Soil is a complex mixture of pulverized rock and decaying organic matter, which covers most of the terrestrial surface of the Earth. Soil not only supports a huge number of organisms below its surface—bacteria, fungi, worms, insects, and small mammals, which all play a role in soil formation—but it is essential to all life on Earth. Soil provides a medium in which plants can grow, supporting their roots and providing them with nutrients for growth, and filters the sky's precipitation through its many layers, recharging the aquifers and groundwater reserves from which we drink.
Background and Scientific Foundations
Soils began to form billions of years ago as rain washed minerals out of the once molten rocks that were cooling on the planet's surface; the rains leached potassium, calcium, and magnesium—minerals essential for plant growth from the rock, creating the conditions in which very simple plants could evolve. Plant life eventually spread and flourished, and as each plant died and decomposed, it added nutrients and energy to the mineral mixture, making the soil more fertile for new plants.
Soil now covers Earth in depths from a few inches to several feet, and these soils are constantly forming and changing. Soils are created from “parent” material, loose earthy matter scattered over Earth by wind, water, or glacial ice, or weathered in place from rocks.
Parent material is turned into soil as other reactions take place on exposed rock surfaces. Water-borne acids react with elements in the rock and slowly change them into soil components; minerals that break down relatively easily—feldspars and micas—become clay, the smallest soil particles with diameters less than 0.0002 mm, while harder minerals like quartz turn into sand (0.05-2.0 mm) and silt (0.0002-0.05 mm).
As the parent material weathers, the nutrients necessary for plant growth are released, and plants begin to establish themselves. As they die, they leave behind organic residues on which animals, bacteria, and fungi feed, and their consumption breaks down the organic matter further, enriching the parent material for plant growth. Over time, more and more organic matter mixes with the parent material.
Wherever soil is found, its development is controlled by five important factors: climate, parent material, living organisms, topography, and time.
A region's climate determines the range and fluctuation of temperature and the amount of precipitation that falls to Earth, which in turn controls the chemical and physical processes responsible for the weathering of parent materials, while weathering controls the rate at which plant nutrients are released. Nutrient flow, along with temperature and precipitation, determines the types of plants a region can support.
A soil’s parent material plays an important role in determining the chemistry and texture of soil. The rate at which water moves through soil is controlled in part by the texture of the soil, with soils from some parent materials weathering more or less quickly than others.
The numbers and kinds of living organisms in a given region help determine the chemical composition of soil; grassland soils are chemically different from those that develop beneath forests, and even within these broad categories of vegetative cover, soil profiles can differ.
Topography, the configuration of Earth's surface, affects soil development because it determines the rate at which precipitation washes over soil and how soils erode. Smooth, flat lands hold water longer than hilly regions, where water moves more quickly down slopes, and swamps, marshes, and bogs are formed as low-lying areas hold water over time.
Time plays an important role in soil development: soils are categorized as young, mature, or old, depending on how the above factors are combined, and the rate at which they work.
Below the surface of the Earth lie layers of soil that are exposed when people dig into the earth, or by natural forces like earthquakes; these cross-sections of soil, called soil profiles, are composed of horizontal layers or horizons of soil of varying thickness and color, each representing a distinct soil that has built up over a long time period. Soil horizons contain soils of similar ages but varying composition, that allow soil scientists to infer about a region's climate, geography, and even agricultural history by reading the story of the region's soil layers.
A soil horizon is a horizontal layer of soil with physical or chemical characteristics that separate it from layers above and below. More simply, each horizon contains chemicals, such as rust-like iron oxides, or soil particles that differ from adjacent layers. Soil scientists generally name these horizons (from top to bottom) O, A, B, C, and R, and often subdivide them to reflect more specific characteristics within each layer; considered together, these horizons constitute a soil profile.
Horizons usually form in residual soils: soils not transported to their present location by water, wind, or glaciers, but formed “in place” by the weathering of the bedrock beneath them. It takes many thousand to a million years to achieve a mature soil with fully developed horizons.
The O horizon (sometimes known as the A0) consists of freshly dead and decaying organic matter—mostly plants but also small (especially microscopic) animals. Below the O lies the A horizon, or topsoil, composed of organic material mixed with soil particles of sand, silt, and clay. Earthworms, small animals, and water mix the soil in the A horizon. Water forced down through the A by gravity carries clay particles and dissolved minerals into the B horizon in a process called leaching; these tiny clay particles zigzag downward through the spaces (pores) between larger particles. Sometimes the lower half of the A horizon is called the E (Eluvial) horizon, meaning it is depleted of clay and dissolved minerals, leaving coarser grains.
The leached material ends up in the B horizon, the Zone of Accumulation. The B horizon, stained red by iron oxides, tends to be quite clay like. If the upper horizons erode, plant roots have a tough time penetrating this clay.
