More than dirt
Soil is one of Earth’s most important resources. It grows nearly all our food, filters water, stores carbon and is home to billions of living things. A single teaspoon of healthy soil can contain more microorganisms than there are people on Earth.
Soil science studies how soil forms, what it’s made of and how to protect it.
What’s in soil?
| Ingredient | Approximate share in healthy soil |
|---|---|
| Minerals (weathered rock) | About 45% |
| Organic matter (humus) | About 5% |
| Water | About 25% |
| Air | About 25% |
Half of good soil is actually empty space filled with air and water! Roots and soil creatures need both.
How soil forms
Soil forms very slowly. It can take hundreds to thousands of years to form just a few centimetres.
- Weathering breaks down bedrock into small pieces. Freezing and thawing, rain, roots and chemical reactions all help.
- Living things move in. Lichens and mosses grow first, followed by plants.
- Decomposers like worms, fungi and bacteria break down dead plants and animals into humus.
- Over time, soil develops distinct layers.
Five factors shape soil: the parent rock, climate, living things, the slope of the land, and time.
Soil layers
A slice through the ground, called a soil profile, shows layers called soil horizons.
| Horizon | Name | Description |
|---|---|---|
| O | Organic layer | Leaves, twigs and decaying material on the surface |
| A | Topsoil | Dark, rich in humus; most roots and soil life |
| B | Subsoil | Lighter; more clay and minerals washed down from above |
| C | Weathered rock | Broken pieces of bedrock |
| R | Bedrock | Solid rock |
Soil texture
Soil texture depends on the size of mineral particles.
| Particle | Size | Feel | Water |
|---|---|---|---|
| Sand | Largest (0.05–2 mm) | Gritty | Drains quickly; holds few nutrients |
| Silt | Medium | Smooth like flour | Holds water moderately |
| Clay | Smallest (under 0.002 mm) | Sticky when wet; hard when dry | Holds lots of water; drains slowly |
Loam has a good balance of sand, silt and clay, plus humus. It holds water and nutrients while still draining well, so it’s ideal for most crops.
The ribbon test
Squeeze moist soil between your thumb and finger:
- Falls apart → sandy
- Feels smooth, short ribbon → silty or loamy
- Makes a long, sticky ribbon → clay
Life in soil
| Organism | Role |
|---|---|
| Earthworms | Mix soil, make tunnels for air and water, and leave nutrient-rich castings |
| Bacteria | Decompose matter; fix nitrogen |
| Fungi | Decompose wood; partner with roots |
| Insects and mites | Break down leaves |
| Moles and gophers | Mix soil layers |
Charles Darwin was so fascinated by earthworms that he wrote a whole book about them in 1881.
Soil pH
Soil can be acidic, neutral or basic (alkaline). Most plants grow best between pH 6 and 7. Blueberries prefer acidic soil, while asparagus prefers slightly alkaline soil. Farmers add lime to make acidic soil less acidic.
Canada’s soils
Canada has many soil types. The rich, dark chernozem soils of the Prairies, formed under grasslands, are some of the most fertile in the world and make Saskatchewan and Alberta major grain producers. Much of the Canadian Shield, scraped bare by glaciers, has thin, rocky soils.
Soil erosion and protection
Soil erosion is the removal of soil by wind or water. In the 1930s, drought and poor farming practices in the Prairies and the U.S. Great Plains led to the Dust Bowl, when huge dust storms blew away topsoil.
| Threat | Protection |
|---|---|
| Wind erosion | Shelterbelts of trees; keeping crop stubble on fields |
| Water erosion | Contour ploughing; terraces; cover crops |
| Loss of nutrients | Crop rotation; compost and manure |
| Compaction from heavy machines | Reducing tillage; controlled traffic |
| Pollution | Careful use of chemicals; safe waste disposal |
No-till farming, where farmers plant seeds without ploughing, is now common on the Prairies and helps protect soil.
Common mistakes
- Thinking soil is just dead dirt. It’s alive with organisms.
- Thinking soil forms quickly. It takes centuries.
- Thinking all soil is the same.
Try it yourself
- Put soil in a jar of water, shake it and let it settle overnight. Measure the layers of sand, silt and clay.
- Dig a small hole and draw the soil layers you see.
- Make a compost bin or a worm farm and observe how food scraps turn into soil.
Quick recap
- Soil is made of minerals, humus, water and air.
- It forms slowly from weathered rock and decayed organisms.
- Soil profiles have layers called horizons.
- Texture depends on sand, silt and clay; loam is ideal for most crops.
- Protecting soil from erosion is vital for growing food.