Soil: To Test or Not to Test

“When are you going to learn?” asked Patrick as I lamented another failed crop in the front-yard garden bed. “There’s a witch buried under there. Nothing will ever grow. It’s just bad mojo.”

“Nonsense,” I replied. “I have no use for your superstition. I’m turning to science. I’m getting my soil tested.”

“When someone tells me they want to have their soil tested, I first ask, ‘Why?’” said Vincent Lazaneo, urban horticulture adviser with the UC Cooperative Extension. (I found Lazaneo by calling the San Diego County Master Gardener Association: 858-694-2860; mastergardenerssandiego.org.)

“If plants or weeds are growing on the site, then a test may not be needed. It might be cheaper to just add a little fertilizer and compost to the soil to help improve plant performance. If it’s very sandy soil, organic amendment may help it hold water, and if it’s a clay-type soil, adding compost may help loosen it up so that plants can grow. And some soil diseases like verticillum wilt or pests like root-knot nematodes won’t be detected by a soil test — you’d have to submit a sample of the plant roots or stem.”

If you are set on getting a soil test, said Lazaneo, “Then it’s important that you submit a representative sample. Collect at least six, maybe ten samples from the crop’s root zone — for vegetables, that’s around six inches down. Dig a small hole and take a thin vertical slice from the side of the hole. Put all the samples in a bucket, and mix them together to get a good composite sample. Then you can submit that sample to a lab or perhaps test it yourself with a kit.”

If you decide to submit your sample to a lab, warned Lazaneo, “Make sure they will provide an interpretation of the results. A number, just by itself, doesn’t really tell you anything. It can vary, depending on what substance the lab used to extract the various nutrients from the soil.”

The first two things a test should indicate, said Lazaneo, is the soil’s pH — how acidic or alkaline it is — and its salinity. “The pH affects the availability of nutrients to the plant. Most plants grow best in slightly acidic soil — pH of six to seven — unless they are ‘acid-loving’ plants such as blueberries. Those need a pH of around five. If the pH is too high, you can correct it by adding soil sulfur or acidifying elements such as peat moss. Salinity tests how salty the soil is. Plants don’t grow well in soil with a high salt content, whether the salt comes from fertilizers, manure, irrigation water, or other sources. Enough water must be applied by either rain or irrigation to leach away the excess salts from the root zone. But if your soil has poor drainage, the water can’t do that.”

Lazaneo said that to check drainage, “You dig a hole around a foot deep, and fill it with water two times. Then you see how long it takes to drain the second time. If it stays in the hole for a day, the plant’s roots will drown in that soil. In that case, you would need to install a drain line or consider planting in a raised bed or a container like a half-barrel.”

Finally, he cautioned against planting near large trees or shrubs. “They can shade your garden, and their root systems are often as wide as the plant is tall. Those roots will be competing with smaller plants for water and nutrients.”

I still wanted a test, and Tiger Palafox at Mission Hills Nursery (619-295-2808; missionhillsnursery.com) told me that he would send my sample to a lab in Ohio for $25.99.

“That’s for the basic test, recommended for anyone doing routine soil-fertility maintenance for turf or ornamental plants. It tests for pH, phosphorus, potassium, nitrogen, phosphates, potash, and lime. There’s another test for $35.99, recommended for when you’re seeding, sodding, or applying new plants. That one tests for all of those plus micronutrients — things such as magnesium and calcium. Also, the availability of those nutrients to the plants. They send the results, and we help you interpret them. We’ll also make suggestions for fixing any problems you might have.” Mission Hills Nursery also sells a home-test kit ($4.99) which tests for alkalinity, nitrogen, phosphorous, and potash.