From a thousand-foot view the recommended formula for increased on-farm soil health sounds surprisingly simple: Cover crops plus reduced tillage equals more soil organic matter and greater water infiltration. For farmers in the Mississippi Delta hoping to preserve a unique legacy of highly productive soils and vast water resources, conscious conservation can be a bit more complex.

“Rarely do we see increased infiltration rates in high residue management systems at a basic plot scale,” said Jason Krutz, director of the Mississippi Water Resources Research Institute. “When you roll it all the way up to a field scale, reduced tillage with cover crops compared to more conventional low surface residue systems had undetectable changes in water infiltration rates.”

Even in studies conducted over many years, the benefits of high residue cover crops combined with reduced or no tillage did not yield the results researchers sought. Extreme weather conditions made cover crop establishment and desiccation difficult. Even in years with good weather, good stands and good yields, the infiltration benefits, especially in highly productive silt loam soil types, did not emerge.

The ultimate answer was to try something different.

Subsoil science

“That led us to do another experiment at the plot scale where we started looking at more subsurface disturbance, because what we thought was happening when we did eliminate a lot of tillage is that we started having compaction issues, particularly subsurface, which hindered the infiltration rates,” Krutz said. “We started looking at systems that we referred to as minimizing surface disturbance and maximizing subsurface disturbance with really narrow shanks dropped down 7 to 10 inches that would fracture plow plans allow greater infiltration rates.”

At the research plot scale, eliminating the compaction with as little surface disturbance as possible improved infiltration rates relative to the control. With that initial finding, researchers realized they were on the right track.

“Silt loam soil seems to be especially susceptible, much more so than heavy clay soils that will crack so there's no problem with infiltration on them in the middle of the summer,” said MWRRI assistant professor Dave Spencer. “Silt loam soils, because of the different texture and the different properties that they have combined with the fact we’re no longer subsoiling to break up that compaction can have bigger issues with infiltration.”

Subsoiling – ripping up deep compaction under a field’s surface – was once a common practice at regular intervals throughout the Mississippi Delta. Today, as producers pursue conservation tillage and prepare fields with fewer field passes and fewer implements, the practice has fallen to the wayside. 

“Subsurface tillage does a lot more for infiltration rates than I think anyone ever actually realized, especially the general public, because a lot of our producers have permanently abandoned the practice - probably less than 50% actually subsoil anymore,” Krutz said. “I think that that would be a simple technique to improve infiltration rates.”

Mississippi State research experiments typically centered around yearly subsoiling or ripping strategies, but Krutz said implementing the practice every 18 months to two years could still be beneficial for improved infiltration.

Cover crops

“We do recognize that having subsurface protection through surface residues does provide some of the benefits we're looking for,” Krutz said. “But it's not the whole answer. It's not the whole picture because there was deeper compaction that was still limiting our infiltration.”

Even when cover crop stands were well established and not limited by weather-related factors like dry autumns and early or late cold snaps impacting winter kill, soil conditions that limited cash crops simultaneously limited the benefits of cover crops.

“It makes sense intuitively that if the compaction layer is limiting infiltration and potentially the cash crops, it probably can limit the cover crop,” Spencer said. “We've seen things like until the root has grown down into the soil, it hits that compaction layer, and it starts pushing it above the soil. There will be several inches of the tuber sticking above the soil because they've encountered hard pack or it'll hit it and turn sideways. We haven't seen them be able to deal with that compaction.”

On large scale cover crop studies, benefits typically emerge over a long span of time. Krutz said even with as many as 15 years dedicated to research for cover crops in the Midsouth region, the evidence still does not support profound benefits.

“Sometimes we could see improved infiltration rates with the cover crops at a small scale but almost universally, we would see somewhere between $50 to $200 an acre loss in returns associated with cover crops on-farm,” Krutz said. “That's more for me as a research environmental agronomist to recommend.”

Above all, Krutz said research that happens on a small scale at the university level also has to perform on-farm from a standpoint of profitability and conservation.

“We are striving to make a production system that does what needs to be done,” Krutz said. “We don't need a lot of surface runoff. We don't need sediment, nitrogen, or phosphorus in lakes, streams and local reservoirs. We need nutrients to stay in place. We just refuse to continue to do the same thing and have the same effect - that is the classic definition of insanity.”