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Use Soil Health Tests Wisely

Photo by: Shane Bugeja

What defines a healthy or unhealthy soil? If you interview five extension agents, you would get ten answers. These could range from how well microbes release plant nutrients to how the soil stands up to a heavy rain. Regardless, most gardeners know an unhealthy soil when they work one, no test required. However, if you take steps to improve your garden beds, you may want to know if you are on the right track.

Seeing this need, companies developed soil health tests for farmers and gardeners. The most famous one is the Haney Test, but newer ones that focus on microbes are quickly arriving on the market. With this flood of new products, it is vital that a grower knows these new tests’ limitations before any shovel breaks ground.

A key reason to be cautious is expense. A soil health test can be as high as $40-80 a sample—two to five times more than a standard fertility test you can get at the extension office. This added expense can be difficult to pencil out, particularly for folks who simply want a productive garden and are not selling to others.

To justify their expense, some products may also give fertilizer recommendations. Currently, the University of Minnesota does not endorse use of these in the field. Any increase in produce from these alternative tests should pay for the application as well as the sampling. Preliminary research conducted by the University of Minnesota suggests the well-known Haney Test could lead to over application of nitrogen and potassium.

Another caveat to watch out for is soil health “scoring”. This too must be taken with a dash of salt. Laboratories tend to calculate these scores differently and do not separate them by state. Frequently, they fail to take into account sampling dates, plant rotation, or soil texture. This could lead to a situation where a “below average” soil in Maine is an “above average” soil in Minnesota.

Additionally, soil health measurements tend to be a “snapshot” of current conditions. For example, moisture and temperature are notorious for affecting microbial or carbon tests. Sampling one field on a dry day and another after a drenching rain could exaggerate any soil health differences between garden beds.

To reduce error, compare soil health measurements between areas that differ only because of decisions you made and not nature. One example is comparing one vegetable plot under cover crops and another without. If you are searching for a goal for your soil health values, consider sampling an uncropped area near the garden. Ideally, this should include a place where living roots are present year-round. Regardless of where you try these tests, sample on consistent dates and weather to minimize variability.

If you are curious about purchasing these products, I recommend discussing them with a Master Gardener or University of Minnesota Extension Educator. Soil heath tests can help fill in knowledge gaps on our land and give an idea regarding how well that land can function. However, the flood of information they provide needs to be sifted with care, and not used as the sole reason for a garden decision.

Works Cited:

  • Stahl, L., Fern├índez, F., and Kaiser, D. 2018. “Can soil health tests determine fertilizer needs?” University of Minnesota Extension.
  • USDA, NRCS. 2018. Soil Health Technical Note SH-XX: Recommended Soil Health Indicators and Associated Laboratory Procedures.
  • Ward Laboratories. 2019. “Soil Health Analysis”.