ABCs of pH: Why, How and When to Soil-Test Your Lawn

soil test

How do you grow and manage a healthy lawn? Well, a lot of the dirty work lies beneath.

Any healthy lawn begins with a well-balanced soil at its base. But before you start adding fertilizer, lime or other soil amendments, dig in and send a soil sample for a pH soil test.

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Jim Friedericks
Jim Friedericks

“Soil testing provides information about the soil that is necessary to manage the soil for good plant growth,” says Jim Friedericks, Director of Outreach and Education at AgSource Laboratories. “It’s like the sensor code reader used by a car mechanic. The warning light may or may not be flashing, but if you plug in the sensor code reader you will find out what is going on with the car and can plan to fix it.”

What is soil pH?

Soil pH is a way to measure the amount of acidity or alkalinity — in your lawn, garden soil or anything else. It is measured in pH units on a scale from 0 to 14. Extreme acidity is at the low end of the scale, extreme alkalinity is at the top end. Soil at the midpoint, number 7, is neutral soil, neither acidic nor alkaline.

The “p” in pH stands for “potential. The “H” is for hydrogen. While the science of pH gets complicated, everyone is familiar with things that are more or less acidic. Pure water is a neutral 7. Materials such as baking soda are alkaline. Alkalines have less hydrogen ion concentration than pure water, and much less than acidic items such as orange juice.

So pH can be defined as the level of hydrogen ion activity in a substance, including soil.

You may have naturally acidic or alkaline soil. There are many localized exceptions, but as a general rule, soils in the Eastern and Southern United States tend to be acidic. The Midwest tends to be more neutral, while soils in the Southwest and West tend toward alkalinity.  The level of acidity originates from three main sources: rain, microbial activity, and nitrogen fertilizers.

  • Rain. Rain is intrinsically acidic, says Friedericks. It carries with it a certain amount of nitric, sulfuric, and carbonic acid absorbed from the atmosphere. Your location makes a difference as well. Rain downwind of a metropolitan area can create a soil pH as low as 4.2. That’s because there is more nitric and sulfuric acid in the atmosphere near a city. And in areas that receive more than 25 inches of rain annually, nutrients such as calcium, magnesium and sodium are leached from the topsoil. That, too, creates acidic conditions. All of which translates to: If you live in a rainy area, your soil may tend to be acidic.
  • Microbial activity reduces soil pH levels. The decomposition of organic matter releases nutrients into the soil, producing soil acidity. As soil microbes decompose plant and other residues, carbon dioxide is produced and stored in the soil. This reacts with the soil moisture to form carbonic acid and results in increased acidity.
  • Fertilizer. Nitrogen fertilizers containing ammonium also lower the soil’s pH, says Friedericks. Ammonium creates acidity when it naturally converts to nitrate in the soil.

“An acid soil has a pH less than 7.0, and as the number decreases the acidity becomes more intense,” says Friedericks. “Soil with a pH of less than 5.0 is considered strongly acidic and is a challenging environment for most plants. As the soil pH increases above 7.0 it becomes more basic, or alkaline. A pH above 8.5 is unusual and also is challenging for plant growth.”

The pH That Turfgrasses Like

Most lawns and turf grasses thrive at a pH range of between 6.0 and 7.0. PH has an effect on plant growth, and important nutrients for plants tend to be available in soil when the pH is at those levels. “With a very low pH there are risks of toxicity for aluminum and manganese as well as interactions between elements that reduce the availability as well,” says Friedericks. Highly acidic soil can also directly damage roots.

Soil pH and plant nutrients
The chart shows how effectively plants take up key nutrients at various pH levels.

Soil Testing: Kit Versus Lab

A soil test kit will let you know if the soil has a low or high pH, or if it suffers from a nutritional deficiency. The University of Illinois Cooperative Extension suggests testing before you establish your lawn, but you should test soil even if that is not possible.

Can you use a store-bought test kit to test your soil? Those kits can tell you the difference between a normal to adequate nutrient supply and a very deficient supply, Friedericks explains, but they are not precise enough to make a specific recommendation to fix the problems. “It’s best to use a soil testing laboratory that follows uniform methods that are directly related to university research and the recommendations that come from that research.”

A soil pH meter from a garden center or a pH test strip could also be misleading with its result. Numbers matter when it comes to the pH of your soil, and even a slight difference on the pH scale can mean a big change to your lawn’s acidity. The best thing to do is to send a sample for testing by a qualified laboratory, which will give you accurate readings of pH levels and nutrients.

Nearly all extension service offices offer low-cost soil testing service. Check with your local Cooperative Extension Office for details on how to take a soil sample and where to send it for a pH test. They can help you find a testing service that is familiar with your particular locale or send it for testing through their office.

Make sure to use the same lab every time you test the soil for pH readings. Soil tests vary by locale. They may use different testing methods and test for different nutrients. Most laboratories will provide interpretive guides for their test results and recommendations.

When Should You Test?

“It is a good idea to sample a lawn or turf area in the spring or early fall,” says Friedericks. “This will allow time to make the fertilizer and lime applications either in the spring as needed or to use the results from the fall sample to plan for the year ahead.”

  • Chill out. Follow the recommendations from the soil pH testing report, but be patient. It could take a year or more to see the results of a lime application, for instance.
  • Split the application. Spread the recommended application into two or three applications over a couple of months. Water the lime into the soil.
  • Season accordingly. Also split nitrogen applications over the spring and summer to avoid leaching loss and to keep the grass healthy. Apply phosphorus in the spring to help establish a strong well-rooted plant.  “Applying some of the potassium in the fall can also help the grass create a good store of energy in the roots to improve overwintering.”
  • It’s a do-over. Keep on top of any problems after the initial test. Experts recommend that you take a soil sample and test soil pH every three to four years.
  • Don’t soak the samples. Dig your soil samples when it is neither too wet nor too dry, so you get a representative sample.

Your Ultimate Goal: Healthy, Loamy Soil

It could take time and repeated soil amendments, but over time you will get the soil under your lawn where it needs to be. It will have a balance of air, organic material, minerals and micronutrients that help grass grow.

Give yourself, and the lawn, time. Plan to test the pH of soil for alkalinity or acidity, make amendments, and level and grade soil before seeding or sodding. ”The ideal soil is considered to be a loam, which is a mix of sand, silt and clay,” according to Cornell University’s Small Farms Program.

“Loams take advantage of the balance of water holding and nutrient availability between the three.”

Rosie Wolf Williams

Rosie Wolf Williams

Rosie Wolf Williams has kept bees, grown vegetables and flowers for farmers markets, and never misses an opportunity to have a conversation with an interesting tree.