Ol

Steppes Semidesert and desert Savannas Tropic forest zone Savannas

Fresh rock

Little chemical alteration Illite-montmorillonite

Fresh rock

Little chemical alteration Illite-montmorillonite

Moisture and temperature both diminish to the north; hence, weathering is slow, organic matter decomposition is slow, and either the slight precipitation (polar desert) or permafrost (tundra) inhibit leaching of mobile constituents

High moisture promotes high leaching; relatively low temperature results in relatively low rate of organic matter decomposition

High temperature aids chemical weathering, but low moisture inhibits vegetation, organic-matter buildup, and leaching of mobile constituents.

High moisture and temperature result in rapid weathering and high rate of leaching of mobile constituents

Inceptisol

Spodosol, Alfisol, Mollisol

Aridisol

Ultisol, Oxisol

FIGURE 2.1 Environmental factors affecting the distribution of terrestrial biomes and formation of soils along a transect from the equator to the north polar region (with permission from Birkeland, 1999).

Mineral soils are formed by the physical and chemical weathering of the rocks and minerals brought to the earth's surface by geological processes. The parent material of mineral soils can be the residual material weathered from solid rock masses or the loose, unconsolidated materials that often have been transported from one location and deposited at another. The disintegration of rocks into smaller mineral particles is a physical process brought about by heating and cooling, freezing and thawing, and abrasion from wind, water, and ice masses. Chemical and biochemical weathering processes are enhanced by the presence of water, oxygen, and the organic compounds resulting from biological activity. These reactions convert primary minerals such as feldspars and micas to secondary minerals such as silicate clays, and they promote the release of constituent elements in soluble forms available to soil organisms and plants.

Physical and chemical weathering of rocks to fine particles with large surface areas and the accompanying release of plant nutrients initiate the soil-forming process (Fig. 2.2).

The initial colonizers of soil parent material are usually organisms capable of both photosynthesis and N2 fixation. Early plant vegetation has intimate root-bacterial/ fungal/actinomycetal associations with soil organisms to assist with supplying nutrients and water.

Soil organisms together with plants constitute one of the five interactive factors responsible for soil formation. By 1870, Russian soil scientists had developed the concept of soils as independent natural bodies each possessing unique properties resulting from parent material, climate, topography, and living matter, interacting

TIME-► WEATHERING -► HUMUS FORMATION -► PROFILE DEVELOPMENT

FIGURE 2.2 Interrelationships of organisms, organic matter, and parent materials in soil development.

TIME-► WEATHERING -► HUMUS FORMATION -► PROFILE DEVELOPMENT

FIGURE 2.2 Interrelationships of organisms, organic matter, and parent materials in soil development.

over time. The approach to describing soil genesis as a biochemical product of organisms participating in their own genesis was quantified by Hans Jenny in his now classic equation of soil-forming factors:

Soil = f [parent material, climate, living organisms, topography, time].

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