Soil pH is an often-overlooked component of soil fertility. While nutrient deficiencies of nitrogen (N), phosphorus (P), potassium (K), and micronutrients can restrict plant growth, an out-of-range soil pH can cause problems as well.
pH is a measurement of hydrogen ions (H+) in a solution. It is measured on a scale of 0 to 14. A pH of 7 is considered neutral, with values less than 7 being acidic (increasing in acidity as it moves toward 0) and more than 7 being basic (increasing in basicity as it moves toward 14).
Most ornamental and vegetable plants prefer a slightly acidic soil pH of 6.0 to 6.5. Azaleas, blueberries, and centipedegrass are a few exceptions to this rule and grow best with a soil pH closer to 5.5.
Negative charges on the surface of soil clay and organic matter particles attract and hold plant nutrients. Hydrogen ions are positively charged. So, when the soil has excessive hydrogen ions, it ‘clogs up’ the soil’s ability to retain other essential plant nutrients to the detriment of plant health.
Lime is a soil amendment made from pulverized limestone. Limestone is high in calcium carbonate CaCO3 (and magnesium carbonate MgCO3 in the case of dolomitic lime). When lime is incorporated into the soil, the chemical reaction sloughs off hydrogen ions from the soil particles to free up negatively charged sites for plant nutrients to adhere. The hydrogen ions combine with oxygen ions to form water, thus reducing overall soil acidity.
Liming the soil is most effective when the lime material is tilled into the soil. The liming material can take three to four months to change soil pH, making fall the ideal time to incorporate lime for next spring’s growing season.
For soils with high soil pH, elemental sulfur can be added to lower the soil pH. When sulfur combines with water (H2O) in the soil, it produces sulfuric acid, which acidifies the soil. Like liming, sulfur is most effective when tilled into the soil and can take several months to change pH. For more information about soil pH, visit HGIC 1650, Changing the pH of Your Soil.
Lime or sulfur can be surface applied in established plantings or lawns since tilling would be impractical. In these instances, it will take longer to change the soil pH as the lime or sulfur will slowly infiltrate the soil profile. Regardless of the application method, it is an excellent practice to retest the soil annually in cases of a pH imbalance to determine if liming or sulfur applications have sufficiently changed the soil pH.
Home soil testing kits are available, but soil testing by the Clemson Agricultural Service Laboratory is a superior method. The Ag Service Lab measures the soil pH and the buffer pH. Buffer pH helps determine how much lime is needed to change to the appropriate soil pH. Additionally, soil testing by the Ag Service Lab is cheaper than most home soil testing kits. Soil samples can be dropped off at your local Clemson Extension office for testing.
This fall, test your soil to gain valuable insight into your soil’s fertility and receive information to make vital adjustments in time for next spring’s growing season. For more information about soil testing, visit HGIC 1652, Soil Testing.