Page 608 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

572 part IV Soils, Ecosystems, and Biomes
Earth’s landscape generally is covered with soil, a dynamic natural material composed of water, air, and fine particles—both mineral fragments (sands, silts, clays) and organic matter—in which plants grow. Soil is the basis for functioning ecosystems: It retains and filters water; is habitat for a host of micro- bial organisms, many of which produce antibiotics that fight human diseases; serves as a source of slow-release nutrients; and stores carbon dioxide and other green- house gases. Nearly 80% of terrestrial organic and inor- ganic carbon is stored in soil, and about a quarter of this amount is stored in wetlands, which are defined by the presence of hydric soils (waterlogged soils, discussed in Chapter 9 and later in this chapter). The soil carbon pool is about three times larger than the atmospheric carbon pool; only the ocean stores more carbon than soil. Remember from Chapter 11 that although about two-thirds of the recent increase in atmospheric car- bon dioxide comes from fossil-fuel burning, one-third comes from the loss of soil associated with land-use changes.
Soils develop over long periods of time; in fact, many soils bear the legacy of climates and geological processes over the last 15000 years or more. Soils do not reproduce, nor can they be re-created—they are a nonrenewable nat- ural resource. This fact means that human use and abuse of soils is happening at rates much faster than soils form or can be replaced.
Soil is a complex substance whose characteristics vary from kilometre to kilometre—and even centime- tre to centimetre. Physical geographers are interested in the spatial distributions of soil types and the physi- cal factors that interact to produce them. Knowledge of soils is also critical for agriculture and food production. Soil science is the interdisciplinary study of soil as a natural resource on Earth’s surface; the field draws on aspects of physics, chemistry, biology, geomorphology, mineralogy, hydrology, taxonomy, climatology, and car- tography. Pedology deals with the origin, classification, distribution, and description of soils (ped is from the Greek pedon, meaning “soil” or “earth”). The soil layer is sometimes called the edaphosphere (edaphos means “soil” or “ground”). Edaphology specifically focuses on the study of soil as a medium for sustaining the growth of higher plants.
In this chapter: We begin with an examination of soil development and the soil horizons of a typical soil profile. We look at the properties that affect soil fertil- ity and determine soil classification, including colour, texture, structure, consistence, porosity, moisture, and chemistry. We also discuss human impacts on soils, and desertification. We conclude with a brief exami- nation of the Canadian system of soil classification, focusing on the principal soil orders and their spatial distribution.
Soil-Formation Factors and Soil Profiles
Soil is composed of about 50% mineral and organic mat- ter; the other 50% is air and water stored in the pore spaces around soil particles. The organic matter, although making up only about 5% of a given soil volume, is criti- cal for soil function and includes living microorganisms and plant roots, dead and partially decomposed plant matter, and fully decomposed plant material that forms a nutrient-rich mixture called humus (discussed ahead).
Soil is an open system with physical inputs of inso- lation, water, rock and sediment, and microorganisms, and outputs of plant ecosystems that sustain animals and human societies, and improve air and water quality. Soil scientists recognize five primary natural soil-forming factors: parent material, climate, organisms, topography and relief, and time. Human activities, especially those related to agriculture and livestock grazing, also affect soil development and are discussed later in the chapter. Soils are assessed and classified using soil cross sections, usually extending from the ground surface to the bedrock or sediments beneath.
Natural Factors in Soil Development
As discussed in Chapter 14, physical and chemical weathering of rocks in the upper lithosphere provides the raw mineral ingredients for soil formation. Bedrock, rock fragments, and sediments are the parent material, and their composition, texture, and chemical nature help determine the type of soil that forms. Clay minerals are the principal weathered by-products in soil.
Climate also influences soil development; in fact, soil types correlate closely with climate types worldwide. The temperature and moisture regimes of climates determine the chemical reactions, organic activity, and movement of water within soils. The present-day climate is important, but many soils also exhibit the imprint of past climates, sometimes over thousands of years. Most notable is the effect of glaciations. Among other contributions, glacia- tion produced the loess soil materials that were wind- blown thousands of kilometres to their present locations (see discussion in Chapter 16).
Biological activity is an essential factor in soil devel- opment (see the Geosystems in Action, Figure GIA 18, on page 577). Vegetation and the activities of animals and bacteria—all the organisms living in, on, and over the soil, such as algae, fungi, worms, and insects—determine the organic content of soil. The chemical characteristics of the vegetation and many other life forms contribute to the acidity or alkalinity of the soil solution (soil pH is discussed in the next section). For example, broad- leaf trees tend to increase alkalinity, whereas needleleaf trees tend to produce higher acidity. When humans move