acid soils

Soil without lime, formed in areas without chalk or limestone. Acid solis tend to breed bacteria more slowly than limy (Alkaline) soils and thus contain more undecayed vegetable matter, which makes them spongy and open but with limited fertility. acid soils is neccessary for most of the huge and important Erica family, which includes rhododendrons, azaleas, the heathers and many other highly decorative plants. Acidity is measured on the pH scale, which counts the number of hydrogen ions. A pH of 7 is neutral. More than 7 is alkaline; less, acid.

Excerpt from The principles of gardening.
Hugh Johnson

www.betuco.be/compost/Soil acid_phosphorus - soil-acidity.pdf

Try this document. It's fascinating and has pictures too!