Second only to sunlight and water, the composition of a garden’s soil can make or break a vegetable garden. The primary aspects of your soil to be on the lookout for are the available nutrient levels, the pH, and the ratio of sand, silt, and clay. If you’re gardening in a city or an area potentially contaminated by heavy metals, it’s also crucial to test the soil for these contaminants before feeding food grown in them to children or pregnant women.
To test the nutrient levels and heavy metal contamination in your soil, the most accurate method is to take a soil sample and send it to a university laboratory, of which there are many around the US that will test your soil for a small fee. To test the soil pH, you can buy at-home soil pH test kits online or at many garden centers. Store-bought pH test kits are accurate as long as you follow the manufacturer’s instructions. Finally, to test the soil ratio of sand, silt, and clay, you can simply use your hands.
The primary elements that limit a plant’s growth are nitrogen, phosphorus, and potassium, also known by their chemical symbols “NPK.” Many land-grant universities have soil labs that will test your soil for these nutrients for as little as $15. The University of Illinois Extension agency has a list of certified labs that you can choose from to submit your soil test.
To collect your soil sample, dig a number of small holes around your growing area and take a small amount of soil from the top six inches of each of the holes. Remove any sticks, dead leaves, or plant material that makes it into your samples, as these will throw off the test results. Mix the soil samples together in a bucket and then set aside a sample from that bucket in a plastic bag to mail to the soil lab. If you have a large growing area and think some areas might have different nutrient levels than others, repeat this process for each of your growing areas and keep the samples well-labeled.
Soil labs will typically return your results within a few weeks, depending on the time of year you submit your sample. These results will come with recommendations for how to amend your soil and what nutrients to add. Often these test results will include the pH and organic matter levels as well.
In order to test your soil for heavy metal contamination, you can follow the same process as you would for the nutrient levels, but be sure to send your soil sample to an accredited laboratory for a lead test. The EPA has a list of accredited lead laboratories that you can choose from.
Additionally, some local agencies and nonprofits hold annual soil lead testing events where community members can bring samples from their vegetable gardens to be tested. Here in Pittsburgh, the Allegheny County Conservation District and the nonprofit Grow Pittsburgh hold annual free soil lead testing events for local gardeners to test their soil. There are also at-home soil lead test kits that you can purchase online, but I have not personally used them and cannot speak to their efficacy.
If you find that you do have dangerous levels of heavy metal contamination in your garden, you can mitigate your exposure by covering the contaminated soil with raised beds full of compost and only growing fruiting plants like tomatoes, peppers, squash, etc. Avoid growing any root crops or low-lying leafy greens because heavy metals are most highly concentrated in the roots and leaves of plants grown in contaminated soils. Very little heavy metal contamination makes it all the way through the plant into the fruit, but you should make sure to wash fruits grown in contaminated soil because any dust from the soil that accumulates on the outside can contain heavy metal contamination.
The pH of your soil tells you where on the spectrum from acidic to alkaline your soil lies. pH is measured on a scale from 0 to 14, with 0 being the most acidic and 14 being the most alkaline. Most garden vegetables do best in a soil with a slightly acidic to neutral pH between 6 and 7, but certain plants like blueberries and cranberries prefer very acidic soil. A soil test from a laboratory will typically tell you the pH of your soil, but if you chose not to submit a soil sample to a lab, you can test your soil’s pH with reliable over-the-counter tests from your local garden center.
If you find that your soil is outside of the ideal 6-7 pH reading, you can raise the pH by amending your soil with an alkaline fertilizer like garden lime or lowering the pH with an organic soil acidifier like sulfur.
Soil scientists categorize soil types by the particle size of the soil, with sand being the largest particle size and clay being the smallest. Your soil type has a huge impact on the ability of your garden to retain moisture, hold onto nutrients, and provide structure for your plant’s roots. Unlike the previous three soil elements, you don’t need anything other than your hands to test your soil type.
The tried and true method for testing your soil type, known as the “squeeze test,” is to dig up a representative sample of your soil and squeeze it between your thumb and index finger while pressing it out to make as long of a strip as possible. If the soil simply falls through your fingers, it’s on the sandy side of the spectrum, but if you’re able to form a long strip that hangs out the end of your hand, you are on the clay side of the spectrum. Most soils will be somewhere in between, and they will clump up when squeezed, but fall apart when you agitate them. This type of soil is called a loam, and it is the ideal soil type for a vegetable garden.
If you find that your soil is a heavy clay you can amend it by working in some sand and compost. On the other end of the spectrum, if your soil is a loose sand you can work in a clay topsoil and compost. You’ll notice I recommended adding compost to mitigate both situations, and that’s because organic matter is always a beneficial addition to your soil as it will help increase nutrient and water retention.
Taking some time to test your garden’s nutrient levels, contamination, pH, and soil type will get you on the right track to setting up a hand’s-off vegetable garden that will provide you and your family with nutrient-dense, healthy homegrown produce for years to come!