This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Owing to the uniform type of soil, rich in acid humus, demanded, heath plants as a whole exhibit a marked similarity in their choice of habitat. In this connection must be considered the relation between the true heath, grass heath, and pasture, on the one hand, and the moister types of habitat of the wet heath, moor, and bog on the other.
There are thus dry habitats and wet habitats, depending upon the altitude to some extent. Wet heaths may also be developed upon a thin peaty or humus soil in hollows in lowland areas, so that the lie of the ground must be considered. Except where pools occur, the heath is more or less homogeneous; but where it is wooded, new conditions are introduced, and shade and sun plants both find a foothold in such areas.