Jun 29, 2023

Skip the Soil Test Kit. Here’s What to Do Instead.

When you’re starting a garden from scratch or prepping your yard for spring planting, it’s important to know the state of your soil. Adding fertilizer with specific nutrients can make up for any deficits or imbalances, and that will help maximize the growth potential of your chosen plants.

But what if your soil contains lead—or something worse?

There are many at-home soil-test kits on the market, and when we began researching this story, we had hoped to find one we could recommend. Unfortunately, all of the kits we tested were complicated, messy, and, often, inaccurate.

Instead, we recommend sending soil samples to a testing lab at an extension office, a university-affiliated community-service organization that’s designed to help people with all sorts of agricultural issues on a local level.

Collecting and mailing dirt from your yard to an extension office is a relatively quick process compared with the hour or more it takes to do a soil test at home. (There are mail-in soil tests you can buy, but we focused our research specifically on the tests kits you do yourself.) Compared with a DIY test’s results, an extension office’s report will not only be far more accurate but also much more thorough in terms of any issues with your soil. It will also provide more-detailed instructions on how to correct any problems based on what you’d like to grow.

The cost for an extension office soil test varies a bit from state to state. Those at the state-university–run labs we looked at ranged from free to $50, while some at private-university–run labs ran as high as $90. (The most expensive DIY kit we researched was $40.) You’ll have to wait about seven to 10 days for the results, but we think receiving a professional analysis and having information on specific next steps to take are well worth the slight delay.

I’ve worked at Wirecutter since 2019, and in addition to covering emergency preparedness and cleaning, I wrote our guide to plant identification apps. I host a podcast about botanical crimes and write a freelance gardening advice column for PopSci. I’m also chair of the plant committee at my community garden, where I’m responsible for finding out what’s in our soil.

Most bags of fertilizer come marked with an NPK label, the elemental shorthand for nitrogen, phosphorus, and potassium (called “potash” in the agricultural world), and those numbers are measures of each nutrient’s percentage by weight. If you buy just any bag of fertilizer and start adding these nutrients into your ground without knowing what’s already in the soil, you can harm your plants or even cause larger environmental damage. Soil-test results allow you to choose fertilizer with NPK levels that balance your existing dirt.

Addressing pH level is another important component of soil adjustment. A pH test is part of every soil test we found, and you can also buy a pH test separately, from soil-test-kit manufacturers or your local hardware store. (We did not test these individual options.) Most plants prefer to grow in soil with a neutral pH, according to the Cornell Cooperative Extension. If pH levels are too high or too low, plants may not be able to soak up nutrients. Generally, you can reduce the pH by adding sulfur or raise it by adding lime.

All of the extension offices I checked with—as well as the instructions on every soil-test kit I looked at—recommend that you test your soil every few years. My community garden tests annually.

I started by researching 112 soil-test kits that I found online. I narrowed that list down by focusing on established companies, test availability, and affordability (below $100, though most were less than half that). And I confirmed that a test provided instructions on how to gather the soil and what to do after the results were determined. At a minimum, a test had to measure NPK and pH levels. This criteria culled the list considerably, down to six soil-test kits.

When the kits arrived, I discovered that several of them were identical—all from a company called Luster Leaf Products but packaged under different names. I performed at least two tests for each model. For the first one, I used fertilizer and vinegar as a control to determine whether each kit could accurately identify the pre-established NPK label and high pH. For the second, I performed the same tests on soil samples from my community garden. All of the tests correctly identified the vinegar’s high acidity.

I had less luck with the control tests for nutrients, using fertilizer with a 6-4-4 NPK label (6% nitrogen, 4% phosphorus, and 4% potash). I struggled to read the results of the fertilizer’s three confirmed nutrient ratio tests. As a result, I had little faith that I would be able to correctly discern the results of my garden-soil test.

The kits were also difficult to use, and they involved multiple steps that left much room for error. And errors there were: Test tubes that needed to be inverted were difficult to seal, resulting in a liquid solution dribbling all over my hands. One test required adding the contents of a capsule to a vial of water, but the capsule crumbled when I pushed the powder out of the casing. Tablets wouldn’t disintegrate properly in water, even after five minutes of vigorous shaking. All of this brought back my worst memories of chemistry class.

I also found the kits’ instructions on correcting problems to be confusing and unhelpfully specific. One test listed the ounces of dried blood needed to make up a nitrogen deficiency. Another soil test simply recommended that I ask an extension agent what to do. And so I did.

Extension services are one of our country’s best-kept secrets. Every state has a university that offers its resources and knowledge about agriculture to the public through an extension agency. Since these experts live in your region, they have more specific and relevant information on the soil in your backyard than any boxed kit can provide. And their labs use impressive tools, such as an optical spectrometer, which vaporizes soil at 10,000 degrees Kelvin and then measures the wavelengths of the elements to identify them.

