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Soil Formation and Composition with Diagram

Soil is the uppermost weathered layer of the Earth’s crust. It is a mixture of weathered mineral rock particles, organic matter (i.e. both living and dead), water and air. Soil is a biologically active matrix and home of diverse organisms. The study of soil is called pedology.

Weathering of Soil

The process of soil formation includes the formation of unconsolidated materials by the weathering process and the soil profile development. Weathering refers to the physical disintegration and chemical decomposition of the rocks and minerals contained in them.

Formation of Soil

Physical disintegration breaks down rock into smaller fragments and eventually into sand and silt particles that are commonly made up of individual minerals. Simultaneously, the minerals decompose chemically, releasing soluble materials and synthesizing new minerals. New minerals form either by minor chemical alterations or by a complete chemical breakdown of the original mineral and resynthesis of new minerals. Based on the location of soil mineral particles formation and deposition, the soils are classified as residual soil and transported soil. 

If the soil mineral particles have been formed in place from the bedrock below, it is called residual soil. If the soil mineral particles have been carried from some other location by wind, water, gravity or ice then it is termed as transported soil. The transported soil can be classified into colluvium (transported by gravity), alluvium (transported by the movement of water), glacial soil (transported by the movement of glaciers) and eolian soil (transported by wind).

Soil Composition

The soil is composed of mineral particles, organic matters, air and water. Mineral particles in the soil include sand (0.05-2.0 mm), silt (0.002-0.05 mm) and clay (<0.002 mm). The relative proportions of sand, silt, and clay in a soil are referred as soil texture. The soil is also composed of organic matter which includes living biomass, detritus and humus. 

Humus is an amorphous and a colloidal mixture of complex organic substances. It is made up of humic and non-humic substances. Non-humic substances include carbohydrates, proteins, lignins, lipids, organic acids etc. Humic substances are stable end products derived from the decomposition of plant and animal residues. It comprises about 80 to 90% of the soil organic matter and characterized by dark-colored amorphous substance. 

The elemental analysis of humic substances reveals that they are primarily composed of carbon, oxygen, hydrogen, nitrogen and sulfur. The three fractions of humic substances are fulvic acid, humic acid and humin. These sub-divisions are arbitrarily based on the solubility of each fraction in water at different pH. Humins are not soluble in water at any pH. Humic acids are insoluble in water under acidic conditions but are soluble in water under alkaline conditions. Fulvic acids are a mixture of weak aliphatic and aromatic organic acids which are soluble in water at all pH.

Soil Formation and Composition with Diagram

Figure 1: Soil components diagram explains soil formation and composition.

Soil air is the mixture of gases that are present in soil pores that are not filled with water. Oxygen and carbon dioxide are important constituents, and their concentration in the soil affects many processes (e.g. nitrification and denitrification).

Soil water can contribute up to 30% of soil volume, and is essential for the activity and physiological functioning of organisms in the soil. The water content in soil depends on soil type. The water-holding capacity of soil is described in terms of field capacity. It represents the soil profile’s ability to retain water after the process of internal drainage has ceased. It is usually measured about two days after an infiltration event. The water holding capacity of a soil depends on soil texture, structure, organic matter content and arrangement of soil pores. Clay soil or soils with a high humus content have a large field capacity.

The water stored in the soil is taken up by the plant roots or evaporated from the topsoil (the topmost mineral horizon which is rich in humus) into the atmosphere. If no additional water is supplied to the soil, it gradually dries out. The dryer the soil becomes, the more tightly the remaining water is retained and the more difficult it is for the plant roots to extract it. At a certain stage, the uptake of water is not sufficient to meet the plant’s needs. The amount of water soil contains at which plants wilt beyond recovery is called the permanent wilting percentage.

The amount of water held by the soil between field capacity and the permanent wilting point is the plant available water or water that is available for uptake by plants. According to the nature of interaction between soil particles and water molecules, soil water may be classified into the following types:

Hygroscopic water: Water present as a thin film around soil particles and remains firmly attached is called the hygroscopic water. It is not utilized by plants hence it is unavailable to plants.

Capillary water: Water present in thin and narrow capillaries formed by soil particles and widely utilized by plants. The water present in soil, which can be utilized by plants is called chresard (available water).

Gravitational water: Water percolates deep into the soil due to the gravitational force of the Earth that constitutes groundwater. It is not available to plants.

Chemically bound water: Water present in the form of hydrated oxides of iron, aluminium, silicon, etc. is described as chemically bound water and is not available to plants.


FAQs on Soil Formation and Composition

Parent material, climate, biota (organisms), geography, and time are all considered contributors to soil formation.

The land’s shape has a big effect on how the soil develops. Because of erosion, soils on the sides of hills tend to be thin. Because of leaching losses, the soil on the tops of hills tends to be deep but lighter in colour. Most of the time, the soil in valleys is deeper, darker, and has more horizons.

Time is one of the soil formation components. As soils age, minerals undergo metamorphosis, chemical compounds and clays are transported lower through the soil profile, and organic matter accumulates. The older a soil is, the more time it has to develop.

How long it takes for soil to form depends on where it is: In places with a mild climate, it takes 200–400 years to make 1 cm of soil. In wet tropical areas, it only takes 200 years to make 1 cm of soil, but it takes 3000 years to build up enough nutrients to make the soil fertile.

Plants are broken down by animals, and eventually, so are the animals who ate them. As a result, the soil is starting to transform. Eventually, organic matter is formed through the breakdown of plant litter and animal wastes and remnants by bacteria, fungus, worms, and other burrowing organisms. Peat, humus, or charcoal are all viable options.

Soil is formed from the following five factors: (a) parent material like sand and rock; (b) weathering processes like heating and cooling, wetting and drying; (c) time; (d) creatures like earthworms; (e) human intervention and geography.

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