Guideline Index

Chapter 4: Soil Properties

4.6 Soil formation

Understanding the soil formation and composition of your soil is important, as the parent material will dictate how the soil will behave. Understanding soil formation also helps in understanding which parts of the landscape certain soil types are likely be found. Having this understanding can help to guide land-use decisions and management.

4.6.1 How soils are formed

Soil formation is a function of regional climate, parent material, topography, relief, biological factors and time. Parent material and landform are the initial reference states for a soil and climate and biological factors determine the rate of soil development. Time determines the stage of the soil forming processes as per Figure 4.8 below.

Figure 4.8  Factors that determine soil formation.  Source: Hans 1980, pg. 10.
Figure 4.8 Factors that determine soil formation. Source: Hans 1980, pg. 10.

Soils are formed when inorganic matter (minerals) and organic matter breaks down into small particles during a weathering process. Weathering can be a mechanical, chemical or biological process.

Inorganic particles are classified by size as gravel or stone, sand, silt, or clay. The size of the inorganic particles determines soil texture.

The inorganic portion of the soil is formed over many years from solid rock (bedrock) found in the earth’s crust. These rocks are classified as:

  • Igneous rock such as granite and basalt, formed from volcanic lava.
  • Sedimentary rock, such as limestone, sandstone, mudstone, shale, dolomite and conglomerates, formed from the deposit and cementation of the weathering products of other rocks.
  • Metamorphic rock, such as gneiss, schist, quartzite, slate and marble, formed from igneous or sedimentary rocks subjected to high temperatures or pressures.

Weathering of the original bedrock produces parent material for mineral soils. Weathering of the bedrock causes fragments to break off and when subject to further weathering become mineral particles. As the mineral particles continue to weather, they are further decreased in size and also release soluble materials, some of which become plant nutrients – See Figure 4.9.

Figure 4.9  Trends in weathering conditions that take place under acid conditions common in humid-temperate regions.  Source: Adapted from Buckman and Brady (1960).
Figure 4.9 Trends in weathering conditions that take place under acid conditions common in humid-temperate regions. Source: Adapted from Buckman and Brady (1960).
4.6.1.1 Mechanical weathering

Mechanical weathering is caused by:

  • Temperature changes, such as freezing of the water in a rock or the different rates of expansion of the minerals composing the rock.
  • Erosion and deposition from water, ice and wind.

Mechanical weathering essentially breaks the bedrock into smaller and smaller pieces and may move it from its place of origin, but it doesn’t change its chemical composition.

4.6.1.2 Chemical weathering

Chemical weathering is caused by:

  • Hydrolysis – the reaction between water and a compound
  • Hydration – the chemical union of water and an ion
  • Carbonation – where carbon dioxide is dissolved into a liquid
  • Oxidation – the loss of an electron by a substance, therefore gaining a positive charge.
  • The solvent action of the soil solution (water and its soluble salts).

Chemical weathering continues to reduce the size of rock fragments and mineral particles and also changes their chemical composition.

 4.6.1.3 Biological weathering

Biological weathering involves chemical or physical weathering processes caused by an organism. For example;

  • Mechanical weathering of rocks by plant roots or burrowing animals
  • Chemical weathering caused by lichen releasing chelating agents

Mechanical weathering also determines whether the parent material is considered to be sedentary or transported – See Figure 4.10. Sedentary parent material is either still at its original site above the bedrock from which it was formed (residual soils) or has been moved by gravity down a slope (colluvial soils). Transported parent material has been moved by water (alluvial, marine, or lacustrine soils), ice (glacial soils) or wind (aeolian soils) from its place of origin.

Figure 4.10   Diagrammatic representation of sedentary and transported soils.   Source: Adapted from Buckman and Brady (1960).
Figure 4.10 Diagrammatic representation of sedentary and transported soils.
Source: Adapted from Buckman and Brady (1960).

4.6.2 How soil formation affects soil properties

The parent material that forms a soil will affect is properties. For example, a quartz-based granite will weather into a sandy soil, which will have a lower water-holding and nutrient-holding capacity than a loam or clay soil. Soil formed from limestone may be alkaline (have a high pH) because limestone consists largely of the mineral calcite (CaCO3).

The weathering process that forms a soil also affects its properties. For example, less chemical weathering occurs in arid (low rainfall) regions than in humid (higher rainfall) regions. This results in the formation of less clay particles and nutrients in arid zones. Rainfall also acts to leach nutrients in higher rainfall areas. This is part of the reason why arid regions often have alkaline soils, and humid regions often have acid soils. It also helps to explain why high rainfall areas often have soils with poor fertility: many of the nutrients have been chemically weathered and then leached from the soil.

The weathering process also influences the soils ability to hold onto nutrients. As soil particles develop during formation, silt and sand sized particles remain relatively inert; however clay sized particles can develop a negative charge. This charge can attract and hold positively charged particles called cations and can be measured as the cation exchange capacity of the soil.