Farmers have long debated which practices do the most to keep their soil healthy. Now, after pulling data from 21 long-term field trials scattered across the United States, a multi-state research team has found cover crop use comes on top.
The study, published in Agriculture, Ecosystems & Environment, was co-led by researchers at the University of Kentucky Martin-Gatton College of Agriculture, Food and Environment thanks to a subaward from the University of Wisconsin. This study, led by the University of Wisconsin, included contributions from researchers at the University of Kentucky MGCAFE and several other universities across the nation.
The team examined trials ranging from four to 50 years old, all built around soybean-based cropping systems, and focused on the top 15 centimeters of soil. They compared four of the most common management decisions a farmer can make: crop rotation, tillage, cover cropping and artificial drainage. Then they measured a suite of soil-health indicators tied to everything from biological activity and soil structure to nutrient levels and pH.
When the numbers were pooled, cover crops were the practice most consistently linked to improvements - specifically, higher levels of mineralizable carbon and water-extractable organic carbon, two markers that tell scientists the soil's microbial community is active and its carbon cycle is humming along.
"That was the part that really stood out," said Hanna Poffenbarger, associate professor of soil nutrient management in the UK Department of Plant and Soil Sciences and a co-author on the paper. "The whole point was to take advantage of long-term trials that were already in the ground and ask, across a wide range of soils and climates, which practices are showing up in the soil-health numbers."
Two-crop rotations also made a difference, at least for one metric: They were associated with higher soil-test phosphorus compared to continuous monoculture. But beyond that, rotation diversity, tillage method and drainage didn't produce clear, consistent shifts in the indicators used - at least not when the results from all those different locations were combined.
One surprising result was that no-till was associated with more acidic soil (lower pH) than conventional tillage in the trials included in the dataset. Poffenbarger said that the trend aligns with a known challenge in some systems: Surface-applied fertilizers can concentrate acidity near the soil surface over time.
"That pH pattern can show up when inputs are staying near the surface," Poffenbarger said. "It's a reminder that conservation practices can come with tradeoffs that need management, like tracking pH and addressing lime needs."
Poffenbarger also said the national scope of the project helps explain why some practices did not "pop" as strongly as many farmers might expect. No-till, for example, is widely promoted for protecting soil, but it does not behave the same way in every region.
"With a dataset this broad, you're seeing a lot of soil types and climates," Poffenbarger said. "No-till isn't necessarily going to look the same everywhere. If some locations benefit and others don't, that can wash out when you're looking at the combined results."
Poffenbarger's own group contributed samples from a long-term rotation trial at UK - with treatments dating back to 1986. Field crews across participating institutions followed a shared sampling plan to make results comparable, which meant collecting standard soil cores alongside shovel-dug samples designed to keep soil aggregates intact for stability testing.
It's worth noting what the study didn't do. The researchers tracked soil-health indicators, not yield or profit. They couldn't say directly whether the improvements they measured translate into better harvests or fatter bottom lines, though Poffenbarger pointed out that a growing body of parallel research is making that connection.
"This study helps tighten up the evidence on which practices are showing up in the soil tests across a lot of real, long-term systems," Poffenbarger said. "Then the next step is pairing that with work that connects soil improvements to outcomes farmers care about day to day."
This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2023-67013-39817. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the Department of Agriculture.
