Understanding pH & Lime

by Marissa L. Tomasic

Many homeowners apply lime to their lawns believing that it will help the lawn become green and healthy. While this isn't necessarily untrue, it's not the whole story either. Without understanding why lime should be applied, how much, and what type to use, these efforts could be spent in vain, using up time and money without having any real benefit. Understanding pH is essential when trying to grow or maintain any plants, whether it be lawn grasses, vegetables or ornamental plantings. 

The main purpose of applying lime is to raise the soil pH. The higher the pH, the more alkaline a soil is; a lower pH means a soil is more acidic. pH is not an indicator of how fertile a soil is, but rather how easily plants are able to take up nutrients from the soilYou could use the best quality fertilizers available, but if the pH levels aren't appropriate for the plants you're trying to grow then it won't have much of an effect. Different plants have different pH levels at which they thrive. Most lawn grasses require a pH between 6 and 7. Over time, natural processes that take place in soil will cause soil to become acid. If your soil pH is much lower than 6, then lime is needed to raise the pH. (A little variance, say up to 0.5, is generally ok.) 

The secondary effect of adding lime to a lawn or garden bed is it's contribution of Calcium and Magnesium, two chemical elements that are present in limestone and essential to plant growth. It's not so much the presence or absence of Calcium and Magnesium that will dictate plant health, but rather the ratio of one to the other. Healthy soils have a Calcium to Magnesium ratio between 7:1 and 10:1. If the ratio is lower than 7:1, dandelions and other weeds will flourish instead of lawn grasses.  

Finding the Calcium to Magnesium ratio in your soil will let you know which type of lime to use. Lime is generally sold as "pelletized lime" or "fast-acting lime". Yes, the fast-acting lime is more efficient (faster) at raising soil pH, but urgency should not dictate your decision in choosing one over the other. If your soil is low in Magnesium, use regular pelletized lime, or dolomitic lime, which has a high Magnesium content. If your soil is high in Magnesium, use fast-acting lime, or calcitic lime, which has a high Calcium content. Calcitic lime is more expensive per bag, but covers more area. It's always a good idea to tally the total cost of the project rather than comparing the price per bag. 

So how much lime should you put on your lawn? That depends entirely on how much you need to raise the pH. The vast majority of people don't use enough lime, or don't apply it for the duration needed to actually have an effect. If you have been applying lime to your lawn or garden and have never tested your soil pH, stop! Pick up a simple, inexpensive home pH test kit or send a soil sample to the UMass Soil and Plant Tissue Testing Lab. The current pH will be your guide in determining how much lime to apply. As a rule of thumb, never apply more than 50 lbs. of dolomitic lime per 1,000 square feet at one time. Each 50 lb per 1,000 square feet application should raise your pH by about one point. (50 lbs of calcitic lime will cover 5,000 square feet.) If you need to raise your pH by more than one point you will need repeat applications, but be sure to wait 2-3 months before adding more lime. 

I highly encourage anyone maintaining a home lawn to have their soil tested by the UMass Soil and Plant Tissue Testing Lab. "What a pain," you think, "why spend any extra time and money on my lawn than I already am." It may seem superfluous, but consider how many hundreds, if not thousands of dollars you've spent over the years on fertilizers, pesticides, fungicides, grass seed and lime treatments, not to mention the days spent applying these products, and the subsequent frustration that the lawn is still filled with weeds or brown in patches despite all of your efforts! (If your lawn looks great, and you've never tested your soil, consider yourself lucky and keep doing what you've been doing!) A laboratory soil test will give you a more accurate pH reading than a home test will provide. It will also give you a clear breakdown of each nutrient contained in the soil, and will indicate if the level is considered high, moderate or low.  There is a lot of information and recommendations included in the soil test results, but focus on the pH, Calcium and Magnesium numbers as preparation for applying lime. The appropriate form is easily available online and the fee is modest. (Here is the link: https://soiltest.umass.edu/ordering-informationAt the very least, try an inexpensive home test (we carry them here at New England Nurseries) to give you an idea of your soil pH before you apply lime. 

Raising your soil pH is a chemical process that takes time. Think of it as a marathon, not a sprint. Dolomitic lime (pelletized lime) will take between six months and one year to take full effectCalcitic lime (fast-acting lime) can take effect in as little as five to eight weeks, but it's high concentration can burn lawns, so be sure to apply it sparingly and follow the recommended application rate on the bag. If you have to adjust your soil pH several points, it may take a few years. Continue to test the soil pH until you reach the appropriate number. 

Fall is a great time to add lime to the lawn since the freeze-thaw cycle in winter helps the lime to break down and get into the soil. You can apply lime at any time during the year. Don't apply lime when your grass is wilted or frost covered, and be sure to water your lawn after the application so that lime does not remain on the blades of grass. 

Doing a bit of homework before applying lawn products will yield better results with less waste. Save your hard earned money and your precious free-time by doing lawn care the right way. And always feel free to direct your questions to us here at the Nursery; we are here to help!