Structure and Composition of Soil

• Soil, covering a substantial portion of the Earth's land surface, is a fundamental natural resource profoundly impacting various facets of the ecosystem. 
• It comprises three primary constituents: minerals of different sizes, organic materials derived from deceased plants and animals, and porous spaces capable of holding water or air.
• An ideal soil for supporting most plant growth typically consists of approximately 45% minerals, including sand, silt, and clay, along with 5% organic matter, 25% air, and 25% water.
• Soils exhibit diverse chemical and physical properties. 
• Factors such as leaching, weathering, and microbial activities combine to yield an extensive array of soil types, each with unique characteristics and abilities.

HOW IS SOIL FORMED

Soil originates from the weathering of rocks, a process carried out in three primary ways:

Mechanical Weathering

• It's a common process occurring near the Earth's surface and is also known as physical weathering. 
• It involves the breakdown of rocks due to physical forces such as wind, water, and temperature, resulting in the fragmentation of rocks into smaller pieces.

Chemical Weathering

• Chemical weathering breaks down rocks through chemical reactions. 
• This type of weathering can alter the chemical composition of the soil and is integral to the transformation of rocks over time.

Biological Weathering

• While not a direct weathering process, living organisms play a role in weakening and disintegrating rocks. 
• For instance, tree roots can grow into rock crevices, expanding them and causing mechanical fractures. 
• Microorganisms also secrete chemicals that can increase a rock's susceptibility to weathering, contributing to the breakdown of rocks.

STRUCTURE OF SOIL

Soil structure is a defining characteristic of soil horizons, influencing the overall arrangement of soil units, also known as soil aggregates. 

This structure varies across different horizons due to factors such as climate and soil-forming processes.

At the surface A horizon, you often find granular and crumb structure.

In the subsoil,the B horizon, subangular blocky, blocky, columnar, or prismatic structures are more common. 

Platy structure might appear on the surface or in the subsoil, while single grain and structureless structures are typically associated with the C horizon.

Soil structure refers to the visible clods and aggregates. It differs from soil texture, which is determined by the way soil particles feel. The arrangement of soil particles into various shapes defines soil structure.

• The physical arrangement of pores and fissures within the solid matrix of soil materials determines soil structure. 
• The pores and fissures form when solid materials bond together. 
• This structure influences essential soil properties like water retention, root penetration, gas exchange, and nutrient accumulation.
• Around half of the soil is made up of solid matter, while the rest comprises pore space. These spaces play a crucial role, serving as areas for water storage, shelter for organisms, and places for the accumulation of organic matter and nutrients. 
• Smaller pores within the aggregates store and protect, while larger pores between the aggregates facilitate movement for liquids, gases, roots, and organisms.

Poor soil structure can lead to drainage issues by clogging soil pores, restricting water flow through the soil. 

Compaction, especially in heavy-textured soils like Ferrosols, can decrease aeration, hindering root development in wet conditions.

Additionally, rapid soaking under irrigation may lead to the disintegration of aggregates and the dispersion of clay material, contributing to the loss of soil structure.

COMPOSITION OF SOIL

Soil is a complex mixture of various components that collectively make up its composition. The components and their percentages in soil are:

• Organic Matter: This constitutes approximately 5% of the soil. Organic matter is derived from decomposed plant and animal materials. It is classified into three categories based on the stage of decomposition: completely decomposed, partially decomposed, and undecomposed organic matter.
• Minerals: Constituting about 45% of the soil, minerals are the solid constituents of soil formed from various elements' atomic compositions. Feldspar and olivine are primary minerals typically found in soil.
• Gaseous Components: The gaseous components in soil occupy approximately 20-30% of the soil's volume. These gases fill the air-filled pores of the soil. Nitrogen and oxygen, similar to those found in the atmosphere, are present in these pores. However, the concentration of carbon dioxide is relatively higher due to microbial activity in the soil.
• Water: Comprising about 20-30% of the soil, water is vital for dissolving minerals and nutrients within the soil. It acts as a transport medium, carrying essential nutrients to different parts of the plants, supporting their growth and development.

