Soil
Soil is the unconsolidated mineral material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. Soil is found on all surfaces except on steep, rugged mountain peaks and areas of perpetual ice and snow.
Soil is related to the earth much as the rind is related to an orange. But this rind of the earth is far less uniform than the rind of the orange. Soil is deep in some places and shallow in others. Soil can be red in Georgia and black in Iowa. It can be sand, loam, or clay. Soil is the link between the rock core of the earth and the living things on its surface. Soil is the foothold for the plants we grow and the foundation for the roads we travel and for the buildings we live in.
Soil consists of mineral and organic matter, air, and water. The proportions vary, but the major components remain the same. Minerals make up 50% of an ideal soil while air and water make up 25% each. Every soil occupies space. Soil extends down into the planet as well as over its surface. Soil has length, breadth, and depth. The concept that a soil occupies a segment of the earth is called the "soil body." A single soil in a soil body is referred to as a "pedon." The soil body is composed of many pedons and is thus called a "polypedon."
Every soil has a profile or a succession of layers (horizons) in a vertical section down into the non-soil zone referred to as the parent material. Parent materials can be soft rock, glacial drift, wind blown sediments, or alluvial materials. The nature of the soil profile is important for determining a soil's potential for root growth, storage of water, and supply of plant nutrients.
Soil formation proceeds in steps and stages, none of which is distinct. It is impossible to determine where one step or stage in soil formation begins and another ends. The two major steps in the formation of soils are the accumulation of soil parent materials and the differentiation of horizons in the profile. Soil horizons develop due to the processes of additions, losses, translocations, and transformations. These processes act on the soil parent material to produce soil horizons. The intensity of any one process will vary from location to location due to five soil-forming factors:
- Soil forms from the parent materials, and they can be rock, sand, glacial till, loess, alluvial sediments, or lacustrine clays.
- Topography is the position on the landscape where the parent material is located. Positions can be summits, sideslopes, and footslopes. Erosion and the depth to the water table will influence soil formation as a result of topography.
- Climate acts on the parent material and determines the rate of rainfall and evapotranspiration, which influence the amount of leaching in the soil profile. The greater the leaching, the faster soil horizons develop.
- Biotic factors include vegetation and animals. Animals are important mixers of the soils and can destroy soil horizons. Vegetation influences soils by determining the amount of organic matter incorporated into the soils. Soils formed under prairie grasses will have a thick black surface layer, compared to soils formed under forests where the surface layer is thin with a light-colored leached zone under it.
- Time refers to the number of years that the parent material has been acted on by climate and vegetation. The older a soil is, the greater the horizon development. Young soils will have very minimal horizon development. The age of a soil is thus dependent on its development and not on the total number of years. Therefore, developed soils in the Midwest may be 10,000 years old, while young soils in the valleys of California will also be 10,000 years old. Developed soils on high terraces in California can be as old as one million years.
Because the soil-forming factors vary from location to location, soils are different across the landscape. The ability to interpret where the soils will change has allowed soil scientists to make a map of these changes. The soil map is thus an interpretation of the soils that occur on a landscape. The soil map is published on a county-by-county basis and is called a soil survey. Soil surveys are very useful documents in planning land use according to the soil's potential.
Soil is an absolutely essential natural resource but one that is both limited and fragile. The soil is often overlooked when natural resources are listed. Throughout history the progress of civilizations has been marked by a trail of windblown or water-washed soils that resulted in barren lands. Continuing to use the soil without appropriate soil conservation management is very destructive to the environment. Protecting the quality of our nation's topsoil is largely within human control. To many soil scientists, saving our soil is much more important than saving oil, coal, or natural gas resources.
Resources
Books
Miller, R. W., and R. L. Donahue. Soils: An Introduction to Soils and Plant Growth. Englewood Cliffs, NJ: Prentice-Hall, 1990.
Other
Stefferund, A., ed. Soils: 1957 Yearbook of Agriculture. Washington, DC: U.S. Government Printing Office, 1957.
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