The soil type is brown earth. This is a fertile soil. In the autumn the leaves fall from the trees. The leaves decompose and help to give the soil its nutrients. Earthworms in the soil help to mix the nutrients, and blend the layers within the soil.
The tree roots are deep and so help to break up the rock below. This helps to give the soil more minerals. The trees take up the nutrients in the soil as they grow. However, more nutrients are put back in the soil when the autumn comes.