usually horizontal. Prismlike peds are shaped like columns. Granular peds are shaped like balls. Blocklike peds are cube-shaped with flat or curved sides. Most soils contain more than one kind of ped.
Chemical conditions.
Soils can be acidic, basic, or neutral. The levels of acid and base in a soil profoundly influence biological processes. Most plants grow best in roughly neutral soils. Highly acidic or basic soils can harm many plants because the chemical conditions in soil influence the availability of nutrients. Many nutrients have a positive electrical charge. Clays and organic matter tend to have a negative electrical charge. The opposite charges attract, causing nutrients to bind to clay and organic matter. This binding prevents nutrients from being washed away in drainage and runoff. It also enables nutrients to enter into the soil solution, making them available to plant roots and microbes. Through a process called cation exchange, the nutrients bound to clay and organic matter can enter the soil solution. This makes the nutrients available to plant roots and microbes. The fertility of soil depends in part on its capacity for cation exchange.
How soils are classified
Soils can be classified in a variety of ways. The United States Department of Agriculture classifies soil using a system of soil taxonomy. This system distinguishes 12 orders (groups) of soils based on color, structure, texture, and chemical composition. The orders are (1) alfisols, (2) andisols, (3) aridisols, (4) entisols, (5) gelisols, (6) histosols, (7) inceptisols, (8) mollisols, (9) oxisols, (10) spodosols, (11) ultisols, and (12) vertisols. For the distribution of soil orders, see the map Soils of the world.
Soil conservation
Soil is a vital natural resource. Most of the plants and animals that human beings eat depend upon the soil. A decline in the productivity of soil would make it difficult to feed large numbers of people. In fact, scientists have found evidence that failure to maintain soils contributed to the collapse of a number of past civilizations.
Farm soils must have adequate nutrients and organic matter to remain productive. If a single kind of crop is planted year after year, the crop may use up nutrients. This lack of nutrients reduces the productivity of the soil. Farmers may use synthetic fertilizers to restore nutrients. Farmers also may add nutrients and organic matter to soil by growing certain crops and applying manure (animal waste) or compost (decomposed plant matter).
Poor use of soil can harm the environment in a number of ways. Poorly maintained soils can erode into waterways, damaging water quality. Excessive use of fertilizers also pollutes the water. Fertilizers can cause explosive growth of plantlike algae. As the algae die, their decay uses up the oxygen in the water, creating a “dead zone” where few animals can survive. By testing the soil for nutrients, farmers can minimize use of fertilizers, reducing their environmental impact.
Much of the world’s soil is in decline because of human activities, especially agriculture. Average worldwide soil erosion rates are difficult to measure. However, some scientists estimate that roughly one-third of the world’s topsoil is eroding faster than it can be replaced. In some areas of the world, topsoil is disappearing tens of times faster than it can be replaced. Topsoil in some places has been damaged so badly that it can no longer support agriculture.
Many farmers use soil conservation methods to control erosion. Farmers may plant cover crops and rows of trees that serve as windbreaks. They also use techniques such as contour plowing, terracing, and minimum tillage (see Conservation (Soil conservation)).
The soils of grazing lands and forests also are vulnerable to erosion. When ranchers allow livestock to overgraze grasslands, the loss of anchoring plants causes soil to erode more quickly. Removing most of the trees from an area of forest can cause rapid soil loss. The soils of grazing lands can be protected by preventing overgrazing. The soils of forests can be protected through the sustainable harvesting of trees.