I live in Brooklyn, New York, so the extension agency for my state is Cornell Cooperative Extension. However, its soil test currently costs $90. So I called the extension office run by Rutgers University, in my neighboring state of New Jersey, to see whether I could get a test for less. Even though I live out of state, Rutgers will test anyone’s soil, and it charges only $20 for its basic test. This is less than the cost of most at-home soil-test-kit options, and I forked out another $18 to get a lead-screening test as well. (Folks who live in Arkansas are lucky—the extension system at the University of Arkansas offers free soil analysis to residents.)

My soil-test results found that the community garden soil was high in most nutrients, but its pH level was a little basic. That means we could add sulfur or adjust the species of plants we plan to grow.

There are two drawbacks to using an extension service’s soil laboratory. First, there’s the inherent delay in mailing your sample. Some of the fastest labs will turn samples around in two to three days. But there’s still the time spent in the mail, and the average turnaround time for a lab is seven to 10 days. Even so, we think the wait is worth it. If you happen to live near an extension office, you can drop off the sample in person. But you might want to call ahead of time to be sure it takes in-person drop-offs.

The second drawback ties directly into the time delay. Stephanie Murphy, from the Rutgers extension office lab, told me that nitrogen (the “N” in NPK) is not part of the lab’s standard testing because the levels in the soil change daily, and it makes more sense to give recommendations based on the type of plants you’re trying to grow. Other extension offices will still test for nitrogen, but Jeffery Waskom, assistant laboratory manager at the Texas A&M soil-testing lab, said that “95% of the time, there’s very little nitrate nitrogen found in the soil.”

Murphy said you can still (and more effectively) find out the correct amount of nitrogen fertilizer to add based on the plants you are going to grow, the time of year, growth stages of the plant, and the weather (for example, if you experience a lot of heavy rain, it could wash out the nitrogen). Combined, this information provides a formula for lawns. For its soil-testing clients, Rutgers recommends fertilizers based on a ratio that it calculates by combining the nitrogen required for the type of plant you’re growing with the amount of phosphorus pentoxide and potassium oxide in your soil (these two compounds were measured in my extension office’s basic soil test).

Rutgers calculated this ratio for me in its report, and it offers a fertilizer finder site that links required nutrients with commercially available fertilizers in New Jersey. A conversation with a representative from your local extension office will most likely yield similarly specific solutions.

When it comes to testing your soil, it may be a good idea to consider lead and other heavy metals, especially if you live in a historically industrial area or in an old building where lead paint might have fallen into the soil. The home soil-test kits we tested don’t analyze lead levels, but many extension agencies will, and they can provide guidance on how to deal with its presence. If your state’s extension office doesn’t test for lead, reach out to extension agencies in neighboring states until you find one that does.

Luster Leaf is the big name in soil-test kits. The Luster Leaf 1601 Rapitest Test Kit is the company’s slightly cheaper option. But we found the Luster Leaf 1662 Professional Soil Kit and the Luster Leaf 1663 Professional Soil Test Kit a bit easier to use. (Although these models are similar, the 1663 has more versions of each test: two extra test tubes, and a special soil-filtering device.) Larry Holbein, chairman of the board at Luster Leaf Products, told me that “the instructions are pretty simple.” I did not find that to be the case, and for me, the results for all were either difficult to interpret or inaccurate.

When we unboxed kits from Flinn Scientific and Rapitest, we found that they were clearly labeled Luster Leaf kits (the 1663 and 1601, respectively). Holbein told me that Rapitest is a brand under Luster Leaf that the company sells to other parties, including Flinn Scientific.

When we started out, we didn’t realize that the Mosser Lee Soil Master Soil Testing Kit would be the only other unique brand we tested. The results of the nitrogen and phosphorus tests showed up clearly and correctly during our control trial. But in all other respects, we had the same issues with the Mosser Lee kit as we had with the others.

This article was edited by Joshua Lyon and Harry Sawyers.

Stephanie Murphy, director of the Soil Testing Laboratory, multiple phone interviews, lab visit, February 2023

Larry Holbein, chairman of the board at Luster Leaf Products, phone interview, February 22, 2023

Jeffery Waskom, extension assistant in the department of soil and crop sciences at Texas A&M University, phone interview, March 14, 2023

Ellen Airhart

Ellen Airhart is an associate writer at Wirecutter, where she covers cleaning and emergency preparedness. Please email her with your biggest messes and most anxious thoughts.

by Wirecutter Staff

These 40(ish) useful things are some of our favorite gardening picks.

by Harry Sawyers

These are the tools and supplies you need to start a container garden in a small area.

by Rachel Cericola

The benefits of smart-home devices don't stop at the door. We found great ones for the backyard and garage.

by Harry Sawyers

Here’s everything you need to keep your yard tidy, no matter what winds up on the ground out there.

Flinn ScientificRapitestMosser Lee Soil Master Soil Testing Kit