Soil pH is a measure of its relative acidity or alkalinity. For most plants to thrive, the pH level needs to be in the range of about 6 to 7, which is just slightly acidic. A lower pH level, indicative of very acidic soil, is problematic because it prevents plants from absorbing nutrients. In a soil with a very acidic pH of 4.5, for example, it is estimated that about 70 percent of applied fertilizer is wasted because plants can't make use of it. Agricultural lime for lawns has alkaline properties and when applied systematically as a soil amendment, it can work to adjust the overall soil pH away from the acidic side and back toward neutral pH.
Signs Your Lawn Needs Lime
Nitrogen absorption of plants is especially affected by soil pH, and this is why lawns are especially sensitive. Nitrogen is the soil nutrient most responsible for green foliage, and turf lawns are nothing but green foliage. A lawn struggling to grow in acidic soil may show the following signs:
- Weak growth
- The presence of lawn moss
- Insect infestation
- Failure to respond after treatment with fertilizer
- Washed-out color
Some species of lawn grass are more tolerant of acid soils. Kentucky bluegrass, for example, likes soil more on the alkaline side, while fescues and bentgrasses will tolerate more acidity.
Testing Soil pH
While acid soil tends to make itself known by lawn grasses that fail to thrive or problems with moss growth, the only way to verify that acid soil is an issue is with a soil pH test. You can buy DIY soil test kits at garden centers and hardware stores, but these tests are often unreliable and the information may not tell you how much lime your lawn needs. For the same amount of money (and a little more time––perhaps two to three weeks), you can have your soil tested at a local extension service. Most university extensions test soil for about $10 to $20 and the report you receive usually offers a much more detailed analysis of your soil's composition and pH level.
Follow the extension's instructions for gathering the soil sample. It's usually best to gather multiple samples from each large lawn area and mix the samples for each area together before bagging it for testing. Be sure to let the tester know that you want to learn about liming your lawn. They will likely perform an SMP buffer test on your sample(s) to indicate how much lime to add.
Soil pH is determined mostly by climate and underlying mineral content of the area. Geographic areas where the topsoil lies over limestone bedrock, for example, tend to have alkaline soils, while areas that get lots of rain tend to have more acidic soils. Your local area may have a history of problems with acidic soil and there may be a long-standing tradition of amending garden and lawn soils to reduce acidity. Consulting any local garden center will tell you if acidic soil is a common problem in your area.
How to Use Lime as a Soil Amendment
There are several types of agricultural lime used as a soil amendment to correct pH, but the form normally applied to lawns is pulverized, powdered limestone or chalk. A soil test can tell you the best type of lime to apply. Lime with a high calcium content is referred to as calcitic lime and it has the benefit of adding calcium to the soil. Some limestone contains a significant amount of magnesium and is referred to as dolomitic lime. Dolomitic lime adds magnesium to the soil and may be recommended if soil tests indicate a magnesium deficiency.
Most types of lime can be applied with a standard lawn spreader. After you've finished liming your lawn, water it thoroughly; this helps the lime reach the soil where it can begin to break down and start working.
Lime can be applied to a lawn any time of year that soil isn't frozen, but it is typically done during spring or fall. It's best to apply lime after aerating the lawn. This aids absorption and allows some of the lime to reach deeper into the soil.
Retest your soil each year until its pH level is satisfactory. After balance has been restored, continue to test your soil every three or four years.