Lime is a soil amendment, typically composed of calcium carbonate or calcium magnesium carbonate, used primarily to reduce soil acidity. This process, known as liming, raises the soil’s potential of hydrogen (\(\text{pH}\)) level, which measures acidity or alkalinity. Maintaining the correct \(\text{pH}\) governs the chemical availability of essential nutrients like nitrogen, phosphorus, and potassium to the grass roots. When the \(\text{pH}\) balance is incorrect, the lawn can appear unhealthy, even if water and fertilizer inputs are adequate.
Visual Clues Your Lawn is Struggling
The appearance of certain plants can suggest a lawn is suffering from overly acidic soil conditions. Acid-tolerant weeds thrive in \(\text{pH}\) levels that inhibit the healthy growth of turfgrass. The proliferation of weeds like red sorrel, plantain, and common yellow oxalis across the lawn is a strong visual indicator that the soil may be too acidic.
Moss is frequently found in acidic soil, particularly in shady and poorly drained areas. Low soil \(\text{pH}\) chemically “locks up” fertilizer nutrients, making them inaccessible to the grass. Homeowners may notice their grass remains thin and yellowish, despite consistent application of fertilizer and regular watering, because the low \(\text{pH}\) prevents the roots from absorbing nutrients. These visual signs are useful starting points, but they are only indicators and not definitive proof of a lime requirement.
The Essential Step: Soil \(\text{pH}\) Testing
Relying solely on visual cues or the presence of acid-loving weeds is insufficient to confirm a lime requirement. A professional soil test is the only reliable method to accurately determine the soil’s \(\text{pH}\) and calculate the precise amount of amendment needed. To get a representative sample, you must collect multiple small sub-samples from random locations across the lawn area. Avoid sampling near driveways, foundation walls, or known problem areas like dead patches, as these spots can skew the overall reading.
For an established lawn, samples should be taken from the top four to six inches of soil, which is the primary rooting zone for turfgrass. Use clean, plastic tools, such as a trowel or a soil probe, to prevent contamination from metals. All sub-samples should be thoroughly mixed in a clean plastic bucket to create a single composite sample. While inexpensive do-it-yourself kits are available, a professional laboratory analysis is highly recommended, as labs provide a complete analysis of nutrient levels and the specific lime requirement.
Interpreting Results and Setting \(\text{pH}\) Targets
Once soil test results are received, locate the \(\text{pH}\) value, which is expressed on a scale of 0 to 14. The optimal \(\text{pH}\) range for most common turfgrasses is slightly acidic to neutral, typically falling between 6.0 and 7.0. A reading below 6.0 indicates the soil is too acidic and requires a lime application to raise the \(\text{pH}\) back into the desired range. Because the \(\text{pH}\) scale is logarithmic, a small numerical difference represents a large change in required treatment; for example, a \(\text{pH}\) of 5.0 is ten times more acidic than 6.0.
The lab report will provide a “lime requirement,” specifying the exact amount of lime needed to reach the target \(\text{pH}\), usually 6.5 or 6.8. This calculation accounts for soil texture, as clay soils are more buffered and require more lime than sandy soils to achieve the same change. The test results also indicate the existing levels of magnesium and calcium in the soil. If the magnesium level is low, the recommendation will likely be for dolomitic lime, which contains both calcium and magnesium carbonates. If the magnesium level is adequate, calcitic lime, which is primarily calcium carbonate, is the better choice.