Soil: Definition, Formation, Composition, Properties & Importance Explained

Study notes August 10, 2025

 

Soil: Definition, Formation, Composition, Properties & Importance

What is Soil?

The term soil originates from the Latin word SOLUM, meaning “earthy material where plants grow.” It refers to the uppermost weathered and fertile layer of the Earth’s crust that supports plant life.

According to Thomson, soil is a natural mixture of minerals, organic matter (humus), water, air, and living organisms, capable of sustaining plant growth. The scientific study of soil is called pedology, and its formation process is known as pedogenesis.

Soil is often called the “skin of the Earth” because it covers most land surfaces and serves as the bridge between living and non-living components of nature.

Composition of Soil

Soil is made up of four main components:

  • Mineral matter (inorganic) – ~45%

  • Organic matter (humus) – 5–10%

  • Soil water – ~25%

  • Soil air – ~25%

Formation of Soil (Pedogenesis)

Soil forms over time through the breakdown of rocks, minerals, and organic matter. This process is called weathering, which can be:

  1. Physical (Mechanical) Weathering – caused by temperature changes, wind, water, ice, and gravity.

  2. Chemical Weathering – caused by oxidation, hydrolysis, carbonation, and acid rain.

  3. Biological Weathering – caused by living organisms like lichens, mosses, plant roots, and human activity.

Importance of Soil

Soil plays a vital role in life on Earth:

  • Stores essential minerals and nutrients for plants.

  • Serves as a habitat for countless organisms.

  • Regulates water storage and supply.

  • Supports plant growth, which produces food, oxygen, and biomass.

  • Filters pollutants and breaks down waste.

  • Acts as the foundation of terrestrial ecosystems.

Properties of Soil

1. Chemical Properties

  • Mineral content (Ca, Mg, Fe, N, P, S, etc.)

  • Organic matter (humus)

  • Soil water & air balance

  • pH level (ideal for plants: 6.6–7.3)

  • Temperature regulation

2. Physical Properties

  • Soil texture – proportion of sand, silt, and clay.

  • Soil structure – arrangement of particles (platy, granular, columnar, etc.).

  • Colour – indicates mineral composition (red from iron oxide, dark from organic matter).

  • Density & porosity – affects water retention and aeration.

Soil Minerals & Their Roles

  • Nitrogen (N) – vital for growth and reproduction; absorbed as nitrates.

  • Calcium (Ca) – strengthens cell walls and activates enzymes.

  • Sulphur (S) – key for vitamins and amino acids like methionine.

  • Phosphorus (P) – important for DNA, cell membranes, and energy transfer.

Soil Types Based on Texture

  1. Sand – Large particles (0.05–2 mm), loose, drains quickly; ideal for peanuts, watermelons, and tulips.

  2. Silt – Medium particles (0.002–0.05 mm), smooth, holds water well.

  3. Clay – Very fine particles (<0.002 mm), high water retention, sticky when wet; suitable for leafy vegetables and tomatoes.

  4. Loam – Balanced mix of sand, silt, and clay; best for most crops.

Conclusion

Soil is more than just dirt — it is a living, dynamic system that sustains life on Earth. Understanding its composition, formation, properties, and types is essential for agriculture, environmental management, and ecosystem balance. Protecting soil health ensures food security and environmental sustainability for future generations.

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