Mineral and Organic Constituents of Soils and their Role | Agriculture Optional Notes for UPSC PDF Download

Soil Formation

Soil represents the Earth's surface composed of broken-down rocks (inorganic elements) and decomposing organic matter. The gradual breakdown of inorganic rocks contributes various essential minerals to the soil, which play a crucial role in sustaining its ability to support life.
Soil formation is a gradual process consisting of two distinct stages:

1. Weathering

This is the process by which large rocks are broken down into smaller mineral particles. Weathering occurs through three primary mechanisms:

• Physical weathering: Involves the crushing of rocks through various means, such as heating, cooling, freezing, abrasion from rain, wave action, wind action, and the movement of stones. Sunlight, water, and wind are the main factors responsible for physical weathering.
  • Sun: During the day, rocks heat up and expand due to solar radiation, contracting at night. As these pieces of rock don't expand and contract uniformly, cracks develop, leading to the fragmentation of rocks.
  • Water: Water infiltrates cracks and, upon freezing, expands, exerting pressure on the rocks and causing them to break.
  • Winds: Strong winds gradually wear away rocks through continuous friction and erosion.
• Chemical weathering: Involves processes such as hydrolysis, hydration, oxidation, and reduction in the breakdown of rocks. The primary end-products of chemical weathering are silica, clay, inorganic salts, and hydrated oxides.
• Biological weathering: Biological weathering is initiated by lichens and mosses. Lichens that grow on rocks produce acids that corrode the rock surface, forming a thin layer of soil. In some cases, tree roots penetrate cracks in rocks, expanding and contributing to the rock's fragmentation.

2. Paedogenesis (soil development)

This refers to the gradual decomposition of organic matter by bacteria and fungi, leading to the processes of humification and mineralization.

• Humification: This is the transformation of detritus, which includes the deceased remains of plants and animals, into a dark-colored, shapeless material known as humus. Humus plays a crucial role in soil fertility as it serves as a rich source of minerals for plants.
• Detritivores: Organisms like nematodes, earthworms, millipedes, and mites are considered detritivores because they consume organic matter and release nitrogen in the process.
• Mineralization: This process involves the conversion of organic substances into inorganic ones, releasing carbon dioxide and various nutrients like ammonium (NH₄⁺), magnesium (Mg⁺⁺), calcium (Ca⁺⁺), among others.

The combination of weathering and paedogenesis of rocks results in the formation of four distinct horizontal layers that constitute the soil profile.

Soil Profile

The soil profile is a vertical slice of the Earth's crust, consisting of a sequence of horizontal layers or horizons. These layers differ in terms of their thickness, color, texture, structure, firmness, permeability, acidity, and composition.

• The A-horizon represents the topsoil and consists of organic material like litter and humus.
• The B-horizon is primarily composed of mineral soil.
• The C-horizon contains loose, unconsolidated parent material.
• The D-horizon consists of solid rock and unaltered parental material.

Soil Composition

• Mineral Matter: 45 – 60%
• Organic Matter: 5 – 10% (living organisms, humans, roots of plants)
• Air: 15 – 25%
• Water: 25 – 35%

Soil Minerals

There are two categories of soil minerals, which make up a significant portion of the soil composition:

• Primary minerals in the soil: These are minerals that have remained chemically unchanged since their initial deposition. Primary soil minerals mainly consist of silica minerals, iron minerals, apatite, volcanic gases, and non-crystalline organic components.
• Secondary minerals in the soil: These are minerals that result from the weathering of primary minerals found in rocks. Examples of secondary soil minerals include hydroxides, carbonates, sulfates, phosphates, and halides.

Soil Forming Minerals

The key elements constituting the majority of minerals in soils are oxygen, silicon, iron, calcium, aluminum, sodium, potassium, and magnesium.
Below, we'll discuss some of the minerals involved in the soil formation:

• Haematite: This mineral ore, derived from iron, is reddish-black in color and has a strong water-absorbing capability, resulting in the formation of ferric oxide.
• Limonite: It's a brownish-black iron ore used as a source of ocher and umber pigments.
• Magnetite: Among iron ores, magnetite stands out for its exceptional magnetic properties.
• Siderite: Found in waterlogged soil, siderite is a valuable iron mineral containing approximately 48% iron. Its color can vary from yellow to dark brown.
• Goethite: This mineral is a weathered product of other iron oxides like limonite and pyrite, often employed as a pigment.
• Gibbsite: An important source of aluminum ore, gibbsite is typically found in highly weathered soils, often forming directly from soil solutions.
• Calcite: A carbonate mineral frequently found in sedimentary rocks, calcite exhibits a range of colors, including white, yellow, red, orange, blue, green, brown, gray, and more.
• Dolomite: This type of limestone is rich in magnesium and calcium carbonate and also contains trace amounts of other mineral elements.
• Gypsum: Commonly known as the sulfur mineral, gypsum is often found in sedimentary rocks.
• Pyrite: A bright yellow iron sulfide mineral ore commonly extracted from sedimentary rocks.

Uses of Soil Minerals

Mineral elements found in soil serve various vital purposes for both plants and humans.
Here are the applications of different mineral elements in plants and humans:

• Nitrogen: Essential for the formation of proteins, nucleic acids, vitamins, and hormones in both plants and humans.
• Phosphorus: An important component of the plasma membrane, specific proteins, nucleic acids, and nucleotides. It plays a crucial role in energy transfer reactions.
• Potassium: Involved in numerous physiological activities, including photosynthesis, respiration, chlorophyll synthesis, and protein production.
• Nitrogen, Phosphorus, and Potassium (NPK): These elements contribute to soil fertility and are primarily absorbed by plants, enhancing agricultural productivity.
• Calcium: Required for spindle fiber formation during cell division, regulating various metabolic processes. In humans, it is a major component of teeth and bones.
• Magnesium: Activates enzymes engaged in respiration and photosynthesis, plays a role in chlorophyll structure, and helps maintain ribosome integrity.
• Sulfur: Enhances root development and nodule formation in plants.
• Iron: An essential component of specific proteins and plays a crucial role in blood function in humans.
• Zinc: Required for auxin synthesis in plants.
• Gypsum: Used both as a soil conditioner in agriculture and for making plasters in construction.

Additionally, secondary minerals have diverse applications in various industrial and agricultural contexts:

• Haematite: Utilized for crafting gemstones, beads, sculptures, and other items.
• Goethite: Valuable in removing cadmium from polluted water bodies and as an iron ore.
• Magnetite: Used in steel manufacturing, as a catalyst in ammonia production, and as a coloring agent in paint and ceramics.
• Limonite: Serves as an economical catalyst in industrial processes and a pigment for art and industrial applications.
• Gibbsite: Employed in the production of aluminum metal.
• Azurite and malachite: Used for ornamental purposes, such as making jewelry and beads.
• Fluorite: Found in metallurgical, ceramic, and chemical industries, and also used for cutting and polishing stones.
• Calcite: Used in cement production and agricultural soil treatment.
• Gypsum: Applied in the manufacture of wallboard, cement, plaster of Paris, and soil conditioning.

Conclusion

Soil is a critical environmental factor, serving as a repository for numerous minerals and mineral elements essential for both plants and animals. Initially, these soil minerals are typically locked within rocks and are not directly accessible for the well-being of plants and animals. It is through processes like soil weathering and soil development (paedogenesis) that these minerals become available for use by plants and animals. Plants, in particular, harness these minerals for food production and growth, while mineral elements play a crucial role in synthesizing various life-sustaining substances like hemoglobin, nucleic acids, and proteins. Furthermore, they are involved in regulating several vital physiological functions.