The purpose of liming

Depending on the crop and OD soil factors, very acid soils may need to be limed to raise their pH and counteract the affects of excessive soil acidity. Very acid soils (below a pH of about 5.0-5.5) may adversely affect crop growth for several reasons:

· Aluminum, manganese, and iron all become more soluble with increasing acidity and may actually become toxic to plants at pH's below 5.5. Many varieties of beans and wheat are especially sensitive to aluminum toxicity, although the true "tropical" soils (see Chapter 1) tend not to release toxic amounts until the pH approaches 5.0. Manganese and iron toxicities can be serious, too, but tend to be more of a problem on soils that are also poorly drained.

· Very acid soils are usually low in available P and have an especially high capacity to tie up added P by forming insoluble compounds with aluminum and iron.

· Although very acid soils usually have enough calcium to supply plant nutrient needs (except for peanuts), they are likely to be low in magnesium, as well as sulfur and available molybdenum (Mo becomes increasingly insoluble as acidity increases).

· Low soil pH depresses the activities of "good-guy" soil bacteria and fungi, such as those that convert the unavailable, organic forms of N, P, and S to available mineral forms. One of the main reasons that soybeans, alfalfa (Lucerne), and many clovers do poorly on acid soils is that their particular types of N-fixing rhizobia bacteria have little tolerance for pH's below 6.0. (On the other hand, many of the rhizobia species associated with the more tropical legumes [e.g. peanuts, cowpeas, and kudzu] can function well at lower pH's.)