Unit 23

FORMATION OF SOILS

Unit Overview

This unit examines the processes responsible for the formation of soils as well as the general characteristics of the structural layering of soils. The main sections are as follows

  • The formation of soil
  • Processes in the soil
  • Soil profiles
  • Soil regimes

Soil is comprised of minerals, organic matter, water, and air. Factors involved in the formation of soil include the parent material, climate, biological agents, topography, and time. Processes acting in the soil are addition, transformation, depletion, and translocation. These processes are highly variable over space and over time. Well-developed soils have soil profiles with distinct soil horizons. Each horizon has a different composition and different properties, which are influenced by processes both within and without the soil. The interplay of the above factors produces different soils throughout the world, but similar to climates, soils can be placed into general classes known as soil regimes.

Unit Objectives

  • To understand the components of soil
  • To outline the factors affecting soil formation
  • To describe and explain a typical soil profile and the processes responsible for the formation of soil horizons


Glossary of Key Terms

Addition The soil-layer formation process involving the gains made by the soil through the adding of organic matter from plant growth, or sometimes when loose surface material moves downslope and comes to rest on the soil; expressed as a dark colored upper layer, whose appearance is attributable to that added organic matter.
Depletion The soil-layer formation process involving the loss of soil components as they are carried downward by water, plus the loss of other material in suspension as the water percolates through the soil from upper to lower layers; while the upper layers are depleted accordingly, the dissolved and suspended materials are re-deposited lower down in the soil.
Eluviation Means "washed out," and refers to the soil process that involves the removal from the A horizon and downward transportation of soluble minerals and microscopic, colloid-sized particles of organic matter, clay, and oxides of aluminum and iron.
Humus Decomposed and partially decomposed organic matter that forms a dark layer at the top of the soil; contributes importantly to a soil's fertility.
Illuviation The soil process in which downward-percolating water carries soluble minerals and colloid-sized particles of organic matter and minerals into the B horizon, where these materials are deposited in pore spaces and against the surfaces of soil grains.
Leaching The soil process in which downward-percolating water dissolves and washes away many of the soil's mineral substances and other ingredients.
Nonrenewable resources One that when used at a certain rate will ultimately be exhausted (metallic ores and petroleum being good examples).
Parent material The rocks of the Earth, and the deposits formed from them, from which the overlying soil is formed.
Renewable resources Resources that can regenerate as they are exploited.
Residual soil The simplest kind of soil formation in which a soil forms directly from underlying rock; when this occurs, the dominant soil minerals bear a direct relationship to that original rock.
Soil A mixture of fragmented and weathered grains of minerals and rocks with variable proportions of air and water; the mixture has a fairly distinct layering, and its development is influenced by climate and living organisms.
Soil horizon Soil layer; the differentiation of soils into layers is called horizonation.
Soil profile The entire array of soil horizons (layers) from top to bottom.
Soil regime The variations in behavior of changeable elements in the soil-formation environment; the term implies there is some regularity in the (spatial) pattern, but recognizes changes within it. Thus, even if their parent material remained constant allover the world, soils would differ because they would form under varying temperature, moisture, biogeographic, and other conditions.
Transformation The soil-layer formation process involving the weathering of rocks and minerals and the continuing decomposition of organic material in the soil; weathering is most advanced in the upper soil layers.
Translocation The soil-layer formation process involving the introduction of dissolved and suspended particles from the upper layers into the lower ones.
Transported soil When a soil is totally independent of the underlying solid rock because the parent material has been transported and deposited by one or more of the gradational agents, often far from its source area.


Unit Outline

  • Introduction to soil
    • Definitions of soil
    • Soil as a renewable resource, capable of regeneration
  • The formation of soil
    • Soil components
      • minerals
      • organic matter
      • water
      • air
    • Factors in the formation of soil
      • parent material
        • residual soils
        • transported soils
      • climate
        • atmospheric temperature
        • soil moisture
        • wind
      • biological agents
        • organic matter, including humus
        • plant nutrients
        • earthworms and other macroorganisms
      • topography
        • slope steepness
        • drainage conditions
      • time
        • natural stability of soil environment
        • human abuses and soil degradation
  • Processes in the soil
    • Addition (of organic matter, especially vegetation)
    • Transformation (rock weathering and decomposition of organic material)
    • Depletion (leaching of soil components by downward-percolating water)
    • Translocation (re-deposition by downward movement of soil particles)
  • Soil profiles
    • Horizonation (layering)
    • Master Horizons: 0, A, E, B, C, R
  • Soil regimes
    • Spatial patterns of soil formation
    • Soil development in different environmental regions


Review Questions

  1. How do the five major factors of soil formation relate to the four processes of soil formation?
  2. What is leaching and what role does it play in soil formation?
  3. Draw a model soil profile containing each of the six master horizons.