Pictured Above: REDUCED TO RUBBLE. After annual ryegrass was grown six times in rotation with soybeans, the altered fragipan (lower profile) has broken into much smaller and looser pieces than the control (upper profile), which is still cemented together.

AFFECTING ABOUT 50 million acres in the U.S., the fragipan is a naturally-occurring, cement-like soil layer that can reduce the water-holding potential of the soil by about 50% compared to other crop-producing soils, and can reduce crop yields by at least 20-25%. 

According to Lloyd Murdock, soil specialist and professor emeritus at the University of Kentucky (UK), fragipans create a barrier that neither roots nor water can penetrate. The fragipan usually sits about 24 inches below the soil surface, leaving a shallow rooting area for crops. In addition, it creates wet field conditions in the spring, which makes planting difficult.

“In most places with a fragipan, evapotranspiration is less than precipitation from November to April. This makes the soil too wet and difficult to work with, reducing yields in summer crops,” he says.

Set in Cement

Research done in the 1980s had shown that fragipans, which consist of hydroxy-aluminum silicate with some iron, were resistant to remediation. Mechanical disturbance was largely ineffective at breaking up the fragipan, though an application of organic matter (sawdust) in addition to disturbance helped somewhat.

Building on research done by the late University of Illinois educator Mike Plumer — who discovered that growing annual ryegrass as a cover crop could increase yields in the following row crops — Murdock and his university colleagues have tested ryegrass along with several common chemicals such as sodium nitrate, potassium chloride and potassium sulfate to try to understand the best way to combat fragipans. 

Experiments in the greenhouse, as well as in the field, show while these chemicals are effective at breaking up the fragipan, annual ryegrass is a more practical solution.

“When we tested the chemicals and the annual ryegrass in the lab, all were found to be effective in breaking down the fragipan. However, greenhouse and field experiments revealed the chemicals were slow to move through the soil profile so annual ryegrass was found to be the most practical solution to date,” says Murdock.

Organic Compounds

While research is ongoing and not yet definitive, Murdock says the researchers believe there is at least one organic compound found in the ryegrass that essentially steals aluminum from the silica, breaking up the cement.

“That’s what we think is happening. We’re not far enough along to know for sure, or to know if there could perhaps be some other mechanism to achieve this other than having a plant grow it. It’s also a possibility you could breed plants other than annual ryegrass to make the particular compound,” he says. “We don’t have all the answers, but we’re very encouraged.”

Murdock says they’ve compared several different varieties of ryegrass, and while all the varieties broke up the fragipan, some were more effective than others. Marshall and Bounty are two varieties Murdock recommends in Kentucky because they’re well adapted to the area and have good root systems. But Bruiser, Centurion and Bardelta have also been very effective, he notes. 

Results Take Time

Murdock says the benefits of ryegrass come both from the chemical makeup of the plants, as well as the force exerted on the fragipan by the robust root system. But he cautions that improvements aren’t immediate.

“It’s not a real rapid process. The first year you grow it, you don’t see much difference. But after about 6 years of growing ryegrass, we’ve found the soil is about 6-7 inches deeper,” he says, noting that the ryegrass doesn’t build the soil, it just changes the nature of the fragipan from cement to soil. Yield response is similar, with little or no yield gain in the first 2 years, but improving after that to an average yield increase of 6.4% after 3-6 years of growing annual ryegrass.

No-tiller Jerry Peery of Clinton, Ky., has also seen the benefits of growing annual ryegrass. He started seeding it as a cover crop after a drought in 2012 that limited corn yields to just 30-50 bushels per acre where he would normally yield 180-200 bushels. He uses a John Deere 1990 air seeder to no-till 6 pounds of ryegrass after soybeans and before corn. Now, 8 years later, he’s seeing a 5-8 bushel yield bump due to the ryegrass, and in 2019 his corn yielded 208 bushels per acre.

At this time, scientific measurements haven’t been made on this farm to determine to what extent the annual ryegrass is degrading the fragipan. However, