By Andrew Margenot & Heidi Allen, University of Illinois Urbana-Champaign 

Research technician Grecia Romero uses an infrared gas analyzer unit to measure the concentration of carbon dioxide released from soil samples to estimate soil mineralizable carbon.

Over the past decade, the agriculture sector has seen growing interest in soil health, including research on conservation programs and, more recently, commercial soil testing. While the value of soil health, however it’s defined, is often intuitive – especially to producers – the best method for measuring it remains debated. Questions persist around cost-effectiveness, interpretability, and sensitivity to management practices that should build soil health.  

Funded by the Illinois Soybean Association (ISA) checkoff program, a team of researchers at the University of Illinois, led by Professor Andrew Margenot, has been evaluating a range of soil health tests based on three criteria: cost effectiveness, interpretability, and sensitivity to management practices. To do this, three sites (Monmouth, Urbana and Ewing) were chosen to reflect the north-to-south soil and climate gradients of Illinois. These sites feature replicated plots with multiple crop rotations (corn-soybean, corn-wheat/soybean), tillage (chisel vs no-till), and cover cropping (cereal rye or none) in all possible combinations. The trials span four growing seasons (’23-’26), beginning in fall 2022 with winter wheat and cover crop planting.  

They’re focusing on one of the most widely used soil health tests: respiration, also known as mineralizable carbon or CO2 burst. The idea is simple: a soil sample is placed in a sealed container and moistened. As soil microbes “wake up” and feed on organic matter, they release carbon dioxide (CO₂). The amount of CO₂ produced is measured. More CO₂ means more microbial activity, which is generally interpreted as greater soil health. Among all soil health tests recognized by USDA NRCS, offered by labs, and studied by researchers, respiration is one of the oldest and most commonly used methods.  

Results from the first two seasons of soil respiration testing in this ISA checkoff-funded project have offered valuable insights. They show how well respiration reflects – or doesn’t reflect – management practices like tillage, cover cropping, and crop rotation, as well as the timing of soil sampling and regional differences across southern, central, and northern Illinois. It’s important to note that for these tests to be useful in tracking changes from soil health practices, they ideally should show a response within two seasons.  

Insight #1: In soils with high organic matter (OM) in locations such as central and northern Illinois, tillage, cover crops, and crop rotation had no effect on soil respiration in the first two years. This suggests it may be better to wait at least two years after making a practice change before testing for soil respiration.  

Insight #2: At Ewing and Urbana, double cropping of wheat-soybean led to 40% and 34% higher respiration in the following corn crop. This suggests that crop rotation may boost microbial activity more than tillage or cover cropping in the first two years after a practice change.  

Insight #3: At the low OM site, the timing of soil sampling had as much, or even more, impact on soil respiration than tillage, cover crops, or crop rotation. Even after accounting for the previous crop, respiration was 44-66% higher in soils sampled in the spring compared to the fall, depending on the crop harvested beforehand.  

Insight #4: In southern Illinois, sampling after soybeans in corn-soybean rotations didn’t affect soil respiration. The same was true for any crop at the high OM sites in central and northern Illinois. This suggests that in low OM soils (like southern Illinois), you may have more flexibility in when you sample after soybeans. And in higher OM soils, timing seems to matter less overall. Keep in mind, no effects of tillage, cover crops, or rotation were seen at these high OM sites during the first two years.  

Follow for more updates on this ISA check-off funded research by visiting the Research section on FieldAdvisor.org.  

This column was originally published in the September 2025 edition of Illinois Field and Bean magazine.