Released: March 11, 2004

Garden Soil Testing a ‘Must’ for Sound Plant Nutrition

OLATHE, Kan. – Whether new to dirty fingernails or an old-hand green thumb, all gardeners should have their soil’s fertility levels firmly planted in the back of their mind.

“Just looking at your plants won’t provide the proper information. Most cheap home soil-test kits won’t, either,” warned Chuck Marr, horticulturist with Kansas State University Research and Extension.

Plants get the carbon and oxygen they require from the air. To be healthy, however, plants also need to be able to extract a whole array of other nutrients from the soil.

“You can get your soil tested for this array. In central Plains soils, however, you rarely need to test for more than phosphorus and potassium availability, plus the soil’s pH level – its acidity or alkalinity,” Marr said. “You can even skip testing for nitrogen. If plants were growing there last year, you can assume nitrogen’s needed now, unless you were growing a nitrogen-fixing crop, such as beans or alfalfa.”

A good soil test report will recommend any needed changes in pH level. It will provide details on what mixes of fertilizer to apply, when to apply them, and at which rates.

“This kind of information is basic for developing a good plant nutrition program,” Marr said. “For example, if your soil doesn’t need phosphorus or potassium – which may very well be the case – your applying it is a waste of time and money. And, if you keep applying it year after year, you eventually could start harming your plants, if not also contributing to your area’s surface water pollution.”

In recent years, the Soils Testing Lab at K-State has found some Kansas landscape samples contained so much phosphorus and potassium that testing couldn’t measure the amounts. The results literally were “off the charts.”

“Nutrient levels can change over time, of course,” Marr said. “That’s why K-State recommends testing garden and crop soils every three to five years. Lawn and pasture soils can go longer between tests, so long as you get that first baseline test and keep fertilizing accordingly.”

Soil tests are only as good as the samples submitted, however, the horticulturist said. So home gardeners should follow these steps:

1. Mentally divide your landscape into areas. Each area should be fairly uniform in soil color, texture and slope, as well as ability to grow plants. Avoid or sample separately any places that could give misleading results: low spots, old fence rows, the soil under shrubs you gave extra fertilizer, and the like.

“You’ll nearly always need to test your garden areas and your lawn separately – if nothing else, because they’ve had such different treatment in terms of cultivation and fertilizer,” Marr said. “You may need more tests than that, however – particularly if your lot is large or if you have obvious differences in soils. For example, you may have a garden that’s fill dirt in one part and native soil in another.”

2. Sample each uniform area, one by one. Each time you’ll need a clean bucket and something with which to collect soil – probably a soil probe, auger, pipe or spade. Take 10 to 15 thin cores or slices per area, making them as close to the same size as possible. For garden areas, include soil down to 6 to 8 inches deep. Lawn samples can extend down just 3 to 4 inches (minus thatch and turf).

“This is easier all the way around if you don’t take samples when the soil’s really wet,” Marr said.

3. For each uniform area, put all samples in your bucket, crumble them and mix thoroughly. Remove any stones, grass, roots and thatch. Then take out a cup to a pint’s worth of soil, air dry it (do NOT use heat), and put it in a sealable container.

“Just spread the soil out on a newspaper to dry,” Marr said. “And when it’s ready to store, put it in a plastic food container or lock-seal plastic bag so it will be easy to ship.”

4. Label each container with your name, complete mailing address and phone number. Also put down the intended use for the soil.

“You can send your sample to a commercial lab. Or, if you prefer university test results, you can take it to your county’s Research and Extension office, to be mailed on from there,” he said. “Either way, the small amount of cost for testing will yield big results in terms of targeted landscape management and healthy plantings.”

Getting test results back can take 10 days to three weeks. The length of time often relates to the time of year, Marr said. Spring and fall bring the labs’ heaviest work loads.

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Kathleen Ward
kward@oznet.ksu.edu
K-State Research& Extension News