11.6: Soil Forming (Pedogenic) Processes

The physical and chemical properties of a soil are determined by the soil forming process under which they form. Though all soils are created by the various horizon development processes of additions, transformations, translocation and removals, it is the soil forming or, pedogenic processes that determines the kind of soil that is ultimately formed.

Laterization

Figure \(\PageIndex{1}\): This ultisol displays the typical features of a soil having undergone laterization (Image source: USDA NRCS. Used with permission)

The deep red to bright orange-red soils of the tropics are a product of laterization. Laterization occurs in the hot, rainy tropics where chemical weathering proceeds at a rapid rate. Soils subject to laterization tend toward the acidic and lack much organic matter as decomposition and leaching is extreme. Exposure of the soil to the hot tropic sun by deforestation bakes the soil dry, reducing infiltration, increasing runoff, and reducing fertility.

Calcification

Figure \(\PageIndex{2}\): Mollisol soil enriched with calcium carbonate (Image source: Agriculture Agri-Food Canada. Used with permission)

Calcification occurs in warm, semi-arid environments, usually under grassland vegetation. Soil tends to be rich in organic matter and high in soluble bases. The B horizon of the soil is enriched with calcium carbonate precipitated from water moving downward through the soil, or upward by capillary action of water from below.

Podzolization

Podzolization occurs in cool and moist climates under pine forests. They are typical of the colder portions of the humid continental and subarctic climates. The E horizon is heavily leached and basically composed a of light colored layer of sand.

Figure \(\PageIndex{3}\): Typical ashy, gray layer left from leaching of sesquioxides is apparent in this podzolized soil. (Image source: Agriculture Agri-Food Canada. Used with permission)

The upper portion of the B horizon is stained reddish color from the accumulation of sesquioxides. The profile gets lighter in color as depth increases. Podzolization of sandy soils in the southern United States has been the result of planting pine plantations.

Salinization

Figure \(\PageIndex{4}\): Accumulation of salts is easily seen in this salinized soil (Image source: Agriculture Agri-Food Canada. Used with permission)

Salinization occurs in warm and dry locations where soluble salts precipitate from water and accumulate in the soil. Saline soils are common in desert and steppe climates. Salt may also accumulate in soils from sea spray. The rapid evaporation of salt-rich irrigation water has devastated thousands of acres of land world-wide.

Gleization

Figure \(\PageIndex{5}\): Cultivated gley soil in southern Canada (Image source: Agriculture Agri-Food Canada. Used with permission)

Gleization occurs in regions of high rainfall and low-lying areas that may be naturally waterlogged. Bacterial activity is slowed in the constantly wet environment thus inhibiting the decomposition of dead vegetation allowing it to accumulate in thick layers. Peat is found in the upper portion of the soil. Decaying plant matter releases organic acids that react with iron in the soil. The iron is reduced rather than oxidized giving the soil a black to bluish - gray color.

Self-Assessment \(\PageIndex{1}\)

Which soil forming process is active in the Amazon Basin of Brazil?

Calcification, Laterization, Podzolization, or Salinization?

Answer

Laterization