Soils & Erosion
What is soil made of? How does erosion happen and how can we stop it? Let's dig in!
🌎 What Is Soil?
Soil is the top layer of the Earth's surface. It is made from tiny bits of broken rock mixed with decomposed plants and animals (called humus), water, air, and millions of tiny living things like worms and bacteria.
Soil is incredibly important — without it, plants could not grow and most life on Earth would not survive!
Topsoil
Dark, full of nutrients and living things. Where plants grow their roots.
Subsoil
Lighter coloured, fewer nutrients. Holds water and minerals.
Weathered Rock
Broken rock fragments that are slowly turning into soil.
Bedrock
Solid rock that forms the base. It takes thousands of years to break down.
Types of Soil
Not all soils are the same. Different soils have different textures, colours, and are useful for different things.
Sandy Soil
Feels gritty. Water drains through quickly. Found on beaches and deserts. Does not hold nutrients well.
Clay Soil
Feels sticky when wet. Holds water for a long time. Can be hard when dry. Rich in minerals.
Loam Soil
A mix of sand, clay, and humus. Great for growing plants! Holds just the right amount of water.
Australian fact: Much of outback Australia has red soil. The red colour comes from iron oxide (rust) in the rock!
🌊 What Is Erosion?
Erosion is when soil or rock is slowly worn away and moved to another place. The main causes of erosion are water, wind, and gravity.
🌊 Water Erosion
Rain and rivers wash soil away. You can see this when muddy water flows into rivers after heavy rain. In Australia, flooding can cause severe water erosion.
🌫️ Wind Erosion
Strong winds blow dry soil away. Dust storms in outback Australia show wind erosion in action. Bare soil with no plants is most at risk.
💡 How Do We Prevent Erosion?
- ● Plant vegetation — roots hold soil in place
- ● Build terraces on hillsides to slow water flow
- ● Mulch gardens to protect topsoil from rain
- ● Windbreaks (rows of trees) block wind from blowing soil away
- ● Avoid overgrazing — too many animals eating plants leaves soil bare
Worked Examples
Example 1: Which soil layer has the most nutrients?
Think: Nutrients are needed for plant growth. Plants grow their roots in the top layer.
Answer: Topsoil has the most nutrients. It is dark in colour because it contains humus (decomposed plants and animals).
Example 2: A farmer notices her paddock is losing soil after rain. What type of erosion is this and how can she stop it?
Think: Rain moves soil away. This is water erosion.
Answer: This is water erosion. The farmer could plant grass or shrubs to hold the soil, or build small terraces on slopes to slow water down.
Example 3: Why is loam soil great for growing vegetables in a garden?
Think: Loam is a mixture of sand, clay, and humus. What does each part provide?
Answer: Loam drains well (from sand), holds nutrients (from clay and humus), and has a texture that roots can easily grow through. It is the best all-round soil for plant growth.
Key Vocabulary
Erosion
The wearing away and movement of soil or rock by water, wind, or gravity.
Humus
Dark, rich material made from decomposed plants and animals. It makes soil fertile.
Topsoil
The uppermost layer of soil. It is the most fertile layer and where most plant roots grow.
Weathering
The breaking down of rocks into smaller pieces by wind, water, or temperature changes.
Knowledge Check
Loading questions…
Key Concepts Summary
- ●Soil is made of broken rock, humus, water, air, and living organisms.
- ●The three main types of soil are sandy, clay, and loam. Loam is best for plants.
- ●Topsoil is the most fertile layer and where plants grow their roots.
- ●Erosion is when soil is worn away by water, wind, or gravity.
- ●Planting vegetation and using windbreaks are effective ways to prevent erosion.