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.
There is one feature of heath vegetation which makes it more uniform than most formations; this is the nature of the soil, which is always of the same character. The plants that grow upon heaths are thus unable to grow elsewhere, and attempts to transplant them without a considerable thickness of the soil, as the gardener well knows, are usually fruitless. This is largely due to the association of a mycorhiza or fungal mycelium with the roots of the plants, especially in the case of the Heath proper.
Peat is an organic soil derived from the remains of plants which accumulate and form a thick deposit above the subsoil. It differs from the mild humus found in woods - which is neutral or alkaline, and well aerated and dry - in being acid or deficient in lime, more or less damp, and poorly ventilated. The passage from humus to peat is largely regulated by the water-content, or state of humidity, and the depth of the deposit. In the last stage a heath passes into a moor.
The soil upon which this organic deposit is formed is usually sandy or gravelly, or of a similar nature. Moorpan is formed by the cementing of gravel by humous acids, and where this is the case trees will not grow. The type of soil of a common or grass heath is usually sandy, and there is at most a very thin layer of humus.