Minerals and Rocks: Soil, Weathering of Rocks, Soil profile (O horizon, A horizon, B horizon, and C horizon)

Minerals and Rocks: Soil, Weathering of Rocks, Soil profile (O horizon, A horizon, B horizon, and C horizon)


Minerals are inorganic crystalline materials that exist naturally and have certain chemical and physical characteristics.  A mineral or group of minerals together to form rocks.Only a small number of the more than 2000 minerals are required to identify the majority of rocks. Rocks and minerals serve as the solids’ building blocks. The soil type is determined by its structure, content, and texture.


It’s a thin layer of material that forms on the earth’s surface. Both microbes and animals that burrow call it home. Additionally, it provides materials to biota inhabitants.

Weathering of Rocks

Rocks that form soil fall into three categories: igneous, sedimentary, and metamorphic rocks. When physical, chemical, or biological forces break down or weather parent rock, soil is created. Little particles known as regoliths are created as a result. Under the guidance of additional pedogenic processes, regoliths eventually transform into mature soil.

 (a) Physical Weathering

 The term “physical weathering” refers to the process by which natural elements like gravity, water, ice, and temperature alter the rocks in regoliths without altering the materials chemically. It grows in high elevations, in latitudes, and in deserts, particularly in areas where the rocks are covered with scant flora.

 (b) Chemical Weathering

New mineral complexes are created by the chemical alteration of the parent mineral. The strongest agent for weathering is water. Water acts as a solvent to weather soluble rocks, such as limestone, gypsum, and rocks containing calcareous materials.

(c) Biological Weathering

The rocks are colonized by specific creatures that transform them into a dynamic system that stores energy and synthesizes organic material. These species include bacteria, protozoa, fungi, nematodes, lichens, and mosses. The rock’s physical structure is altered by their actions. Weathered rock transforms into regolith, which subsequently transforms back into soil. As a result, worn material goes through several intricate processes known as pedogenesis. Most of the time, pedogenesis is a biological phenomenon. Living things including bacteria, algae, fungi, lichens, insects, and molluses all contribute to various geochemical, biochemical, and biophysical reactions that occur during this process. Through these processes, the worn earth crust is transformed into genuine soil, which is made up of a mineral matrix combined with a variety of organic compounds that sustain a diverse range of microbes. Because the processes are ongoing, organic matter and materials in the form of layers are constantly being added to the growing soil. As a result, the fully grown soil has multiple horizons that extend from the surface downward. A soil profile is formed by these horizons. Soil consists of these major components:

Components of soil:

The four major components of soil are shown: inorganic minerals, organic matter, water, and air. Approximately 25% of the soil volume is made up of air, approximately 40% to 45% of the soil volume is made up of organic matter, approximately 5% of the soil volume is water, and approximately 25% of the soil volume is made up of inorganic mineral stuff.

Based on their horizons, soils are given names and categorized. There are four different strata in the soil profile:

Soil profile: This soil profile shows the different soil layers (O horizon, A horizon, B horizon, and C horizon) found in typical soils

1. The O horizon is characterized by decayed vegetation at its base and freshly decomposing organic debris, or humus, at its surface. Humus improves soil moisture retention by adding nutrients to the soil. The top layer of dirt, or topsoil, typically has a depth of two to three inches, however, this can vary greatly. For example, deep layers of topsoil are found in river deltas, like the Mississippi River delta. Organic matter is abundant in topsoil. There, microbial processes take place, which is how plants are produced.

2. The A horizon indicates the start of real mineral soil and is composed of a mixture of inorganic weathering products and organic particles. The organic stuff present causes this horizon to have a dark colour. Rainwater seeps into the soil in this location, carrying things from the surface.

3. The B horizon, also known as the subsurface, is a dense layer of soil that is created by a downward accumulation of primarily fine material. There may be calcium carbonate or nodules in the B horizon of some soils.

4. The C horizon, or soil base, The organic and inorganic material that decomposes to create soil, along with the parent material, make up the C horizon, also known as the soil base. The original material may have been brought to its current position from somewhere else or may have been formed in its natural habitat. Bedrock is located below the C horizon.

Biota of the Soil

A complex microflora and fauna as well as frequently a complex biota of higher communities are supported by the organic matter in the soil.

These can be classified as follows:

1. Microflora: It consists of algae, bacteria, and soil fungi. While soil fungi and animals develop better in acidic soils, soil bacteria do pretty well in neutral soil. While fungi rely on the decomposing organic materials in the soil, symbiotic fungi reside on the roots of plants.

2. Microfauna: The micro-fauna is made up of copepods, nematodes, mites, rotifers, and protozoa. These animals range in size from 20µ to 200µ. Protozoans that are found close to the soil’s surface include amoeba, ciliates, and flagellates.

3. Mesofauna: Mesofauna are animals that fall between 200 µ and 1 cm in size. The primary soil insect is the collembola. Ants are the most significant land animals in the Hymenoptera. In organic soil that is damp, mites thrive.

4. Macroflora: A vast range of plants, including trees, shrubs, and herbs, are supported by soil. We refer to plants that thrive in acidic soils as oxylophytes. Halophytes are plants that grow in salty soils. We refer to the plants growing on the sand as Psammophytes. Lithophytes are plants that grow on the surface of rocks, and charmophytes are plants that grow in the cracks in rocks.

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