Is dirt the same as soil?

People often say dirt, but scientists prefer the word soil. Soil is alive and full of nutrients and tiny creatures, while we usually call it dirt only when it is somewhere we do not want it, like on our clothes.

Why is soil different colours in different places?

The colour depends on what the soil is made of. Dark soil is usually rich in rotted plants (humus). Red soil often contains iron, and pale, sandy soil has more tiny rock grains and less rotted material.

Soil and Why It Matters | Nature

The amazing stuff beneath your feet

When you walk across a garden, a park or a field, you are walking on soil. It might look like plain brown dirt, but soil is one of the most important materials on the whole planet. Without it, there would be almost no plants, no farms and very little food. Let's dig in and find out what soil really is.

What is soil made of?

Soil is a clever mixture of several different ingredients all mixed together:

Tiny pieces of broken rock. These give soil its sand, silt and clay. They form the gritty, solid part of the soil.
Humus. This is the dark, crumbly material made from dead plants and animals that have rotted down. Humus is packed with nutrients that plants need to grow.
Water. Soil holds water in the gaps between its pieces, ready for plant roots to drink.
Air. There are also gaps full of air. Roots and tiny soil creatures need to breathe, just like we do.

If any of these parts is missing, plants struggle to grow. Good soil has a healthy balance of all four.

How soil forms

Soil does not appear quickly. It takes a very, very long time to make.

It all begins with rock. Over many years, wind, rain, ice and changing temperatures slowly break solid rock into smaller and smaller pieces. This breaking down is called weathering. The tiny rock pieces are the start of soil.

Next, plants such as moss begin to grow in the cracks. When plants and small animals die, they rot and turn into humus, mixing with the rock pieces. Creatures like worms churn it all together. Layer by layer, soil slowly builds up.

This is so slow that it can take hundreds of years to make just a few centimetres of soil. That is one big reason we must look after the soil we have — we cannot simply make more in a hurry. The same slow Earth processes that build soil also shape rocks, which you can read about in the rock cycle.

A world that is alive

Here is something surprising: soil is alive. A single handful of healthy soil can hold more tiny living things than there are people on Earth.

Earthworms dig tunnels that let air and water reach plant roots. As they feed, they mix and enrich the soil. Charles Darwin, the famous scientist, spent years studying how earthworms make soil healthy.
Beetles, ants and centipedes break down leaves and dead matter into smaller pieces.
Fungi and bacteria, far too small to see, rot down dead plants and release nutrients back into the soil.

All these creatures work together, like a busy underground team, to keep the soil rich and ready for plants.

Why soil matters so much

Soil might be quiet and hidden, but our lives depend on it.

Food. Almost everything we eat starts with a plant growing in soil — wheat for bread, rice, vegetables and the grass that feeds cows and sheep.
Clean water. Soil acts like a giant sponge and filter. It soaks up rainwater and cleans it as it trickles down.
Homes for wildlife. Countless animals, from worms to rabbits, live in or on the soil.
Fighting climate change. Soil stores huge amounts of carbon, helping to keep it out of the air.

Without healthy soil, farms would fail and forests would disappear. That is why soil is sometimes called the "skin of the Earth". The plants it supports are also the start of nearly every food chain and ecosystem.

Looking after our soil

Because soil forms so slowly, it is precious and easy to damage. Heavy rain can wash bare soil away, a problem called soil erosion. Too much building and pollution can harm it too.

People help protect soil by planting trees and grass to hold it in place with their roots, by adding compost made from food scraps to feed it, and by not using too many harsh chemicals. You can help by starting a compost bin or by never dropping litter on the ground.

Try it yourself: investigate a jar of soil

This simple experiment shows you what soil is really made of.

Half-fill a clear jar with soil from a garden (ask permission first).
Add water until the jar is nearly full, leaving a little space at the top.
Put on the lid tightly and shake the jar hard for one minute.
Place the jar somewhere safe and watch it over a few hours, then check again the next day.

You will see the soil settle into layers. The heavy sand sinks to the bottom, then finer silt, then clay on top, with bits of floating humus and a layer of cleaner water above. You have just separated soil into the ingredients it is made from, exactly the way soil scientists study it.

Soil and Why It Matters

Key takeaways