What Is Soil?

Soil is a mixture of weathered rock, minerals and other living and non-living things. In between the small pieces that make up soil are spaces that contain air and water.

An important part of soil is humus. Humus contains the remains of dead plants and animals and is often darker in color than other parts of soil. Humus adds nutrients to the soil which are essential to plants and other living things.

Many plants need soil to survive. Plant roots grow down into the soil and take in the nutrients, minerals and water the plants need to live and grow. As plants are the main source of food for people and other animals, soil is important to all life on Earth.

Many animals, such as ants, worms and other burrowing animals also live in soil.

AB Activity 8.5 Think Deeply

When gardening, people often add extra ‘topsoil’ to gardens. How does this help plants grow?

Did You Know?

Compost is a soil very high in nutrients. It is made from the decomposition of organic matter – usually plant parts. You can make compost by putting food scraps in a compost bin in your garden. A compost bin is like an upside-down trash can.

compost

How Soil Forms

Soil forms when wind, water and changes in temperature break down rocks into small pieces. This process is called weathering. The pieces of weathered rock are carried by wind and water to new places. The remains of dead plants and animals also form part of the developing soil. As more weathered rock particles are added over time, soil begins to form in layers. The soil layer closest to the surface of the Earth is called topsoil. This part of the soil contains lots of humus and is rich in nutrients. Topsoil also has large spaces for air and water. Most plants and soil-living animals get the things they need from the topsoil.

Below the topsoil is subsoil. Subsoil contains less nutrients and smaller spaces for air and water. Below the subsoil is a layer of weathered rock fragments that sit on a layer of bedrock.

Soil forms in layers.

topsoil (organic layer)

subsoil

weathered rock particles

bedrock

AB Activity 8.6 Try This!

Use a clear plastic cup and art and craft materials to build a model to show how soil forms in layers.