If You’re Questioning the State of Your Soil, Here’s How to Test It

We sorted through more than 100 options to find kits that were easily accessible, provided clear collection instructions, and were not too expensive. Nutrient-testing kits had to assess NPK levels and pH to be the most useful to gardeners. Lead tests had to meet the US Environmental Protection Agency’s testing standards and provide specific numerical reads rather than simply indicating the presence of lead. This is important, because while no level of lead exposure is safe, the EPA says households can have up to 200 parts per million (ppm) of lead in their soil or multiple spots with 100 ppm before remediation is necessary (though California’s threshold is lower, at 80 ppm). And if you simply look for the presence of lead in soils in an urban area like Los Angeles, you’re likely to find it — so you’ll still have to test again to get more specific information about your risk level.

Picking kits: Of the options we found that fit that bill, we ordered four nutrient test kits. Though all of the tests correctly identified our test soil’s high acidity, none of the kits we tried delivered the clear results we desired. Test tubes dribbled solution, capsules crumbled, and tablets failed to disintegrate. Results and recommendations were difficult to interpret.

Though these results were discouraging, ultimately leading us to university extensions for gardening-soil assessments, we decided to give kits another try when we looked at lead. Specifically, mail-in kits seemed like they could offer the mix of accessibility and expert-level analysis we were seeking, so we sourced four to test.

Sourcing soil: Our initial test found us in a Brooklyn, New York, community garden where we collected samples to assess their nutrient breakdown. As a control, we mixed vinegar and a store-bought fertilizer with a known NPK breakdown to see if the kits could identify the correct nutrient levels as well as a high pH.

To test contaminants, I turned to my colleague, Michael B. Cohen, and Wirecutter contributor Gregory Han for advice when it came time to gather samples. Both were impacted by the Eaton Fire and have shared their experiences dealing with the aftermath. Michael lost his home, while Gregory’s survived but was left with lead contamination issues. Both were eager to get more insight into their soil, so I took samples from each property.

Scanning and scooping: I collected soil from both sites on the same hot, dry August day. To mitigate my own exposure, I wore goggles, a mask, and disposable gloves, which I switched out between taking samples from different areas. Some of the kits came with small scoops, but I opted to use my own trowel, which I wiped down between uses.

Unlike our tests for soil nutrients, there really wasn’t a control option available, as lead in soil can vary widely, even across the same property. So I asked experts for their advice on how to sample the spaces. Thomas Borch, a soil chemist at Colorado State University, told me how to do what he called “judgmental sampling.” That means zeroing in on areas where higher-risk items might have burned, such as a lawn mower, electric vehicle, or car. In addition to identifying those potential hot spots, soil scientist Lynn Fang advised I focus on areas that residents interact with regularly, such as entryways or where kids play. Andrew Whelton, a professor of civil, environmental, and ecological engineering at Purdue University, walked me through what he described as the “gold standard” of representative sampling: Divide your property up into even sections, and then take multiple samples from each and combine them, giving you a composite from each area.

Before I dug in, I asked Michael and Gregory about any previous testing they had performed, as well as areas of potential concern. Michael pointed out where he had a solar battery, cars, and a bathroom with older plumbing; Gregory showed me areas that have remained untouched since the fire. I collected samples from each hot spot they shared with me, and for Michael, I gathered a representative sample from across the footprint of his house. I focused on scraping the top levels of the soil since that is where any fire-related contamination was most likely to be found, and I tried to shake or pick out any rocks and other debris.

Once I collected the samples, I split them up between the kits. Some requested samples from a single spot, while others recommended composites, so I followed those instructions and sealed them up for their journeys. Then I registered them online and waited.

What goes on at a soil-test lab, anyway?

While I was waiting for my mail-in results, I met with Josh West and Seth John, two professors who are part of the Contaminant Level Evaluation and Analysis for Neighborhoods (CLEAN) project at the University of Southern California. This is one of multiple free soil-testing initiatives that universities and local government agencies have created in the aftermath of the Los Angeles wildfires. Residents collect samples and either mail or drop them off, or they can arrange for collection to be done on their behalf.

At their USC lab, West and John walked me through how they use an X-ray fluorescence (XRF) analyzer to check for lead levels. To prep the samples, they dry them out in an oven and then sift them before storing them in a clear container. Then they run a control sample through the XRF machine to make sure its reading is similar to previous attempts before giving the new samples a go.

If a sample reads above 200 ppm — the EPA health limit — it’s checked twice more and receives a shake between tests, just in case a fleck of paint or something else was skewing the results. Those numbers are then averaged to get the final result.

West noted that this method has a wider margin of error than the dissolution process performed in the labs where I sent my mail-in results, but it delivers results quickly and with enough accuracy to allow people to take informed next steps. Each result recipient receives a guide to lead levels and recommendations for mitigation. West, who was impacted by the fire, was able to speak about undergoing this process personally. That’s the sort of expertise and access that would be difficult for a mail-in kit company to match, and it’s another reason why we suggest seeking help from your local experts.