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Research Spotlight: Soil Health: Many Tests, One Goal

Posted: August 31, 2015

Although researchers continue to study which soil test provides the best results for nutrients recommendations, they all agree on one thing: soil health is a priority.
Soil testing in the field

Soil testing in the field

Here at Penn State, researchers in the College of Agricultural Sciences are exploring which tests provide the most informative results for farmers to have healthy soil now and for future generations.  

Soil health testing can be time consuming and expensive for a laboratory to complete, as many different analyses are conducted to evaluate the biological, physical and chemical components of soil health.  How do you develop a soil test package that is cost-effective? Once farmers get the results, how do you use the numbers as a management tool?

To answer these questions, Charlie White, a Penn State extension associate in sustainable agriculture, and extension educators on the Field and Forage Crop Extension team, have been evaluating the different soil health testing packages that are currently available to farmers.

One finding from the research group is that soil health is driven by organic matter. Many soil health processes, such as microbial respiration, nitrogen mineralization, and aggregate stability, can be predicted by simply knowing the organic matter content of a soil.  Thus a simple organic matter test, which costs as little as $5 from the Penn State Agricultural Analytical Services Lab and many other soil testing labs, can be used as a general indicator of soil health.

The soil test currently used by the Penn State Agricultural Analytical Services Lab is known as the Mehlich 3 test, used to determine Phosphorus, Magnesium, Calcium, Potassium, and micronutrient content in a soil sample. While there are several different extracting solutions used for soil tests around the world, the Mehlich 3 test was developed for soils in warm, humid climates and provides results that have been calibrated to determine optimum nutrient levels for crop production in Pennsylvania. Soil tested in a different part of the country, may use a different extractant for the soil test, with results that have not been calibrated to the climate and soils of Pennsylvania.

Another of the soil health testing packages being evaluated includes a suite of analyses informally known as the “Haney Test.” The Haney Test was developed by Dr. Rick Haney, a soil scientist for the USDA Agricultural Research Service. The Haney Test is broken down into two different tests, the H3A nutrient extraction and the water soluble organic carbon and nitrogen extraction.

The H3A extract is a weak acid solution that simulates the ability of root exudates to dissolve nutrients into the soil solution. This can measure the amount of nutrients that might be available for plant use at a given moment in time. The results then can predict if the amount of nutrients is below or above the plant's requirement, although calibration of the test in Pennsylvania growing conditions is limited.

Comparing the H3A extract and Mehlich 3 extract, White has determined there is a high correlation between both tests. He concluded that both tests measure from the same pool of nutrients, and thus yield similar outcomes for nutrient recommendations.

The second part of the Haney Test is the water extractable carbon and nitrogen, where the soil gets shaken with water and then organic carbon, organic nitrogen and inorganic nitrogen amounts are determined. It offers “a new way to look at” Carbon to Nitrogen (C:N) ratios in the soil, according to White. Results may reveal the C:N ratio of the organic matter being consumed by microbes, helping to fine tune predictions of nitrogen supply from soil organic matter and cover crops. This holds promise for improved nutrient management and soil health. Researchers are still evaluating the reliability of the test results for these purposes.

To learn more about soil health testing, check out the soil testing page.