IMPORTANCE OF SOIL

Soil holds immense significance and contributes in various ways to the environment, organisms, and human activities. Here are the key points highlighting the importance of soil:

• Plant Growth: Fertile soil promotes healthy plant growth, supporting the cultivation of crops, which serve as sources of food, textiles, wood for furniture, and raw materials for medicines.
• Microbial Support: Soil sustains a diverse array of microorganisms, including bacteria, fungi, and algae. These organisms contribute to environmental equilibrium by maintaining moisture levels and decomposing organic matter.
• Habitat Support: The topsoil provides an environment for various organisms to perform vital life activities, including reproduction, nesting, and breeding.
• Fertility Enhancement: The organic matter within the soil increases its fertility, providing essential minerals and elements necessary for plant growth and cellular processes.
• Construction Material: Soil is a source of materials such as gravel, clay, and sand used in constructing homes, roads, and buildings. It also plays a role in making items like cups, utensils, and tiles.
• Resource Extraction: Valuable mineral medicines like calcium and iron, as well as substances like petroleum jelly used in cosmetics, are extracted from soil deposits.
• Water Regulation: Soil has the ability to absorb rainwater. This absorbed water is later released through evaporation, which contributes to cooling the atmosphere during sunny days, helping to maintain a balanced environment.

FAQs regarding soil structure and composition

1. What is soil, and what makes it up?

Ans. Soil is the surface layer of the Earth's crust and is composed of minerals, organic materials, and open spaces filled with water or air. Ideal soil contains about 45% minerals (sand, silt, and clay), 5% organic matter, and equal parts of water and air.

2. How is soil formed?

Ans. Soil is formed by the weathering of rocks, which occurs through mechanical, chemical, and biological processes. Mechanical weathering includes physical forces like wind and water, while chemical weathering involves chemical reactions. Biological weathering occurs when living organisms contribute to rock breakdown.

3. What defines soil structure?

Ans. Soil structure refers to the arrangement of soil particles into aggregates or clods. It's influenced by climate and soil-forming processes and can differ across various soil horizons. The arrangement of pores and fissures within the solid matrix determines soil structure, affecting essential properties like water retention, root penetration, and gas exchange.

4. What are the primary components of soil?

Ans. The soil is composed of organic matter (5%), minerals (45%), gaseous components (20-30%), and water (20-30%). Organic matter is derived from decomposed plant and animal materials, minerals are solid constituents formed from various elements, and the gaseous components consist of atmospheric gases like nitrogen, oxygen, and carbon dioxide in different proportions.

5. Why is soil important?

Ans. Soil plays a crucial role in fostering plant growth, supporting microbial life, and serving as habitat and resource material. It is a key medium for construction materials, extraction of valuable minerals, and water regulation, impacting the environment and supporting human activities.

6. What are the dangers of poor soil structure?

Ans. Poor soil structure can lead to drainage issues, compaction that affects aeration and root development, and the disintegration of aggregates under rapid soaking, causing a loss of soil structure.

7. How does soil affect plant growth?

Ans. Fertile soil provides a conducive environment for healthy plant growth by offering essential nutrients, minerals, and aeration, supporting the production of food, textiles, and medicines.

8. What makes up the gaseous components in soil?

Ans. Soil's gaseous components contain atmospheric gases like nitrogen, oxygen, and a higher concentration of carbon dioxide due to microbial activities in the soil pores.

9. How does soil contribute to the environment?

Ans. Soil aids in maintaining environmental balance through moisture retention, decomposition of organic matter, and evaporation of rainwater, which cools the atmosphere during sunny days.

10. Can soil be used for resources other than agriculture?

Ans. Yes, soil serves as a resource for construction materials, valuable minerals, and is a source of raw materials for manufacturing various products like cups, utensils, and tiles.

11. What is the role of water in the soil?

Ans. Water is essential in soil for dissolving minerals and nutrients and acts as a carrier to distribute these crucial components to different parts of plants for growth and development.

12. Why is the arrangement of soil particles and pores significant?

Ans. The arrangement of pores and fissures affects soil properties like water retention, root penetration, and nutrient accumulation, providing essential conditions for plant growth and microbial activity.