St. Augustine grass (Stenotaphrum secundatum) is a popular warm-season turf, valued for its dense, carpet-like growth and shade tolerance, making it a favorite for lawns in the southern United States. Achieving a healthy St. Augustine lawn depends on the soil’s chemistry. Soil health is determined by its pH, which measures acidity or alkalinity. This pH level directly controls the availability of nutrients and is a primary factor in successful turfgrass maintenance.
The Ideal pH Range for St. Augustine Grass
For optimal growth, St. Augustine grass requires a soil environment that is slightly acidic to neutral. The recommended pH range is typically between 6.0 and 7.5. Many experts suggest it thrives best closer to the neutral end of that spectrum, specifically 6.5 to 7.0. When the soil pH moves outside this optimal window, becoming too acidic or too alkaline, the grass struggles to absorb the necessary elements for growth.
Understanding How pH Affects Nutrient Uptake
Soil pH plays a central role in turfgrass health because it determines the solubility of essential nutrients. Nutrients present in the soil can become chemically locked up and unavailable to the grass roots if the pH is too high or too low. For St. Augustine grass, the most common issue occurs in alkaline soils (pH above 7.0).
When the soil is overly alkaline, micronutrients like iron (Fe) and manganese (Mn) become less soluble. This reduced solubility means the grass roots cannot absorb these elements efficiently. The resulting deficiency is known as iron chlorosis, which causes the grass blades to turn yellow while the leaf veins remain green. St. Augustine grass often shows signs of chlorosis when the pH exceeds 7.5.
How to Measure Your Lawn’s Soil pH
Determining the current pH of your lawn is the necessary first step before attempting corrective action. The most accurate way to establish a baseline is by submitting a sample to a professional soil testing laboratory, often operated by a university extension office. This process involves collecting multiple soil cores from various spots across the lawn to create a representative composite sample. Samples should be taken from the top four to six inches of soil, and any grass or root matter must be removed before air-drying.
The laboratory analysis provides a precise pH reading, a comprehensive report on nutrient levels, and specific recommendations for amendments. While at-home kits, pH meters, or test strips offer quicker results, they are generally less accurate than professional lab tests. These consumer-grade options are useful for quick spot-checks, but they should not be the sole basis for large-scale soil adjustments.
Correcting Soil pH Imbalances
Adjusting soil pH is a gradual process that requires patience and should always be based on the results of a soil test.
Raising pH (Acidic Soil)
To raise the pH of overly acidic soil, materials like agricultural limestone are applied. Calcitic lime is primarily calcium carbonate, while dolomitic lime adds magnesium along with calcium. Both effectively increase alkalinity. For existing lawns, applying pelletized lime is generally preferred because it is less dusty and easier to spread.
Lowering pH (Alkaline Soil)
For alkaline soils, the goal is to lower the pH by increasing acidity. This is most often accomplished using elemental sulfur. Soil bacteria slowly convert the elemental sulfur into sulfuric acid, which naturally lowers the soil pH over time. Alternatively, acidifying fertilizers containing ammonium sulfate can be used, providing a gradual pH reduction while also feeding the lawn.
Application Guidelines
Large pH changes should not be attempted in a single application, as this can shock the turf. Elemental sulfur acts slowly, sometimes taking six months to a year to show significant results. It should be applied in conservative amounts, typically no more than two pounds per 100 square feet at a time. After any amendment application, the soil should be retested every three to twelve months to monitor the change and determine if further applications are necessary.