Adventures in Soil Testing: Do DIY Kits Match Pro Results?

Lisa's garden area where she tested the soilWhich test kits work better — DIY or professional? We compared them with soil samples from this garden. Image: Lisa Kaplan Gordon

Testing your soil should be a first step in any landscaping plan. Yet, few of us take the time to know our soil up close and personal. Here’s how to remedy that.

Have you tested your soil? Do you know its pH? Phosphorous level?

Me neither, and after 20 years of gardening, I should know better.

The U.S. Department of Agriculture says soil testing is an “essential” tool that provides information on fertility and pH. Testing also indicates what amendments — lime (raises soil pH), fertilizer (increases nutrients) — you should add to perk up tired soil.

So I finally decided to take the test — professionally and with DIY kits.

Testing by the pros

Public soil testing in Virginia, where I live, is conducted in laboratories at Virginia Tech on behalf of the Virginia Cooperative Extension. Your county extension agent can direct you to soil testers in your area.

Testing the soil in my vegetable garden, where I’ve never been able to grow a decent tomato, was an easy, five-step process.

1. I picked up a free soil-sampling kit at my local library — a small white box for the soil, and a form that requested information on the trees, shrubs, and plants I wanted to grow. To find out where you can get a sampling kit, call your extension agent.

2. I filled out a form that asked what I planned to grow — grass, trees, shrubs, or, as in my case, vegetables. The form also asked when pH-raising lime was last added to my soil, which was never.

3. Using a metal trowel, I made a vertical cut 4 inches down, scooped out soil, and placed it into the sampling box. I removed the worms, because my garden wants all the industrious worms it can get. 

4. Wrote a check for $16, which bought me a routine mineral and pH analysis ($10), plus information on organic matter ($4) and soluble salts ($2).

5. I mailed the sample to the lab.

DIY testing

You can buy DIY soil-testing kits at hardware stores for about $7, or at garden centers that will charge up to $30 for more elaborate setups (even the cashier was surprised they cost so much).

I randomly bought three types of testers:

  • pH meter ($23): Stuck a probe into the soil, and it registered pH on an attached meter.

pH meter

  • pH soil tester ($5): Mixed soil with water; poured it into a plastic container, added powder from capsule, and shook. Compared the color of water to the adjacent color scale, which indicated the pH.
  • Soil test kit ($25): Same as the pH tester, only it adds three extra containers and capsules to measure nitrogen, phosphorus, and potash. Note: The state lab did not test for nitrogen because it moves in and out of soil so quickly that testing would be meaningless.

Testing options

What I found out from Virginia Tech

Test results

  • pH: 5.8, which is slightly more acidic than tomatoes like (6.0 to 6.5 pH), which could be why my crop never flourishes.
  • Phosphorous, calcium, and magnesium: Very high, which is good and can be maintained with fertilizer.
  • Micronutrients(such as iron and zinc): sufficient
  • Organic matter: 7.8%, which is higher than normal (yea!) for Virginia gardens (.5 to 2.5%) and the result of endlessly digging in compost from my pile.
  • Soluble salts (indicates if too much fertilizer has been applied): Low

The report suggested that I apply 8 lbs. of ground or pulverized limestone per 100 square feet to make the garden less acidic. It also suggested that I apply a nitrogen-only fertilizer.

What DIY kits told me

Basically, nothing.

pH results

The pH meter registered 7.0, which is neutral and more than a point higher than the extension service testing indicated.

The pH soil tester came up with a pea green color that most closely matched the color gauge for a “slightly acid” pH of 6.5, closer but not the same as extension service results.

And the third kit, with containers that test for potash, phosphorous, and nitrogen (even though the state says nitrogen tests are meaningless), didn’t register at all. Only dingy water remained.

So none of the three kits I tried offered the detail or same results as the state test.

Lesson learned: Go with your state extension service or other reliable professional testing organization.

Have you ever tested your soil? Did the results surprise you?