Study sheds light on sustainability practices that make for more resilient soybean

Over the last 25 years, Tennessee growers have experienced pretty much every kind of extreme weather: droughts, floods and heavy rainfall, extreme heat, sudden cold snaps and freezes, tornadoes, hail—you name it, says Virginia Sykes, a plant scientist at the University of Tennessee, Knoxville. The university’s test plot soybean fields lived through it all. The fields recorded how the crop responds to those extremes as well as to more normal weather, how soil health changes over time and how the crop responds to that, and how both the crop and soil health respond to various treatments and management practices scientists have thrown at the test plots. 

Scientists have been working on sustainability options with these plots since 2001—introducing different cover crops, biocovers, and crop rotations. Several things have become clear, Sykes says. One, “these long-term studies are really critical” for seeing how management practices change the soils over time. Two, poultry litter helps increase soybean yield and should be strongly considered by farmers. And three, “these sustainability practices make for a more resilient system that is more stable in the long term.”

Now, results from the two-decade experiment show just how important those management choices can be. Sykes and her colleague Isaac Mirahki, then a post-doc in Sykes’ lab, found that combining crop rotations with biocovers such as legume cover crops and poultry litter consistently improved soybean yields compared with simpler no-till systems alone. The findings suggest that diversified management systems can strengthen both soil health and crop resilience over time, reinforcing that no-till alone may not deliver the full benefits often associated with conservation agriculture.

The power of diversification

No-till farming is a cornerstone of sustainable agriculture. As of 2022, 93% of Tennessee farm acreage was managed under no-till conditions, far exceeding the national average. In keeping with the state trend, the two Knoxville research plots have both been managed under continuous no-tillage practices since the start of the long-running experiment, which was started by Fred Allen in 2001 and taken over by Sykes in 2016. For this study, published in Agronomy Journal in March, Sykes and Mirahki wanted to see how additional sustainable agriculture practices would affect soybean yield and soil health. 

The pair assessed 10 crop rotation sequences of corn, cotton, and soybean at one site and seven rotation sequences of corn and soybean in a different rotation at the other site. Soils at both sites were silt loam. Over time, they found that crop rotations alone resulted in yield gains between 1 and 32%. The greatest increase in soybean yield occurred with a corn–soybean rotation as the two crops work together to enhance soil nutrient cycling and promote microbial diversity. 

Overall, diversified crop rotations proved better than soybean monocultures at both sites, Sykes says. "To have a soybean year after year after year, you're going to start getting some pretty significant yield declines." By rotating them, farmers will have better harvests and a more sustainable approach, Sykes and Mirahki wrote in Agronomy Journal.

Crop rotations suppress pests, diseases, and weeds; enhance soil fertility; boost crop yields; mitigate nutrient depletion; reduce erosion risk; and improve soil physical properties like structure and permeability. “By systematically alternating crops in temporal and spatial sequences, this method mitigates nutrient depletion, as different species vary in their demands for soil resources, thereby preventing the exhaustion associated with monocultures,” Sykes and Mirahki wrote. 

Sykes and Mirahki also compared several biocovers—two legumes (Austrian winter pea and hairy vetch), wheat, and poultry litter—against a winter fallow control. Generally, cover crops did not offer a yield advantage compared with the winter fallow. Wheat showed variable performance year over year, with small yield deficits within a few rotations, Sykes says. This could be due to nitrogen immobilization, she says, which can occur with cover crops that have a higher carbon-to-nitrogen ratio. Legumes also showed variable performance but generally did not increase soybean yield. However, Sykes says, “while we aren’t increasing yield, we also aren’t reducing it, which is an important factor.” And there are other benefits to cover crops, she says. 

Poultry litter, however, was a standout: Using it as a natural fertilizer, poultry litter yielded up to 31% more soybean than the fallow site with gains year after year. Poultry litter likely provided bioavailable nitrogen, phosphorus, and potassium, which boost soil fertility. It also likely improved soil structure and water retention. Additionally, Sykes says, courtesy of the long-running study, the researchers could tell there is a cumulative effect on system productivity from decades of organic amendment application.

Why it’s not an either-or choice

The results of this study thus appear to favor poultry litter amendments over cover crops. But it’s not exactly a fair comparison, Sykes says. For one thing, in this study, the cover crops are planted as late as possible and terminated as early as possible. So “what we saw is probably the minimum benefit you could get from cover crops,” she says. If planted earlier and terminated later, farmers could maximize cover crop benefits, she notes. “In those systems, you would get more biomass and with cover crops, more biomass generally means more benefits and faster accumulation of those benefits.” Such benefits include pollinator habitat, slowly building soil organic matter, improving soil structure, preventing erosion and runoff, suppressing weeds, and retaining nutrients in the system without reducing yield, she says. 

Yields will vary year to year, depending on conditions, and the benefits farmers will see from amendments, cover crops, and crop rotations will also vary, Sykes says. “Some years, one of these sustainability practices will be better than another, but in general, they're all going to be better than the conventional approach.” 

Putting decades of data to work

Choosing the right sustainability practices can be challenging. Farmers should work with their CCA or local extension specialists to determine which crop rotations, cover crops, and soil amendments make the most sense for their soils and climates since practices that work well in Tennessee may not perform the same way in Iowa or California. Farmers can also turn to resources like the North American Project to Evaluate Soil Health Measurements, which runs a database of long-term soil health studies from across the country, including this one. 

Sykes also hopes that one day, scientists can turn these decades’ worth of research data into practical decision-making tools for farmers. Precision agriculture researchers have already developed tools to help growers choose cover crops, optimize planting dates, and determine ideal termination timing, she says. She would like to see similar tools for crop rotations—programs that could pull from long-term datasets and help farmers compare how different rotation sequences might affect yields over time. “We’ve got the data,” Sykes says. “We need to take those data and make tools that are usable for people.”

Along those lines, Sykes also hopes researchers can eventually identify the most effective crop rotation sequences for different regions and conditions. She is currently working on a paper examining corn and cotton yields from the rotations used in this study and would ultimately love to give farmers clearer guidance on combinations such as a 1-2-1-2 corn–soybean–corn–soybean rotation or a 1-2-3 corn–soybean–cotton repeat, or whatever combination proves most effective. We’re not there yet, she says, but that’s definitely a direction for future research. She is also working on a follow-up paper examining how these treatments affect soil health over time, particularly the buildup of soil organic carbon, nitrogen, and other key nutrients. 

For now, the two-decade-long study in Tennessee is providing evidence that building healthier, more resilient soils is less about any single practice and more about how multiple strategies work together over time.