Soil is the thin, dynamic, natural layer covering much of the Earth’s land surface. It is composed of mineral particles, organic matter, water, air, and living organisms. Soil develops through the interaction of the lithosphere, atmosphere, hydrosphere and biosphere.
Soil is often described as the “skin of the Earth” because it forms the interface between the solid Earth and the atmosphere and supports terrestrial life.
Composition of Soil
A typical mineral soil contains approximately 50% solid matter and 50% pore space, although the proportions vary according to soil type, depth and moisture.
| Component | Approx. volume | Major function |
|---|---|---|
| Mineral matter | 45% | Provides the physical framework and supplies mineral nutrients |
| Organic matter | 5% | Improves fertility, structure, water retention and biological activity |
| Soil water | 25% | Dissolves and transports nutrients and supports plant growth |
| Soil air | 25% | Supplies oxygen for roots and soil organisms |
Important: The 45–5–25–25 proportions are a conventional approximation, not a fixed composition of every soil.
3. Major Soil Components
Mineral matter:
Includes particles of sand, silt and clay derived mainly from the weathering of rocks.
Organic matter:
Includes decomposing plant and animal remains. Its relatively stable component is called humus. Organic matter improves soil structure, fertility and moisture retention.
Water:
Soil water occupies some of the pore spaces and carries dissolved nutrients to plant roots.
Air:
The remaining pore spaces contain soil air. Oxygen is essential for plant roots and many soil organisms.
Living organisms:
Bacteria, fungi, earthworms, insects, nematodes and other organisms contribute to decomposition, nutrient cycling and soil formation.
4. Soil Texture and Major Types
Soils are commonly differentiated according to the relative proportions of sand, silt and clay.
A. Sandy Soil
- Contains a high proportion of relatively large sand particles.
- Has a coarse and gritty texture.
- Has large pore spaces.
- Drains water rapidly.
- Generally has relatively low water- and nutrient-holding capacity.
- Commonly supports plants adapted to dry conditions.
Example: Many desert soils are sandy.
B. Clayey Soil
- Contains a high proportion of very fine clay particles.
- Has a smooth texture when moist.
- Becomes sticky when wet and hard or compact when dry.
- Has high water-holding capacity.
- Drainage can be relatively slow.
C. Silty Soil
- Contains a high proportion of silt particles.
- Has particles intermediate in size between sand and clay.
- Generally feels smooth or flour-like when dry.
- Can be relatively fertile because it can retain nutrients and moisture.
- May be susceptible to erosion.
D. Loamy Soil
Loam contains a relatively balanced mixture of sand, silt and clay, together with organic matter.
It generally provides:
- good drainage,
- adequate moisture retention,
- good aeration,
- relatively high fertility, and
- favourable conditions for root development.
This is why loamy soils are widely regarded as suitable for many agricultural and horticultural purposes.
E. Peaty Soil
Peaty soils contain a large amount of partially decomposed organic material.
Characteristics include:
- dark colour,
- high organic-matter content,
- high water-holding capacity,
- generally acidic conditions in many peat environments, and
- formation commonly associated with waterlogged environments.
5. Functions of Soil
1. Supports Plant Growth
Soil provides plants with anchorage, water, air and nutrients required for growth.
2. Nutrient Cycling
Soil stores and cycles essential nutrients such as nitrogen, phosphorus, potassium, calcium and sulphur.
3. Water Storage and Regulation
Soil stores water and regulates its movement through the infiltration, retention and drainage processes.
4. Habitat for Organisms
Soil is an important habitat for enormous numbers of microorganisms and larger organisms such as earthworms, insects and nematodes.
5. Decomposition
Microorganisms decompose dead organic material and return nutrients to the soil.
6. Carbon Storage
Soils store substantial quantities of organic carbon, making them important components of the global carbon cycle.
7. Water Filtration
As water moves through soil, physical, chemical and biological processes can filter, transform or retain some pollutants and nutrients.
8. Foundation for Human Activities
Soil provides the physical base for agriculture, forestry, settlements and infrastructure, although construction also modifies and can degrade soil.
Readings:
Factors Affecting Process of Soil Formation
Processes of Soil Formation
Podzolization of Soil
Soil Compaction
