Garden soils are often described as acidic (sour) or alkaline (sweet). Unfortunately, the meanings of these terms and their relationship to plant growth are not clearly understood by some gardeners. The relative acidity or alkalinity of soil is indicated by its pH. The pH scale runs from 0 to 14. Any pH reading below 7 is acidic and any pH above 7 is alkaline. A pH of 7 indicates a neutral soil. The pH is important because it influences the availability of essential nutrients. Most horticultural crops will grow satisfactorily in soils having a pH between 6 (slightly acid) and 7.5 (slightly alkaline). Since most garden soils in Iowa are in this range, most gardeners experience few problems with soil pH.
There are a few plants that require a soil pH of 4.5 to 5.5. These "acid-loving" plants include azaleas, rhododendrons, and blueberries. The soil pH for these plants can be lowered by incorporating elemental sulfur (S) into the soil. Since the soil acidifying response to elemental sulfur is slow, it should be applied and incorporated a year before planting. Working Canadian sphagnum peat into the soil is another method to lower pH.
For most plants, however, a soil pH below 6.0 is undesirable. Strongly acid soils need to be limed to raise the pH to near neutral levels. Liming materials include ground limestone which is mainly calcium carbonate (CaCO3) and dolomitic limestone which contains CaCO3 and some magnesium carbonate (MgCO3). Since most Iowa soils do not need to be limed, apply liming materials only when recommended by a soil test.
A highly visible example of the role of soil pH in the availability of nutrients can often be seen in pin oaks. Many pin oaks in the Midwest suffer from iron chlorosis which is characterized by yellowish green foliage. The yellowing of the leaves is caused by a deficiency of iron within the plant. Most Iowa soils contain sufficient quantities of iron for good plant growth and leaf coloration. Unfortunately, much of the iron is in an insoluble form and is unavailable to pin oaks when the soil pH is above 7.0. Iron is readily available to pin oaks at a pH range of 5 to 6.5. Since efforts to correct iron chlorosis are extremely difficult, gardeners should avoid planting pin oaks in alkaline soils.
Additional information on the use of lime and sulfur can be found in PM-1487 Modifying Soils in Iowa Lawns and Gardens .
This article originally appeared in the May 24, 2002 issue, p. 72.