The pH scale...
Just about every substance is acidic, basic or neutral. The acid or base nature of a substance is measured by a pH scale that runs from 0 to 14. Substances from 0 to 7 are considered acid; substances from 7 to 14 are basic. Seven, the pH of pure water, is considered "neutral." The pH of your blood is about 7.35. Most plants grow best around pH 7.0. Some--like blueberries, azaleas and rhododendrons--like acid soil with a pH from about 5 to 6.