Soil Science Fundamentals

Soil Composition and Properties

• Soil is a complex, dynamic, and natural resource that forms at the interface of the Earth's lithosphere, atmosphere, hydrosphere, and biosphere.
• Soil consists of mineral particles, organic matter, water, air, and living organisms.
• Mineral particles include sand, silt, and clay particles, resulting from the weathering of rocks and minerals over time.
• Organic matter enhances soil fertility, structure, and microbial activity.

Soil Functions

• Soil serves as a critical medium for plant growth, providing physical support, nutrients, and water.
• Soil acts as a reservoir for essential nutrients, cycling them between organic and inorganic forms to support plant and microbial nutrition.
• Soil regulates water movement, filtering and storing water crucial for sustaining plant life, groundwater recharge, and preventing surface runoff.
• Soil provides a habitat for living organisms, supporting a diverse community of microorganisms, insects, and plant roots.
• Soil plays a crucial role in the global carbon cycle by storing and sequestering carbon through the decomposition of organic matter.

Soil as a Three-Phase System

• Soil is often described as a three-phase system, consisting of solid, liquid, and gas phases.
• The solid phase comprises mineral particles and organic matter, providing physical structure and support for plant growth.
• The liquid phase consists of water, dissolved minerals, and various solutes, serving as a medium for nutrient transport and chemical reactions.
• The gas phase consists mainly of air, which fills the pore spaces between soil particles, supporting root respiration, microbial activity, and gas exchange.

Soil Properties and Classification

• Bulk density is the mass of soil per unit bulk volume, including both solids and voids, and is a measure of soil compaction.
• Dry density is the mass of solids per unit bulk volume, calculated under dry conditions.
• Saturation density is the maximum density achieved when all voids are filled with water.
• Specific gravity is the ratio of the mass of a given volume of soil solids to the mass of an equal volume of water.
• Water content is the ratio of the mass of water to the mass of solids in the soil, expressed as a percentage.
• Void ratio is the ratio of the volume of voids to the volume of solids in a soil mass, providing insights into soil compressibility.
• Porosity is the ratio of the volume of voids to the total volume of soil, measuring the soil's ability to store water.

BIS Classification of Fine-Grained and Coarse-Grained Soils

• Fine-grained soils are classified as clay, with high plasticity, according to the Indian Standard (IS:2720 Part 5).
• Coarse-grained soils are classified based on their particle size distribution.