Soil Chemistry

Introduction

Plants depend on soil for their growth because it supplies physical support for their roots and nutrients. Soil is a complex substance that is produced by the ongoing weathering of rocks. Its characteristics vary from place to place. The acidity, texture, and nutrient content of a soil affect what kind of crop can grow in it.

The nutrient content of soil cannot usually keep up with the demands for high productivity imposed by modern agriculture. Therefore, either natural or chemical fertilizers are often added to boost fertility. These can become polluting if they run off into local surface water. Soil is also prone to erosion by wind or rain and also human activities like logging, road building, mining, and overgrazing. It can also absorb pollution from air or water. Soil is a precious resource, and sustainable agriculture and pollution control are the best ways of preserving it.

Historical Background and Scientific Foundations

Soil is a complex mixture of organic and mineral components. The organic content comes mainly from humus, a material produced by the decay of the plants living in the soil. Humus gives soil its characteristic brown color and provides food for the microbial communities that live in the soil. The mineral component comes from the underlying rocks and contains various metals that can also be essential nutrients for plants growing in the soil. Humus coats the mineral content, giving soil a crumblike and spongy texture, holding onto water and nutrients that are taken up by the roots of plants. Soil may have very little organic content, such as sand, or may be almost 100% organic content, like peat.

Soil is a renewable resource. It is produced by the effects of weathering upon the underlying rocks. The weathering process is both physical and chemical. Physical weathering fragments rock into smaller particles. Chemical weathering involves a range of chemical reactions between the rock and surrounding environmental components. A soil is defined according to the size of its particles. Clay comprises the smallest particles, which are 0.002 mm in diameter, while gravel particles are the largest, with a diameter of 2 mm or more. Silt and sand are components with particle diameters that lie between these extremes.

The range of particle size in a soil determines its texture which, in turn, influences the number of pores it contains that can carry water. This is important when it comes to the accumulation of groundwater and the supply of water to plant roots. Soils dominated by clay hold water, while those dominated by sand have good drainage and dry out easily. Another important characteristic of soil is its pH, which is a measure of acidity. Few plants like to grow on an acidic soil, but pH can be corrected by the addition of chemicals.

A mature soil is layered. The topsoil is usually around 4 in (10 cm) deep and it contains material such as fallen leaves and partially decomposed organic matter as well as some minerals and living organisms. Then there is a layer between topsoil and subsoil through which dissolved or suspended matter moves. The subsoil, below this, is where humic compounds, clay, iron, and aluminum may accumulate after leaching from the upper layers. Finally there is the bedrock, which is the source of the soil through weathering processes.

Impacts and Issues

Soil is a renewable resource, for it is constantly being formed from the underlying rocks. However, weathering

WORDS TO KNOW

CLAY: The portion of soil comprising the smallest particles, resulting from the weathering and breakdown of rocks and minerals.

GRAVEL: The most coarse particles in soil.

PEAT: Partially carbonized vegetable matter that can be cut and dried for use as fuel.

SOIL: Unconsolidated materials above bedrock.

SUSTAINABLE AGRICULTURE: Agricultural use that meets the needs and aspirations of the present generation without compromising those of future ones.

WEATHERING: The natural processes by which the actions of atmospheric and other environmental agents, such as wind, rain, and temperature changes result in the physical disintegration and chemical decomposition of rocks and earth materials in place, with little or no transport of the loosened or altered material.

can take many years to produce new soil and, in the meantime, soil can be degraded by various processes. Soil erosion occurs through wind, rain, and activities like mining and agriculture. Some farming practices, such as planting rows of corn and soybean, or the eradication of all weeds with herbicides, can leave the soil very exposed to erosion. The eroded soil often ends up as sediment in surface waters, which may cause problems for aquatic ecosystems. Soil erosion may also lead to desertification, which is the conversion of fertile soil into desert.

The fertility of soil depends on its levels of nitrogen, phosphorus, and organic matter. These may not be enough to support long-term crop productivity. Some crops, like corn, tobacco, and cotton, take a lot of nutrients out of the soil. This can be replenished in various ways. For instance, crop rotation involves planting legumes, whose roots fix nitrogen into the soil. Often, fertilizer is added to enrich the soil. These might be natural, like compost or manure, or chemical, like ammonium nitrate or phosphate. Fertilizer runoff can be a major pollutant for surface waters because it can enrich them and encourage the overgrowth of algae, which can upset aquatic ecosystems.

See Also Agricultural Practice Impacts; Desertification; Geochemistry

BIBLIOGRAPHY

Books

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

Williams, I. Environmental Chemistry. Chichester: Wiley, 2001.

Web Sites

U.S. Department of Agriculture Natural Resources Conservation Service. “What Is Soil?” http://soils.usda.gov/education/facts/soil.html (accessed April 10, 2008).