Sometimes the top of the B horizon develops a dense layer called a fragipan—a claypan (compacted by vehicles) or a hardpan (cemented by minerals). In arid climates, intense evaporation sucks water and dissolved minerals upward; this accumulation creates a hardpan impenetrable to any rain percolating downward, resulting in easily evaporated pools or rapid runoff. Fragipans are extremely difficult for crop roots and water to penetrate. The A and B horizons together make up the solum, or true soil.
Partially weathered bedrock composes the C horizon. Variously sized chunks of the rock below are surrounded by smaller bits of rock and clay weathered from those chunks; some of the original rock is intact, but other parts have been chemically changed into new minerals.
The R layer (D horizon) is the bedrock, or sometimes, the sediment from which the other horizons develop. Originally, this rock lay exposed at the surface where it weathered rapidly into soil. The depth from the surface to the R layer depends on the interrelationships between the climate, the age of the soil, the slope, and the number of organisms. Most people do not consider the R layer soil, but include it in the profile anyway, since the weathering of this bedrock usually produces the soil above it.
In a perfect world, all soils demonstrate these horizons, however, some soils, like transported soils (moved to their present locations by water, wind, or glaciers), lack horizons because of mixing while moving or because of youth, while in other soils, the A and B rest on bedrock, or erosion strips an A, or other complicated variations.
Like all living things, soils age, with exposure to wind, rain, sun, and fluctuating temperatures combining to push soils through four stages of development: parent material, immature soil, mature soil, and old-age soil.
Parent materials are loose materials weathered from rocks. As plants establish themselves in parent material, organics accumulate, and the upper soil layer becomes richer and darker, and evolves into an A horizon, at which point, the soil has only A and C horizons and is in the immature stage, which it usually reaches in less than 100 years.
Through continued weathering and plant growth, the soil gathers more nutrients, and can support more demanding species. Soils break down into smaller particles such as clay, and as water moves down through the matrix, it carries these fine soil particles with it; as they accumulate in the underlying layer, these particles form a B horizon. Soils that have A, B, and C layers are described as mature.
Gradually, as weathering continues, plant growth and water percolation remove nearly all mineral nutrients from soil, and acidic by-products begin to develop; when a soil lacks the nutrients or contains enough acids that plant growth is slowed, the soil has reached old age.
Soil scientists have developed systems for identifying and classifying soils. Some broad systems of soil classification are used worldwide, with one of the most widely applied developed by the U.S. Department of Agriculture, which includes 11 major soil orders: alfisols, andisols, aridisols, entisols, histosols, inceptisols, mollisols, oxisols, spodisols, ultisols, and vertisols. Each major order is subdivided into suborders, groups, subgroups, families, and series.
Soils are also classified at an extremely specific level: soils are named after a local landmark such as a town, school, church, or stream near where the soil is first identified. Soils that share characteristics that fall within defined limits share the same name, and these soils form a soil series.
Plants have adapted to a variety of soils and can grow in almost every soil and under all variations of weather, yet plants grow better in some places than others, especially in places where nutrients are most readily available from the soil.
The tropical belt around Earth's equator contains the globe's “oldest” soils. Under heavy rainfalls and high temperatures, most nutrients have leached out of these soils, and they generally contain high levels of iron oxides, which is why most tropical and subtropical (lateritic) soils are red in color. Yet many tropical soils are able to support rich, dense forests because organic matter is readily available on the surface of the soil as tropical vegetation falls to the ground and decays quickly, but when tropical forests are cleared, the hot sun and heavy rains destroy the exposed organics, leaving very hard, dry soil that is poor for cultivation.
Soils in desert regions are usually formed from sandstone and shale parent rocks, and like tropical soils, contain little organic matter, in this case because the sparse rainfall in arid regions limits plant growth. Desert soils are generally light in color and shallow; desert subsoils may also contain high levels of salts, which discourage plant growth, and rise to the surface under rains and irrigation, forming a white crust as the water evaporates.
Tundra soils are dark mucky soils that cover treeless plains in arctic and subarctic regions. Below the A horizon lie darker subsoils, and below that, in arctic regions, lies permafrost. While these soils are difficult to farm because of their high water content and because permafrost prevents plant roots from penetrating very deeply, tundras naturally support a dense growth of flowering plants.
Below the great flat plains of the Midwestern United States and the grassy plains of South Africa, Russia, and Canada lie deep layers of black soil atop a limestone like layer, which has leached out of the soil into the subsoil. These soils are termed chernozem soils, (from the Russian word for “black earth”) and are highly productive.
The most productive soils for agriculture are alluvial soils, which are found alongside rivers and at their mouths, where floods bring sediments containing sand, silt, and clay up onto the surrounding lands. These are young soils high in mineral content, which act as nutrients to plants.