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The Oft-ignored Yet Vital Process of Soil Formation Explained

Process of Soil Formation Explained
The process of soil formation is lengthy, occurring over a period of thousands of years. We shall explain how soil is formed, and at the same time shed light on the five factors that play a crucial role in determining the soil type.
ScienceStruck Staff
Last Updated: Mar 26, 2018
Did You Know?
The National Cooperative Soil Survey (NRCS) identifies over 20,000 different types of soils in the United States alone.
Soil is one of the three resources that are essential for the presence of life on Earth; air and water being the other two. It is basically a mixture of minerals, air, water, and organic matter (i.e., humus and microscopic organisms) which supports plant life. On an average, it comprises 45 percent mineral matter, 25 percent each of water and air, and 5 percent organic matter. Interestingly, the composition of soil is closely related to its formation process.
Soil Formation Process
It begins with the process of mechanical weathering disintegrating the parent rock to form parent material, which then comes in contact with moss and organic matter to produce a thin layer of soil. That may come across as simple, but it occurs over the course of thousands of years. When it comes to soil formation, five factors come into play: (i) parent material, (ii) climate, (iii) living organisms, (iv) topography, and (v) time. Each of these factors play a crucial role in determining the rate at which soil formation takes place, and the soil type.
Stages of Soil Formation
Mechanical Weathering
» Mechanical weathering starts disintegrating the parent rock.
Moss Growth
» Pioneer species add organic matter to the parent material.
Role of Plants
» Living organisms accelerate disintegration, and horizons start forming.
Thick Vegetation Growth
» Once the soil is fully developed, it starts supporting thick vegetation.
Parent Material: The parent material can be alluvium, colluvium, loess, lacustrine and marine sediments, or residual sediments. It forms as a result of physical, chemical, or biological weathering of the parent rock into smaller particles.

✦ Physical weathering occurs when the parent material breaks down due to the wear and tear resulting from changes in temperature, glacial activity, etc.

✦ Chemical weathering occurs when minerals within the parent material are exposed to air or water, with which they react and cause it to disintegrate.

✦ Biological weathering can be attributed to the action of living organisms such as plants, with their roots penetrating the parent rock to disintegrate it, and burrowing animals.

Climate: Besides determining what type of soil will form, climate affects soil formation by dictating the rate at which weathering occurs. If basalt breaks down under moist conditions, it results in the formation of fertile clay soil. If moist conditions are aided by high temperature, the rate of weathering gets a boost. While warm weather boosts chemical weathering by accelerating chemical reactions, cold weather boosts physical weathering by accelerating wear and tear.

Similarly, in the event of high precipitation, rainwater washes away soluble components of the soil, in what is known as leaching, thus reducing the fertility of the soil.

Living Organisms: Biota or living organisms comprise plants, animals, and microorganisms. When pioneer species die after colonizing a previously barren ecosystem, they create plant litter (i.e., dead plant matter). This plant litter eventually comes in contact with parent material and forms soil, which is necessary for the growth of new species.

Furthermore, protists to invertebrates break down detritus and contribute to the process. By resorting to the process of composting and aiding aeration, earthworms alter the soil profile and add to its fertility. High temperature increases the rate at which organisms decompose, and thus, adds to its organic value.

Topography: In geography, topography refers to the arrangement of the surface features of the region. The gradient of slope, for instance, determines the rate of drainage, erosion, and deposition, thus playing a crucial role in the soil formation process. If the soil in a valley is deeper than soil on a slope, it's because the slope is subjected to large-scale erosion, while the valley is subjected to deposition. Similarly, leaching of soil at the top of the hill as a result continuous drainage leaves it light-colored and less fertile.

Time: The soil profile is constantly changing as a result of all the aforementioned factors coming into play. The rate of change is directly dependent on the intensity of other four factors. The length of time in this case can range from decades to thousands of years, starting from the period when the said land surface became stable and enabled the process. Unlike old soil, young soil is not fully developed.
Active and Passive Factors of Soil Formation
As you can see, climate and living organisms play a direct role in mechanical weathering of the parent rock and its subsequent disintegration, and thus, are considered active factors of soil formation. In contrast, topography and time play an indirect role in this process, and thus, are considered passive factors.
These soil forming factors continue to influence soils throughout their existence, either slowly or rapidly. As long as all the intensity of these factors is the same, they are likely to produce a particular type of soil. If the intensity of any one factor decreases, or it is absent, then it will result in the formation of a different type of soil. Thus, if you want to find similar soils, you need to focus on finding areas which have similar soil forming factors.