Unit 25

CLASSIFICATION AND MAPPING OF SOILS

Unit Overview

This unit describes the soil orders. The main sections are as follows:

  • Classifying soils
  • The soil orders
  • The spatial distribution of soils

Soils are complex entities because the same parent materials develop differently under different environmental conditions. However, soils are not classified genetically. Instead, soils are classified based on their actual, present-day characteristics.

The current Soil Taxonomy has twelve soil orders. They are entisols, histosols, vertisols, inceptisols, gelisols, andisols, aridisols, mollisols, alfisols, spodosols, ultisols, and oxisols. A soil order is a general grouping of soils with broadly similar compositions, the presence or absence of specific diagnostic horizons, and similar degrees of horizon development, weathering, and leaching.

Unit Objectives

  • To present a brief history of soil science and highlight problems in achieving a universal soil classification scheme
  • To outline the current Canadian system of soil classification
  • To outline the current U.S. Soil Taxonomy
  • To survey the 12 Soil Orders in the CSSC and examine their regional patterns on the North American and world map


Glossary of Key Terms

Alfisol One of the 11 Soil Orders of the Soil Taxonomy, found in moister, less continental climate zones than the mollisols; characterized by high mineral content, moistness, and sizeable clay accumulation in the B horizon.
Andisol One of the 11 Soil Orders of the Soil Taxonomy; established to include certain weakly developed, parent-material, controlled soils, notably those developed on volcanic ash that are very finely distributed throughout the Pacific Ring of Fire, Hawaii, and the world's other volcanic zones.
Aridisol One of the 11 Soil Orders of the Soil Taxonomy, and the most widespread on the world's landmasses; dry soil (unless irrigated) associated with arid climates, light in colour, and often contains horizons rich in calcium, clay, or salt minerals.
Entisol One of the 11 Soil Orders of the Soil Taxonomy, which contains all the soils that do not fit into the other 10 Soil Orders; is of recent origin, evinces little or modest development, and is found in many different environments.
Gelisol The newest of the 11 Soil Orders of the Soil Taxonomy, added in 1998, defined as high-latitude or high-altitude soils that have permafrost within 100 cm of the soil surface, or gelic materials within 100 cm of the surface and permafrost within 200 cm.
Histosol One of the 11 Orders of the Soil Taxonomy; organic soil associated with poorly drained, flat-lying areas that, when drained, can become quite productive in rootcrop agriculture.
Inceptisol One of the 11 Soil Orders of the Soil Taxonomy; forms quickly, is relatively young (though older than an entisol), has the beginnings of a B horizon, and contains significant organic matter.
Mollisol One of the 11 Soil Orders of the Soil Taxonomy, found in the world's semiarid climate zones; characterized by a thick, dark surface layer and high alkaline content.
Oxisol One of the 11 Soil Orders of the Soil Taxonomy, found in tropical areas with high rainfall; heavily leached and usually characterized by a pronounced oxic horizon, red or orange in colour.
Soil Order In the Soil Taxonomy, the broadest possible classification of the Earth's soils into one of the 11 major categories; a very general grouping of soils with broadly similar composition, the presence or absence of certain diagnostic horizons, and similar degrees of horizon development, weathering, and leaching.
Soil Taxonomy The soil classification scheme used by contemporary pedologists and soil geographers; evolved from the Comprehensive Soil Classification Scheme (CSCS) that was derived during the 1950s.
Spodosol One of the 11 Soil Orders of the Soil Taxonomy, which develops where organic soil acids associated with pine needle decay cause the depletion of most A horizon minerals; that A horizon is characterized by an ash-gray colour, the signature of silica that is resistant to dissolving by organic acids.
Ultisol One of the 11 Soil Orders of the Soil Taxonomy; usually quite old, not especially fertile, and located in warm subtropical environments with pronounced wet seasons.
Vertisol One of the 11 Soil Orders of the Soil Taxonomy, found in tropical as well as mesothermal wet-and-dry climates; this soil type is heavy in clay composition, cracking during the dry season and swelling with moisture when the rains return.


Unit Outline

  • Classifying soils
    • Russian beginnings
    • The Marbut System
    • Comprehensive Soil Classification System (CSCS)
    • Soil Taxonomy used today
  • The Soil Taxonomy
    • Hierarchical organization (Table 25.1)
    • Soil Orders, their properties, and their components (Table 25.2)
    • Soil distribution on a hypothetical continent
    • The 12 Soil Orders
      • entisols
      • histosols
      • vertisols
      • inceptisols
      • gelisols
      • andisols
      • aridisols
      • mollisols
      • alfisols
      • spodosols
      • ultisols
      • oxisols
  • Spatial distribution of soils
    • U.S. regional patterns (Fig. 25.1)
    • The global soil map (pp. 332-333)


Review Questions

  1. Trace the development of the Soil Taxonomy.
  2. List and briefly describe the major features of each Soil Order.
  3. Compare and contrast the regional distribution of soils across the hypothetical continent, the conterminous United States, and the Earth's landmasses.