Soil pH is a measure of its relative acidity or alkalinity. For most plants, including the turf grasses used in lawns, to thrive, the pH levels need to be in a range of about 6 to 7, which is just slightly acidic. (The pH scale is a numerical rating from 1 to 14, with 7 being neutral, and lower numbers indicating more acidic soils, and higher numbers, more alkaline levels.) A lower pH level, representing very acidic soil, is especially 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 is a soil amendment with 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.
Acid Soils and Lawns
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 very acidic soils may show weak growth, the presence of moss, and it may fail to respond to fertilizer applications. Growing in acidic soils, lawn grass may have a washed-out color, and the lawn may be unusually susceptible to weeds and pest problems.
Some species of lawn grass are more tolerant of acid soils. While Kentucky bluegrass, for example, likes soil more on the alkaline side, while fescues and bent grasses will tolerate more acidity.
What Determines Soil pH
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. So 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.
Testing for 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. A good kit costs about $15 to $20 and tests for pH as well as nutrients, such as nitrogen and phosphorus. The accuracy of the results from these tests is difficult to predict, 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.
Lime as a Soil Amendment
Adding lime is the most common method of changing the pH of the soil. 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 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.
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.