Research Highlights

Research Highlights
Testing Soil Health Tests

Soil from no-till field that uses cover crops. Photo: Ohio NRCS

By Laura Temple

Farmers often associate soil health with characteristics they observe in their fields and crop productivity. USDA defines soil health as the continued capacity of soil to function as a vital living ecosystem that sustains plants, animals and people. While these definitions don’t completely align, farmers ask questions to better understand what soil health means for yields and how to track changes over time.

“The farmers I work with asked about the best soil tests to measure soil health,” says Elizabeth Hawkins, a field specialist in agronomic systems with Ohio State University Extension. “As I looked at what was available, I learned that we really didn’t have data to answer those questions.”

The Ohio Soybean Council agreed to fund research to better understand what currently available soil health tests can tell farmers about soil health. Hawkins started this work in 2020 with an in-depth conversation with farmers to understand their soil health test needs.

“Tests to track soil health need to be affordable, reliable and repeatable,” she says. “Farmers made it clear that they want to be able to see trends in their soil health over time, and those three characteristics summarize what they need.”

Through the discussion, Hawkins also learned how Ohio farmers define soil health. For example, she found they associate soil health with improved water infiltration, a characteristic that reduces ponding and helps soils handle heavy rain events with minimal erosion and flooding. 

Measuring Soil Health

Based on farmer direction and her research on available tests, Hawkins and her team identified four soil tests to effectively track what is happening in the soil. 

  1. A basic organic matter test provides information about total carbon in the soil.
  2. The active carbon test measures biologically active carbon, the part of the soil where microbes work to make nutrients readily available to plants. It is commonly called a POXC test, short for permanganate oxidizable carbon.
  3. A respiration test estimates soil microbe activity. It measures the carbon dioxide being released by microbes living in the soil. 
  4. Wet aggregate stability measures how the soil holds together in water, which is closely related to water infiltration. This is the most complex, variable test in the study, because soil handling from the field to the lab influences results.

The study funded soil testing for 250 to 300 farmers each year in return for in-depth information on long-term management practices and crop yield. Local extension agents gathered soil samples to do the tests, and farmers shared how they manage those fields and crop yield history. To build a more comprehensive database of soil data, the team partnered with other researchers doing soil tests to include the information they gathered. 

“We targeted fields under different practices for varying amounts of time to begin correlating them with soil test results,” Hawkins says. “For example, we considered conventional tillage systems, no-till, incorporating cover crops and applying manure. This data provided a way to study the types of potential changes farmers could expect from different practices.”

Correlating Soil Test Results, Management Practices and Yield

As Hawkins linked soil test results with information from farmers, she first concluded that soil type matters. Soil test results correlated with soil texture and soil cation exchange capacity, or CEC, the ability of soil to store specific nutrients.

Ohio farmer Dave Brandt digs up a field pea cover crop. Photo: Ohio NRCS

“All the test results related first to soil type, so when looking at soil test results, we can’t directly compare fields with different soils,” she says. “Any improvement in soil health over time will vary based on that.”

To help farmers determine realistic soil health goals for different soil types, Hawkins and her team developed a baseline tool showing soil characteristics by soil texture and CEC. It provides more context for farmers as they consider how their current management practices impact soil health and how additional management could support further long-term improvements.

Though soil type dictates much of how soil responds, similar general trends resulting from management practices appeared across soil types. 

“Many of the links we’ve found between management and soil test results support what we expected,” she continues. “Cover crops show a strong correlation with building soil organic matter and increasing aggregate stability. Manure application also correlates positively with organic matter.”

However, other findings have challenged assumptions about soil health.

“Though aggregate stability increases under no-till, we didn’t find a positive correlation between no-till and organic matter or active carbon,” Hawkins explains. “While residue on the soil surface minimizes erosion, it isn’t getting deep into the soil.”

To understand the value of this soil health data to farmers, the team began gathering yield data in 2022. Organic matter content, active carbon from the POXC test and respiration all show positive correlations with crop yield.

“While we expect to wait five or more years for measurable changes, we’ve seen that cover crops have a relatively quick impact on organic matter content,” she says. “Understanding how some of those measures link to yield tells farmers what to look for in their soils over time.”

Elizabeth Hawkins profile

Published: Apr 17, 2023

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.