Processes and factors of soil formation

Soils develop as a result of the interplay of 5 factors; Parent material, climate, organisms, relief and time.
Soil formation or soil genesis refers to changes of soil properties with time in one direction: the content of one component or mineral in a certain horizon decreases or increases‚ sedimentary layering disappears‚ etc. Mostly‚ such changes are slow and can be seen only after decades to millennia. So‚ most soil properties that change during soil formation are relatively stable. Sometimes‚ however‚ effects of soil formation can be seen within weeks or months.

Pedogenesis  is the process of soil formation as regulated by the effects of place, environment, and history. Biogeochemical processes act to both create and destroy order (anisotropy) within soils. These alterations lead to the development of layers, termed soil horizons, distinguished by differences in color, structure, texture, and chemistry. These features occur in patterns of soil type distribution, forming in response to differences in soil forming factors.

The nature of the parent material strongly influences soil properties such as texture, pH, fertility, and mineralogy. For example, coarse-grained, quartz-rich parent material such as glacial outwash generates soils that are often gravely and with a coarse (sandy) texture.

Soil parent material may be broadly grouped into the following classes:

  1. Residual or sedentary – developed in place (in situ) from the underlying rock. Typically it experienced long and intense weathering. Residual parent materials can be found overlying any rock type – provided that the landscape has been stable for a sufficient period of time for weathering to occur. This situation and type of parent material are uncommon in Canada.
  2. Transported – loose sediments or surficial materials (i.e., weathering products of rocks that are not cemented or consolidated) that have been transported and deposited by gravity, water, ice, or wind. These materials are classified on the basis of the agents responsible for their movement and deposition (see table below).
  3. Cumulose – organic deposits that have developed in place from plant residues and have been preserved by a high water table (or some other factor retarding decomposition). These deposits are widespread and not restricted to any climatic zone. Examples include peat (undecomposed or slightly decomposed organic matter) and muck (highly decomposed organic material).

Climate involves both local (microclimatic) and global (macroclimatic) considerations. The key components of climate in soil formation are moisture and temperature.

Soil moisture depends on several factors:

  • The form and intensity of precipitation (water, snow, sleet)
  • Its seasonal variability
  • The transpiration and evaporation rate
  • Slope
  • Aspect
  • Depth of soil profile
  • Soil texture / permeability of the parent material

Temperature affects the rate of mineral weathering and synthesis, and the biological processes of growth and decomposition. Weathering is intensified by high temperatures, hence weathering is stronger in the tropics than in humid regions. Temperature also influences the degree of thawing and freezing (physical weathering) in cold regions. Biological processes are intensified by rising temperatures.

Plants, animals and micro-organisms (fungi and bacteria) all affect soil formation by producing or contributing to humus production. The amount of humus in a soil is a result of how much plant material has been incorporated into it. If vegetation is sparse a soil will be low in humus and less fertile.

Relief is not static; it is a dynamic system (its study is called geomorphology). Relief influences soil formation in several ways:
• It influences soil profile thickness i.e. as angle of slope increases so does the erosion hazard
• it has an effect on climate which is also a soil forming factor
• gradient affects run-off, percolation and mass movement
• it influences aspect which creates microclimatic conditions