Lime isn't something every lawn needs, so test the soil before deciding whether your grass needs a lime boost. Lime changes the soil's pH level, or acidity. When your soil isn't at an optimum pH for the grass, adding lime might help, but it can also hurt your lawn if the pH is already in the proper range. When you decide your lawn needs some lime, apply it anytime during the growing season.
Plants enjoy different levels of soil acidity, but most grass species like a soil pH of 6.0 to 7.0, which means the soil is slightly acidic -- 7.0 is a neutral pH. Many garden supply stores offer soil test kits that help you discover your soil's pH. If it's below 6.0, adding lime could help your grass absorb soil nutrients better by raising the pH to an acceptable level. Having your soil professionally tested, such as with tests run by a cooperative extension service, can tell you if lime is needed and how much to apply -- home kits simply give the pH level, not how much lime you need to correct a low pH.
Time of Year
When you need to add lime to your lawn, pick a time after the last frost of spring but before the first frost of fall. For soils that naturally have low pH levels, you might need to apply lime twice, preferably in the spring and fall. For a single application, anytime during the growing season is acceptable. Avoid frost or freezing temperatures when applying lime.
Other Times to Avoid
Adding lime when the grass is already wilted and weak can harm the grass, burning the blades if not washed off quickly. When the grass is too wet, the lime might not reach the soil -- it could stick to the grass blades -- or might wash away without being properly absorbed.
Although you can apply lime before you plant new grass by turning it into the top 5 inches of soil, you also can spread it evenly over existing grass with a seed spreader. The amount you need varies based on how drastically you need to change the soil pH, but when you're only slightly lower than 6.0, a 40-pound bag of lime should be enough to properly cover 1,000 square feet of lawn. Aerating the lawn with a mechanical plug aerator before you add lime helps the soil absorb the lime deeper than simply spreading it on the surface.