Everything You Need to Know About Soil Testing

June 30, 2020

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Soil Testing

If you read our Turf-Tech blog, you may notice that we sometimes talk about soil tests. Many garden stores and big-box home improvement stores sell DIY soil test kits. However, you can also get your soil tested at your local extension. The advantage of testing at a local extension is that you get more accurate results, and the results come with some amount of interpretation and suggestions for improving your soil. In general, your local extension is an excellent resource for lawn care and turfgrass knowledge, so it’s a good idea to learn more about what they offer.

Why Test Your Soil?

Farmers regularly test their soil as part of their agricultural process. It helps them learn about the current state of their soil and decide how to amend the soil before the growing season. For farmers, soil tests can help guide thousands of dollars of investments in fertilizers and other types of soil modification. And making those modifications can have significant results on crop yields and a farmer’s bottom line.

For amateur turfgrass enthusiasts or anyone who wants a beautiful home lawn, the results of a soil test may have much less of a financial impact. But if you want a perfect, green lawn year after year, you’ll still need to test your soil. Trying to fertilize or make any other soil adjustments without a soil test is like trying to season a dish without tasting it. You have some idea of what to do, but there’s no way to know that you’re doing it right and in the right amounts.

If you’ve never tested your soil or are starting a new lawn from scratch—for example, laying down all new sod—now is the time to test your soil. Once you have a baseline reading, you can test your soil every 2-3 years to keep track of how your soil is reacting to growing conditions and the amendments you are making throughout the growing season.

How to Test Your Soil

The most crucial factor in testing your soil is getting a good sample. Without a high-quality sample, your results won’t mean much.

The first concern when testing your soil is that you get a representative sample of the soil in your lawn. Taking soil from just one spot in your lawn and testing it won’t tell you much about the overall state of the soil under your lawn. Similarly, avoid taking samples from unusual areas of your lawn, such as right next to a fence, near an area where you store fertilizer, or next to a compost pile.

To take your samples, you need to sample at least for areas around your lawn. Start with a digging spade and dig six inches into the ground. Then dispose of that first shovel-full of dirt. The cut you make will be conical and will have a disproportionate amount of soil from the surface. Once you have a hole six inches deep, carefully dig along the side of the hole to take a sample that is even across all six inches. Put the soil in a plastic bag and move on to your next sampling location. You will need a total of about 2 cups of combined samples to send to a testing laboratory.

Once you have four samples, you need to prepare them and mix them. First, spread out each sample on a baking pan in the sun. Let the soil air dry. Then, remove any twigs, pebbles, or other debris besides dirt. Once you have good, clean soil, sift together the four samples and prepare about two cups to send for testing.

Additional Tips for Preparing Samples to Test Your Soil

Four sampling sites are enough for a typical lawn. If you have a very large lawn, you can take up to eight samples to produce a representative sample.

You may need to prepare two separate tests if you have very different conditions in different areas of your lawn.

Make sure that your soil samples are not contaminated. Take samples before fertilizing your lawn. Make sure that you have clean hands and a clean, not-rusty shovel. Also, avoid taking samples when the soil is wet.

Everything You Need to Know About Soil Testing [infographic]

Interpreting Soil Test Results

Once you get your results, you may find that you have a lot more information than you know what to do with. If you send your samples for testing at a local extension, your results should include some amount of interpretation and recommendations. However, it is best to know a little bit about what your results mean so that you can understand the recommendations. Knowing what your results mean will also help you figure out how to implement the recommendations for how to amend your soil.

Soil pH

One of the most relevant results you get from a soil test is your soil pH. For most plants, including turfgrass, the ideal growing pH is about 6.8, or slightly acidic. The way your turfgrass absorbs nutrients involves a complex web of chemical reactions, and those all work best in a somewhat acidic environment.

P, K, Ca, Mg Levels

Your soil test results will include measures of phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg). These are all considered macronutrients, and they are required in relatively high amounts for ideal plant growth.

For turfgrass, you should aim for 25 to 50 ppm of phosphorus. However, avoid adding too much phosphorus. Phosphorus that runs off into natural waterways is a potent pollutant. Too much phosphorus in waterways can lead to algae blooms that consume oxygen and choke out all other plant and animal life.

The ideal level for potassium is 40 to 80 ppm. However, the research around the effect of potassium levels on turfgrass indicated that turfgrass can grow well with a wide range of potassium levels. Potassium primarily helps your grass to regulate water content within the plant. If your soil is sandy, you should aim to keep your potassium levels at the higher end of the scale. If you leave grass clippings on your lawn, you can reduce your potassium application since some potassium returns to the soil as the grass decays.

Acceptable calcium and magnesium levels will probably be noted in the results of your soil test.

Cation Exchange Capacity (CEC)

Cations are nutrients with a positive charge. These include hydrogen (H+), calcium (Ca++), magnesium (Mg++), and potassium (K+). The cation exchange capacity is the ability of your soil to transfer these nutrients to your turf plants. Typically, clayey soil has a higher CEC than sandy soil. The higher the CEC, the more fertilizer is necessary or the soil to be considered fertile. However, one of the best ways to improve the exchange capacity of your soil is by adding organic matter, such as compost or manure.

Base Saturation

Many soil tests will include a measure of the base saturation of your soil. Base saturation is a measure of the percentage of your CEC that is taken up by calcium, magnesium, and potassium. The base saturation is considered significant based on a theory developed in the 1940s that the correct ratio of the calcium, magnesium, and potassium in the soil will improve plant growth. However, a large amount of research since then has shown this theory to be incorrect. So although the base saturation is included with your test results, you can mostly ignore it.

Soil Testing Recommendations

Based on your test results, most extensions will include recommendations for soil amendments. These may consist of suggestions about pH adjustments and fertilizer applications. Now that you understand a little more about the meaning of your test results, the suggestions should make a little more sense. If you don’t understand any part of your results or the recommendations for improving your soil, talk to your extension. These are educational institutions, and there is usually staff who are happy to discuss your lawn care needs and answer questions. Many of these extensions also offer classes, so check if there are any that interest you. An extension is an excellent resource for learning more about how to keep your lawn picture-perfect all season long.

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