Soil is a mixture of tiny pieces of rock that have broken away from the rock below. But soil is home to countless living things, and they are responsible for changing the soil near the surface. Animals move around as they live, loosening the soil and chewing up dead bits of plant.

What comes out of the back end of the animals is all mashed up. We call it dung. But it is a rich source of nourishment, or fertiliser, for plants. Because the roots of the plants are right in amongst where the animals are at work, the roots find it easy to get the dung.

So the plants provide the food for the soil animals and the soil animals eat it, and release nourishment for the plants to reuse. It is all a wonderful cycle of nature.

Animals like earthworms bring dead pieces of leaf into the soil and mix them with the tiny rock grains of clay, silt and sand. The dead plant material is dark brown, so that is why the soil near the surface is dark brown. Lower down, where the earthworms and other animals rarely go, there is no mixing, and so what you see is just tiny bits of rock, and it will have a lighter colour. The layers in a soil are called soil horizons.

But beware: if you harvest the plants (take them away to eat, for example), there will be nothing left for the soil animals to eat. So the whole natural cycle will start to break down. And that is why a garden soil and a farm soil need compost added to them. Simple, once you know how – and why!