When it comes to soil health, there's been a lot of discussion surrounding perennials. Of course, extended crop rotations and cover crops can provide a number of soil health benefits, like soil water infiltration, reduced soil erosion, and over time, improved organic matter — all benefits that come with having living roots in the soil for a longer period. However, with perennials in the rotation, those benefits are extended even further — and there's much less disturbance involved.

The soil health and agronomic benefits of perennials in the rotation are well-known places like the Pampas region of Argentina, but have also been observed in the Midwest and Great Plains when planting corn following alfalfa, or into land that was previously pasture or prairie.

But what if a perennial grain crop could provide a profit? Manbir Kaur Rakkar, a postdoctoral associate in the University of Minnesota-Twin Cities Department of Agronomy and Plant Genetics, has been researching Kernza — a perennial grain crop with a deep, dense root system. Rakkar discussed some of the University of Minnesota's preliminary research findings in a recent Iowa Learning Farms webinar.

Kernza study

Rakkar and her fellow researchers tested Kernza, or intermediate wheatgrass, during a three-year organic transition period to study the agronomic and environmental impact of the crop at three Minnesota sites: St. Paul, Rosemont and Lamberton. Of course, during this period, no pesticides or inorganic fertilizers can be used, but no premium is earned. The study, which began in 2017, used six treatments: three annual cropping systems, soybean-corn, soybean-corn plus a cover crop, and winter wheat-red clover. These were compared against three perennial systems: alfalfa, intermediate wheatgrass, and a bi-culture of intermediate wheatgrass and alfalfa. After two years of these rotations, the plots were tilled, and corn and soybeans were planted to determine any legacy effect from the previous cropping system.

"The intermediate wheatgrass, or intermediate wheatgrass plus alfalfa system had way more root biomass compared to the annual cropping systems," Rakkar says. "If we took the cumulative root biomass all the way to 60 centimeters [soil depth], it was about six times higher. If you look at 0 to 15 cm, the upper 5 inches, in the very first year, there was about 39 times more root biomass in Kernza systems compared to the corn and soybean system."

The study also measured physical soil health indicators, as well as chemical and biological indicators. Physical indicators include mean weight diameter — an indicator of how strong soil aggregates and soil structure are, and how resistant the soil is to erosion.

"Within these two years, from 2017 to 2019, we did observe higher mean weight diameter in intermediate wheatgrass systems compared to the annual cropping system at two of the sites. It was about 1.55 to 1.64 times higher than annuals, on average," Rakkar says. "We think that happened because of higher root biomass, year-round root coverage and root input into the soil to stabilize those aggregates, to stabilize that soil structure."

Another metric used was sorptivity — an indicator of soil water infiltration.

"It was about 2.5 times higher than the annual cropping system at one of the sites. Other sites had similar trends, but the differences were insignificant," Rakkar adds. "We attribute that to an extensive root system, presence of macroporosity and flow of water in the root systems."

On the chemical side, soil pH, potassium, nitrogen, phosphorus and carbon were measured.

"The intermediate wheatgrass in St. Paul and Lamberton compared to alfalfa and wheat-red clover systems was able to maintain soil pH at a near-neutral level, which is 7," Rakkar says. "One of the reasons for that difference is that legumes are actually known to reduce soil pH a little bit because of the process by how they uptake nitrogen through rhizobia. Being a grass species, intermediate wheatgrass may be able to maintain soil pH near neutrality in low-pH soils."

There were also higher soil potassium levels in intermediate wheatgrass systems compared to alfalfa and wheat-red clover, while nitrogen and phosphorus differences were insignificant.

While there weren't significant differences in enzymatic activity, there was a higher proportion of fungal biomass, actinomycete biomass, and gram-positive and -negative bacteria in the intermediate wheatgrass-plus-alfalfa system compared to an alfalfa-only system.

For both corn and soybeans, there was no subsequent yield decrease following intermediate wheatgrass — which Rakkar says may be due to an improved soil environment, with improved soil structure and water infiltration. Intermediate wheatgrass also helped reduce weed pressure.

"After the organic transition periods, intermediate wheatgrass had no negative effect on crop yields, whereas alfalfa or soy-corn can reduce subsequent yields," she says. "Whether it was soybean or corn, the intermediate wheatgrass system had much lower weed pressure compared to other cropping systems."

Economic viability

Of course, for farmers to grow a crop for grain, it has to be economically viable. Rakkar says the next steps in the study will involve researching the profitability of intermediate wheatgrass in a crop rotation. Profitability might mean establishing markets for Kernza grain, which would need to become available in a timely manner for growers to sell it. However, there will likely be economic advantages associated with the soil health benefits, as well as the reduced weed pressure, with a perennial crop rotation.

"Is that system profitable? That's what we are working on right now," Rakkar says. "We will be doing a whole-system analysis of every single cropping system we had in our experiment, and we will calculate which system was most profitable."