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Do I need to test my garden’s soil?

By Maddy Rohr, K-State Research and Extension news service

MANHATTAN, Kan. — Most gardeners think soil testing only determines nutrient deficiencies, but the tests also help gardeners understand whether the soil contains adequate nutrients. Basic tests check the soil’s pH, and the phosphorus and potassium levels.

“Most of the lawn and garden soil tests that come out of our soil-testing lab show more than adequate levels of both phosphorus and potassium,” said Kansas State University horticultural expert Ward Upham. “If those nutrients are not needed, applying them is a waste of money and can be a source of pollution.”

Upham urges gardeners to test their garden’s soil before beginning spring gardening, particularly if the soil hasn’t been tested in several years.

To do so, he says, take a sample from multiple locations in the garden and lawn, and which are representative of the soil. Samples should be taken at a depth between the surface and eight inches. Next, mix the samples together to total one pint of soil.

More information on taking an accurate soil test is available online from the K-State Agronomy Soil Analysis.

According to Upham, the soil sample can then be submitted to your local K-State Research and Extension office to have tests done at the K-State soil-testing laboratory for a fee.

“A soil test determines fertility problems, not other conditions that may exist, such as poor drainage, poor soil structure, soil borne diseases or insects, chemical contaminants or damage,” Upham said. “All of these conditions may reduce plant performance but cannot be evaluated by a soil test.”

Upham recommends submitting dry soil samples, as wet soil has different precautions.

“Wet soil samples should be air-dried before being submitted for testing,” Upham said. “Do not use artificial means of drying such as an oven or microwave as such treatment may result in inaccurate readings of nutrient levels.”

Soil tests should be used as a tool to identify nutrient deficiencies, Upham said, but often they do not tell the whole story of other factors affecting plant growth.

Factors that can affect plant growth that are not due to nutrient deficiencies or pH include:

  • Not enough sun
  • Poor soil physical characteristics
  • Walnut trees (walnuts give off a natural herbicide that interferes with the growth of some plants, such as tomatoes).
  • Tree roots.
  • Shallow soils.
  • Improper watering.
  • Overwatering.

Upham and his colleagues in K-State’s Department of Horticulture and Natural Resources produce a weekly Horticulture Newsletter with tips for maintaining home landscapes and gardens. The newsletter is available to view online or can be delivered by email each week.

Interested persons can also send their garden and yard-related questions to Upham at wupham@ksu.edu, or contact your local K-State Research and Extension office.