Soil Forming Factors

It has been pointed out that developing a solid understanding of soil requires a consideration of the history of soil formation as well as an ability to describe the current condition and possible future scenarios. Soil as it exists at any one time on the Earth's surface is formed by a combination of five factors, called the 'factors of soil formation':

1. parent material,

2. climate,

3. organisms,

4. topography, and

Since alteration of geologic material typically begins the process of soil formation, exactly what constitutes soil parent material is a matter of some contention among soil scientists. Where soil forms on solid rock, change is initially dominated by disintegration called 'physical weathering' due to thermal expansion and contraction of particles (i.e., individual grains of different minerals in rock expand and contract at different rates in relation to each other), movement and abrasion, pressure from freezing of water, and root penetration and swelling. As physical weathering proceeds, changes are primarily physical, and the original rock minerals persist. Disintegration increases the surface area exposed to further weathering by chemical as well as physical processes. In this case, the zone of highest change from original rock properties is closest to the surface where the material has the greatest exposure to the effects of climate and organisms. Soil becomes increasingly like the original rock from which the soil formed with additional depth. Where relatively little time has passed since the rock has been exposed (i.e., recent deglaciation or accelerated soil erosion), the properties of surface soil material can be quite similar to the properties of the rock, with soil minerals being largely unchanged chemically.

Climate has an impact on soil formation by regulating the rate of physical, chemical, and biological activity. For instance, the soil profile will form more quickly under conditions of high temperature and high precipitation. Higher temperatures will increase reaction rates and higher precipitation can more rapidly move the products of soil weathering deeper into the soil profile. The soil profile forms particularly quickly in humid tropical regions, whereas soil forms very slowly in very cold or very dry regions.

The primary influence of topography is through the impact of movement of material by erosion from zones of high slope to areas lower in the landscape. Where the products of soil formation are removed through erosion, soil profiles will be shallower, and where the products of erosion deposit, soils will be deeper.

The relationship between organisms and the soil is complex. As stated previously, nutrients stored in soil are an extremely important resource regulating the productivity of plants. However, organisms also influence soil formation. Carbon captured through photosynthesis is the ultimate source of all soil organic matter. Organic matter increases aggregation, porosity, and the water- and nutrient-holding capacity of soils. In addition, organic acids, which are exuded by plant roots, play an important role in the chemical weathering of many minerals. The influence of organisms on soil formation is not limited to plants. Insects and microorganisms are constantly reprocessing organic matter that comes into contact with the soil surface. Additionally, macrofauna play an important role in the vertical and horizontal mixing of the soil. For instance, the arctic ground squirrel (Citellus undulates) can excavate over 15 t of soil per hectare per year for the construction of their dens. The common earthworm can consume up to a third of its own body weight in soil per day.

All of these effects are integrated over time. When the soil-forming processes have had little time to change the parent material, soil properties will be very similar to the parent material, and contain largely the same original minerals. Where larger amounts of time have passed, and soil formation has proceeded further, the soil may bear no physical or chemical resemblance to the parent material. In fact, in some soils all of the minerals originally present have been altered and new minerals make up the entire surface soil horizons.

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