One essential overlooked by aspiring growers is the need for a certain fungus which grows on the roots of these plants. This organism, called mycorrhiza, lives on the roots of all the plants I have mentioned, in a symbiotic relationship. This means that the fungus is not a parasite, but contributes to the welfare of the host plant, and in turn is sheltered and fed by it.
Just how they feed the plant is not clearly understood. We know that an azalea grown from seed in sphagnum moss produces root hairs like other plants. When transplanted to soil it loses these root hairs. Experts believe that the inycorrhizae function as root hairs and help dissolve soil chemicals so they can be used by the host.
An even more obscure phase of acid soil plant nutrition is their need for hormones from outside the plant itself. Most broad-leaved evergreens belong to what is known as the forest floor group. That is, they live in light shade on forest floors. There is considerable evidence that such plants do not manufacture enough of their own hormones for normal growth. I suspect they extract these from decaying plant material that originated in sunshine. Certainly, the use of hormone compounds like Transplantonc produces a much greater response in plants of this group than in plants growing in full sun.
Since fungi survive only in the presence of abundant organic matter, we know that one way to insure survival is to mix in at least 25 per cent humus, peat, compost, etc., when making the bed.
To wrap up the culture of acid soil plants in a single, oversimplified statement, we might say that it is largely the culture of mycorrhizae. What is good for the fungus is good for the plant. True, we find wild plants in nature growing under conditions which favor mycorrhizal activity. Practically all of them have rather shallow roots covered with a mulch of fallen leaves, and grow in sections where high humidity and mist keep the soil constantly moist, yet where drainage is so sharp that moisture never stands.