What is soil?

Soil is much more than “dirt.” It’s a living ecosystem made from five main components:

1. Mineral particles (about 45%)

• Weathered rock broken down into:

• Sand (large particles)

• Silt (medium particles)

• Clay (tiny particles)

• These determine drainage, nutrient-holding capacity, and texture.

2. Organic matter (about 5%)

• Decaying leaves, wood, roots, fungi, dead insects, and other organisms.

• Fully decomposed organic matter is called humus.

• This improves fertility, water retention, and soil structure.

3. Water (about 25%)

• Occupies pore spaces between soil particles.

• Carries dissolved nutrients to plant roots.

4. Air (about 25%)

• Healthy soil contains oxygen that roots and microorganisms need to survive.

5. Living organisms

• Billions of bacteria, fungi, archaea, protozoa, nematodes, insects, mites, worms, and plant roots.

• These organisms recycle nutrients, build soil structure, and help plants defend against disease.

Soil isn’t the same everywhere

Different proportions of sand, silt, and clay create different soil types:

• Sandy soil: Drains quickly but doesn’t hold nutrients well.

• Clay soil: Holds lots of water and nutrients but can become compacted.

• Silty soil: Smooth, fertile, and retains moisture.

• Loam: A balanced mixture of sand, silt, clay, and organic matter. It’s generally considered ideal for growing most plants.

How soil forms

Soil develops over hundreds to thousands of years through:

• Weathering of rock by wind, water, temperature changes, and plant roots.

• Accumulation of dead plants and animals.

• Activity of fungi, bacteria, insects, and earthworms that mix and transform organic matter.

A useful way to think about it is:

Rock + time + climate + living organisms + topography = soil.

Fungi are major soil engineers. Their hyphae bind soil particles into stable aggregates, decompose complex organic matter like lignin and cellulose, transport nutrients, and form mycorrhizal partnerships with plant roots. In many forest soils, fungal networks are among the most important organisms responsible for creating and maintaining healthy soil structure.