Soil Resources

Introduction

The coverage of Earth’s surface with soil varies from place to place. Tropical forests have thin soils that are poor in nutrients, while grasslands in temperate regions have soils that are rich and well able to support crops. There are several different classes of soil depending on how the soil is formed and where it is located. An understanding of which class of soil is found in a particular location is an important foundation to obtaining successful crop yields.

Soil resources need conserving as much as water resources do. Erosion is the key process by which soil is created from rock and destroyed. Wind and rain are the main factors that cause erosion of soil from agricultural areas, possibly converting them into new desert. Some modern agricultural practices leave soil exposed to the elements and thereby risk loss of long-term productivity because of soil erosion.

Historical Background and Scientific Foundations

Soil is as important as water as a resource. It provides nutrients and an anchor to the roots of plants and is therefore essential to their healthy growth and yield of food. It is a complex mixture of organic and mineral content which is constantly being formed by the weathering of rocks.

Soil has a layered structure, with the topsoil being around 4 in (10 cm) deep and rich in organic material. Then there is a layer between topsoil and subsoil through which dissolved or suspended matter moves. Below this is the subsoil, where humic compounds, clay, iron, and aluminum may accumulate after leaching from the upper layers. Finally there is the actual bedrock, which is the source of the soil through weathering processes. Soil is also classified according to the size of the particles it contains. A soil with a lot of clay has fine particles, while one with a lot of gravel is coarser.

A knowledge of soil resources, which is concerned with the type and distribution of soils around the world, is an important factor in global food supply. Soils are classified according to their type and composition. In the United States, these considerations give rise to twelve soil orders and these can be related to the type of environment where the soil is located. The best soils for farming are known as mollisols and alfisols, both of which are rich in organic matter. Both of them form wherever rainfall and precipitation are moderate. Spodosol is another class of soil; it is formed under pine forests, where the acidic needle litter will form a characteristic white and ashy looking middle layer.

Hot and rainy environments have quite different soils. Oxisols and ultisols are the two classes found in these locations. They are severely depleted of nutrients and they are red in color because they contain a lot of iron-rich minerals. In arid environments, aridosols are the predominant soil class. These are characterized by their low organic content and the presence of accumulated salt. Some classes of soil are defined by how they are formed. Andisol comes from volcanic material and vertisol from clay-rich material, for example.

Impacts and Issues

Soil is a renewable resource. It is constantly being formed and destroyed, mainly by erosion processes. Although erosion can spread rich soils by wearing down mountains, it can also lead to removal of top soils from agricultural areas. When erosion removes too much soil, the farmer will need to apply more fertilizer, which can cause pollution problems through runoff. Annual soil loss from agricultural lands amounts to as much as 25 billion metric tons, although this only leads to a loss of about 1% in crop production, because of compensating applications of water and fertilizer.

Wind and water are the main agents of soil erosion. Their impact is accelerated by removal of vegetation, as in deforestation. Soil erosion is not always compensated by the farmer and may therefore lead to desertification, in which productive agricultural land is gradually converted into desert. Desertification is particularly marked in China, Canada, and the United States. The U.S. Department of Agriculture states that 170 million acres of U.S. farmland are eroding their soil at a rate that is sure to reduce long-term productivity. Intensive farming makes a large contribution to soil erosion because practices like sowing row crops leaves soil exposed to wind and rain.

See Also Desertification; Soil Chemistry

BIBLIOGRAPHY

Web Sites

Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

Kaufmann, R., and C. Cleveland. Environmental Science. New York: McGraw-Hill International Edition, 2008.

WORDS TO KNOW

ALFISOIL: Rich soil formed under deciduous forest.

ARIDOSOL: Type of soil found in arid environments.

DESERTIFICATION: Transformation of arid or semiarid productive land into desert.

EROSION: The wearing away of the soil or rock over time.

MOLLISOL: Rich soil formed under grasslands.

RENEWABLE RESOURCE: Any resource that is renewed or replaced fairly rapidly (on human historical time-scales) by natural or managed processes.

RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.

SPODOSOL: Acidic soil formed under pine forests.

Web Sites

National Aeronautics and Space Administration (NASA). “Soil and the Environment.” August 15, 2001. http://soil.gsfc.nasa.gov/env.htm (accessed April 23, 2008).