What is Soil?

Soil covers much of the Earth's land surface area, and it is comprised of minerals (rock, sand, silt, clay), air, water, and organic matter (living organisms and the remains of plants and animals).

In a very basic sense, soil can be defined as unconsolidated mineral and organic matter on the Earth's surface that can be used as a medium for plant growth (SSSA, 1996).

A more complicated and comprehensive definition of soil is as follows: The unconsolidated mineral and organic matter on the surface of the earth that has been subjected to and shows effects of genetic and environmental factors such as climate, organisms, relief or topography, parent material, and time (SSSA, 1996).

Scientists that study various aspects of soil are referred to as soil scientists. These men and women study soils in natural landscapes and/or the laboratory to make recommendations and interpretations about soil fertility and plant growth, land use suitability, pollutant fate and transport, soil quality, soil formation, and a host of other soil physical, chemical and biological properties.

Many colleges and universities provide training and courses in the exciting field of soil science, including the Department of Soil, Environmental, and Atmospheric Sciences at the University of Missouri.

Soil Composition

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Soil consists of solids, liquids, and gases. The solid material consists of minerals and organic materials. Pores between the solid materials hold liquids and gases that are essential for plant and microorganism growth, and solid materials hold or provide nutrients that can become accessible for organism growth.

During a precipitation event or snowmelt, water infiltrates into soil through the pores space and pores become filled. As the soil begins to drain or dry, air replaces water in the pores. On average, soil contains 50% pore space which can be evenly divided between air and water.

The Soil Body

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Soil is a collection of living and non-living matter that forms a three-dimensional body (soil body) that covers the surface of the earth. The soil body begins where the atmosphere meets the soil surface and ends when bedrock or unweathered parent material is encountered.

In order to study soil, scientists examine the soil pedon. A pedon is the smallest unit than can be called a soil. A pedon is a three-dimensional sample that extends from the soil surface to the deepest roots or genetic soil horizons. The area covered by a pedon varies from 10 - 100 square feet, depending on changes in soil properties. Groups of pedons with very similar characteristics that are closely associated in the landscape are called polypedons. Polypedons that have a common set of characteristics that fall within a particular range are referred to as a soil series (Brady and Weil, 1999). Soil series are the basic units used by the U.S.D.A Natural Resources Conservation Service to map soils within the United States.

How is Soil Formed?

Soil is formed slowly through the weathering of geologic materials called parent material, and parent material is defined as bedrock or unconsolidated mineral and organic matter from which soils develop. As bedrock erodes into smaller particles near the earth's surface, organic matter decays and mixes with inorganic material (rock fragments, soil minerals, water, and gases) to form soil.

The processes of weathering include both physical and chemical weathering reactions.

Physical Weathering

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Physical weathering includes the effects of physical factors (temperature, water, and wind) on rock weathering or breakdown. For example, frost-wedging occurs when water freezes and expands in a crack within rock. When water enters into a rock and freezes it can expand up to 10% and effectively pry the rock apart. With sufficient time this process can cause the rock to fracture and break into smaller pieces. In addition to daily freeze and thaw of water in the winter, and heating and cooling of rock throughout the year also fractures rock to generate smaller particles.

Chemical Weathering

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Chemical weathering occurs when rock dissolves or the composition of a rock changes through chemical processes. The simplest chemical process is called dissolution or the dissolving of a rock or mineral. Slightly acidic rain reacting with rock will slowly dissolve rock and release the chemical components of the rock into solution.

Another form of chemical weathering is called hydrolysis. Hydrolysis reactions involve the splitting of water molecules into hydrogen (H+) and hydroxyl (OH-) ions. Hydrogen ions may then replace other cations (positively charged ions) contained within rock forming minerals.

A third form of chemical weathering involves oxidation-reduction reactions. These reactions involve the transfer of electrons to and from minerals. Most rock forming minerals (primary minerals) contain reduced iron (Fe2+), and when this iron is exposed to oxygen gas (O2) the iron becomes oxidized (iron loses an electron; Fe3+). This process disrupts the structure of primary mineral and weakens the mineral.

Why are Soils Important for Missouri and the World?

Soil is Essential in the Hydrologic, Carbon, and Nutrient Cycles.

The Hydrologic Cycle

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A large proportion of precipitation falling onto Earth's land surface area falls directly or indirectly onto soil. Depending on soil factors such as soil moisture status, texture, organic matter content, and soil structure, precipitation may infiltrate into soil where it may be stored for plant use. Additionally, some of the water may percolate downward through the soil or run over the soil surface (surface runoff). Percolation plays an important role in recharging groundwater aquifers, whereas, surface runoff often flows directly into surface water bodies (rivers, streams, and lakes) where it may be used to support aquatic ecosystems and provide sustainable water supplies.

The Carbon Cycle

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Soil is an integral component of carbon cycling on Earth. Soil provides a medium for plant growth, thus permitting plants to assimilate carbon into their biomass through photosynthesis. Upon plant or animal death, soil organisms decompose organism remains and incorporate a portion of carbon stored within these organisms into soil to form soil organic matter or humus. A portion of carbon not incorporated into soil is released to the atmosphere as carbon dioxide (CO2) via decomposer respiration. Soils have a great capacity to store soil carbon in the form of soil organic matter. Therefore, proper soil management can be used to increase soil organic matter content thus reducing atmospheric CO2 concentrations. This is very beneficial because CO2 is a known greenhouse gas that contributes to global warming.

The Nutrient Cycle

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Soils have a large capacity to retain and provide nutrients for plant growth in natural or managed ecosystems. Nutrients in soils may be derived from atmospheric deposition, plant decomposition, and rock and soil mineral weathering. These nutrients may then be retained on clay surfaces or organic matter where they can be slowly released into soil water. Plants and other organisms can then uptake nutrients from soil water to sustain and enhance growth. However, a portion of these nutrients may be lost from soils or the plant rooting zone through a process called leaching.

The Five Functions of Soil

Medium for Plant Growth

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Soil provides a physical substance that supports plants and enhances plant growth. Soil contains pores which allow gases, such as oxygen and carbon dioxide, to enter and escape from soils. Pores also absorb water and retain it for plant use. The water-holding capacity of a soil is essential for the survival of plants and soil organisms. Deep soils with the proper mixture of sand, silt, and clay can allow plants to survive during periods of drought. To sustain plant growth, soil must provide the following characteristics, which determine its productivity:

Habitat for Soil Organisms

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Soil provides ecological habitats for soil microorganisms, mammals, insects, and reptiles.

Recycling of Nutrients and Organic Wastes

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Soil is nature's recycling system for nutrients and organic wastes. Waste and organic materials from plants, animals, and humans are decomposed and assimilated into soils to from humus. Soils are able to adsorb and protect large quantities of humus and nutrients released during humus formation are converted into usable forms for plants and other organisms.

Water Supply and Purification

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Water that infiltrates into soil during rainfall can be used by vegetation or enter into groundwater aquifers (porous rock formations that store water). Pollutants in precipitation or those that enter soil solution upon water infiltration into surface soil horizons can be removed from percolating waters at deeper soil depths, thus purifying and cleansing the water. There are also processes occurring within soil that can kill potential disease-causing organisms.

Engineering Medium

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Soil is an important building material providing earthen fill and materials to create building materials (bricks), but soil is also the foundation for our daily lives. Soil supplies the foundation for roads, bridges, airports, buildings, and homes.

There are numerous soil properties that need to be considered when constructing structures on soil. Engineers must consider the bearing strength, compressibility, sheer strength, and stability of soils. Swelling properties of soils are also important because home foundations and roads can crack due to seasonal soil shrinking and swelling.