Theory:

Almost all the things we see around us come from the soil. We get different kinds of resources from it. So in this topic, we are going to study about the compositions of soil.
Formation of soil and types of soils
Formation of soil:
 
Soil is formed by breaking down of rocks into smaller particles by wind, water and climate. This process is called weathering. The texture of the soil depend entirely on their parent rock.
 
Composition of soil:
 
Generally, soil can be defined by its constituents as a mixture of rock particles and humus.Soil consists of minerals, humus, water, air and living organisms.
 
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Soil particles: Based on their sizes, the soil particles can be divided into four major groups viz., Clay, Silt, Sand and Gravel.
 
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Clay: Clays are formed from the smallest rock particles. These are so small that we cannot see a single clay particle. It feels smooth in texture.
 
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Silt: These are made up of rock and mineral particles larger than that of clay particles but smaller than that of sand particles. It does not feel so smooth like clay. It is deposited at the bottom of the water body.
 
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Sand: These are composed of particles larger than that of silt. These are rough to touch because of their larger size.
 
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Gravel: These are the largest sized rock particles that are present in the soil. They present as tiny stones in topsoil with minimal quantity.
 
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Humus: Microorganisms like bacteria, fungi act on the dead plants and animals and change the complicated matter into simple organic molecules. This organic matter is called humus. It makes the soil fertile due to the presence of nutrients. Manure is added into the soil to improve the water holding capacity.
 
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Soil water:
  • Spaces between the soil is filled with either water or air.
  • These spaces are narrow, and water moves down through these spaces and readily available to the plant roots.
  • Soil water is also known as capillary water because it rises through the soil's spaces using capillary action.
Below picture shows the water level rising in different soils. We can see that water level rise in clayey soil is more than that of other soils.
Because of small spaces between the particles of clayey soil than others, capillary action will be more in clayey soil compared to other soils. So the water level is high in Clayey soil.
  
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Soil air:
Soil contains air in its spaces. Soil air contains more moisture, less carbon dioxide and less oxygen as compared to the atmospheric air. During rainy seasons the soil spaces are filled with water instead of air. Due to the evaporation of water during summer, the moisture content in the soil decreases and air fills the soil spaces. This air is required for the respiration of plants and other organisms living in the soil.
 
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Living organisms:
  • Soil has a variety of living organisms such as flora, fauna, and decomposers.
  • The plants, algae, fungi, and bacteria that grow naturally in a particular area are called flora. E.g. Bacteria, actinomycetes, fungi, and algae.
  • The animals which live naturally in a particular area are called fauna of that area. E.g. Springtails, mites, nematodes, earthworms, ants, insects.
    Soil fauna and flora are crucial components of all soils.
  • Healthy soil contains various organisms that decompose plant and animal material into organic matter. These organisms include bacteria, earthworms and fungi, and are called as decomposers. Decomposers help to consume organic matter and convert it into healthier soil and remove harmful elements from the above-ground food web.
  • Earthworms move through the soil in the farms and fields. They feed on the debris in the soil. They also aid in decomposition of the organic matter. When the soil is loosened due to their activities, the roots of the crops grow well. They also enrich the soil by their excreta which act as a fertilizer. All these facts make earthworm, a farmer's friend.
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Reference:
https://commons.wikimedia.org/wiki/File:Components_of_Soil_Pie_Chart.png
https://www.gwisd.us/vimages/shared/vnews/stories/4ebbddc12d3a0/Porosity%20and%20Permeability%20Lab%20and%20Aquifer%20Lab2012.pdf
https://www.facebook.com/TenCateGeo/photos/capillary-action-will-leave-you-on-the-edge-of-your-seat-it-gives-water-the-powe/10158093715688082/