(introduction...)

In the soil there are dead leaves and roots.
They rot and change into humus.
You cannot see humus as you see sand, clay and silt.

· Dead plants change into humus.

Leaves, branches and dead trees rot in the soil.
We say they decompose.
Even big trees rot in a few years on wet soil.
Many worms and insects live in a rotting tree. You can often see them.
But other living things cannot be seen. They are too small.
These are called microbes.
There are very, very many of them. In a lump of earth as big as a lump of sugar there are millions and millions of microbes.
They feed on leaves, on dead branches, on organic matter (see Booklet No. 2, page 23).
They also need air to breathe, and water.
If there is no air and water, the plant does not rot. The organic matter does not decompose.

Organic matter decomposed by the microbes in the soil is humus.