Page 618 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

582 part IV Soils, Ecosystems, and Biomes
CRITICAlthinking 18.1 Soil losses—What to Do?
Using the information you just read in “Human impacts on Soils,” break down the issue of soil loss into the forcing causes, impacts produced, and possible actions to slow soil degradation and loss. Next, consider the possible actions in terms of scale, moving from individual to local to regional to state and finally to national in scope. Given the problem and the scale at which you determine solutions are best imple- mented, what actions do you suggest to reverse the situation of soil degradation here and abroad? •
conservation strategies. Many of the highest-risk lands are in India and central Asia. In fact, a 2009 mapping project showed that 25% of India is undergoing desertification.
Soil Classification
Soil classification is complicated by the variety of interactions that create thousands of distinct soils— well over 15000 soil types in Canada and the United States alone. Not surprisingly, a number of different classification systems are in use worldwide, includ- ing those from Canada, the United States, the United Kingdom, Germany, Australia, Russia, and the United Nations Food and Agricultural Organization (FAO). Each system reflects the environment of the coun- try or countries in which it originated. Because of the involvement of interacting variables, classifying soils is similar to classifying climates.
The National Soil Survey Committee of Canada developed a system suited to its great expanses of boreal forest, tundra, and cool climatic regimes. The remainder of this chapter will outline the Canadian System of Soil Classification (CSSC). Appendix B contains information about the U.S. Soil Taxonomy system for classifying soils. For information on tropical soils not in the CSSC, refer to the Soil Taxonomy in Appendix B.
The basis for a soil classification system is field observation of soil properties and morphology (appear- ance, form, and structure). The smallest unit of soil used in soil surveys is a pedon, a hexagonal column mea- suring 1 to 10 m2 in top surface area (see Figure 18.2a). A pedon is considered a soil individual, and a soil pro- file within it is used to evaluate its soil horizons. Pedons with similar characteristics are grouped to form a soil series, the lowest and most precise level of the classifica- tion system.
Diagnostic Soil Horizons
Soil scientists use diagnostic soil horizons to group soils into each soil series. A diagnostic horizon has distinctive physical properties (colour, texture, structure, consist- ence, porosity, moisture) or a dominant soil process (dis- cussed below with the soil types). A diagnostic horizon
that occurs at the surface or just below it is an epipedon. It may extend downward through the A horizon and may even include all or part of an illuviated B horizon. It is visibly darkened by organic matter and sometimes is leached of minerals. Excluded from consideration as part of the epipedon are alluvial deposits, eolian deposits, and cultivated areas, because these are relatively short-lived surfaces that soil-forming processes would eventually erase. A diagnostic horizon that forms below the soil sur- face at varying depths is called a diagnostic subsurface horizon. It may include part of the A or B horizon or both.
Pedogenic Regimes
Soil-forming processes are linked to climatic regions, a connection that serves as the basis for pedogenic regimes. Before the present soil classification systems in Canada and the United States, these pedogenic regimes were used to describe soils. Although each pedogenic process may be active in several soil orders and in different climates, not all are present in Canada, such as the lateritic pro- cesses in the tropics. Such climate-based regimes are con- venient for relating climate and soil processes. However, both the Canadian and U.S. soil classification systems recognize the great uncertainty and inconsistency in bas- ing soil classification on such climatic variables. Aspects of several pedogenic processes are discussed with appro- priate soil orders:
• Laterization: a leaching process active in humid and warm climates (in the tropics and subtropics, but not in Canada);
• Salinization: a process that concentrates salts in soils in climates with excessive potential evapotranspira- tion (PE) rates, discussed with the Solonetzic order;
• Calcification: a process that produces an illuviated accumulation of calcium carbonates in continen- tal climates, discussed with the Chernozemic order (Figure 18.13);
• Podsolization: a process of soil acidification associated with forest soils in cool climates, discussed with the Podzolic order (Figure 18.21);
• Gleysation: a process that includes an accumulation of humus and a thick, water-saturated grey layer of clay beneath, and is usually found in cold, wet climates and poor drainage conditions. It is discussed with the Gleysolic order (Figure 18.17).
The Canadian System of Soil
Classification (CSSC)
Canadian efforts at soil classification began in 1914 with the partial mapping of Ontario’s soils by A. J. Galbraith. Efforts to develop a taxonomic system spread coun- trywide, anchored by universities in each province. Regional differences in soil classification emerged, fur- ther confused by a lack of specific soil details. By 1936, only 1.7% of Canadian soil (15 million hectares) had been surveyed.