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 always clearly understood by some gardeners.
The pH Scale
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.
pH is a measure of the number of hydron (H+) ions. More hydrogen ions mean more acidic conditions and fewer mean more basic. The scale in which pH is measured is logarithmic (not linear). This means that the unit of distance between each value is not equal but is raised to a power of ten. For example, on a linear scale, the difference between 4 and 5 is one, the distance between 4 and 6 is two, 4 and 7 is three, and so on. In the logarithmic scale of pH, if the difference between 4 and 5 is one, the distance between 4 and 6 is ten, 4 and 7 is one hundred, 4 and 8 is one thousand, and so on. This is, in part, why it can be difficult to change the pH of the soil in a big way.
Why pH is Important
The soil pH is important because it influences the availability of essential nutrients. Figure 1 below depicts the pH values where nutrients are available for the plant to uptake. The width of the bar indicates how available the nutrient is. For example, Phosphorus is available for plant uptake on soil pH that ranges from 6.5 to 7.5; outside of this range, P availability decreases. Manganese becomes less available if the pH is greater than 6.5.
Most horticultural crops will grow satisfactorily in soils having a pH between 6 (slightly acidic) and 7.5 (slightly alkaline). In Iowa, soil pH typically ranges from 5.5 to 7.5, with many areas of the state trending toward the higher end of that range. Since most garden soils in Iowa are in the satisfactory range for horticultural plants, most gardeners experience few problems with soil pH.
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 it is difficult to alter the soil pH in the entire rooting zone of a mature tree efforts to correct iron chlorosis are extremely difficult. It's best to avoid planting pin oaks in alkaline soils.
Changing Soil pH
When a soil test confirms a pH outside the desired range, amendments can be added to alter the soil pH. Learn more in this article: How to Change Your Soil's pH
Lowering 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.
Raising 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. Large-scale commercial agricultural fields apply lime on a regular basis. This practice is not typically needed in garden settings. Farmers raise soil pH in corn fields due to low pH soils caused by fertilizers and other amendments. Garden soils are not managed this way and rarely need their pH raised.
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.
More Information
- How to Change Your Soil's pH
- Managing Garden Soil
- Soil Testing Resources for Home Gardeners
- Using Wood Ashes in the Home Garden
Photo credits: 1: AlDa.videophoto/AdobeStock; 2: University of Massachusetts Amherst Center for Agriculture, Food, and the Environment