Soil – Forming Factors – Parent Material

Parent material refers to the substance from which the soil has been derived. While in most cases it is of geological origin, parent material can also be organic. The nature of the parent material can have a profound influence on the characteristics of the soil. For example, the texture of sandy soils is determined largely by the presence of quartz grains in the parent material, which, in turn, controls the movement of water through the soil.

The mineralogy of the parent material is mirrored in the soil and can determine the weathering process and control the natural vegetation composition. For example, lime – rich soils are generally derived from calcareous rocks (e.g. limestone and chalk) or sediments derived from such deposits. In turn, lime – rich soils can offset the development of acidic conditions but may not support organisms and plants that are not tolerant of alkaline soil conditions (e.g. rhododendrons).

There are three types of parent material:

1) Unconsolidated deposits that have been transported by ice, water, wind or gravity;
2) Weathered materials directly overlying consolidated hard rock from which they originate;
3) Organic material composed of decaying or partially decayed plant remains.

In the former two cases, the parent material can be weathered through physical destruction of rock (freezing or drying cycles) or chemical reactions (dissolution of elements). Weathered parent material is often referred to assaprolite.

While the forces created by the expansion and contraction of minerals, induced by daily temperature variations, cause rocks to shatter and exfoliate (especially in hot deserts), in most cases water is the dominant agent in weathering processes. Water can cause rocks to shatter through repeated freezing and thawing of water trapped in rock cavities. Water also initiates solution and hydrolysis (the destruction of a compound through a reaction with water that produces an acid and a base) that liberate minerals contained within the rock. Water also supports life which, in certain situations, is a major contributor to the weathering process.

Plant roots can cause physical weathering as they grow and expand inside cracks in the rocks. Roots and decaying vegetation also produce organic compounds such as solvents, acids and alkalines that enhance the actions of percolating rainwater.

The degree of weathering depends on a number of environmental factors, such as temperature (determined by climate, exposure and altitude), the rate of water percolation (determined by texture, relief, climate), the presence of oxygen (again texture and climate), the surface area of the parent material (largely determined by the geological structure) and the mineralogy of the parent material (for example, quartz is much more stable than olivine).

Weathering of minerals continues in the soil following a sequence from the least to the most stable minerals. Minerals undergo changes that cause the formation of secondary minerals and other compounds that are soluble in water (to varying degrees).

Soil is considered organic if it contains more than 20 % of organic matter. By contrast, mineral soils contain less than 20 % organic matter but can possess organic surface horizons.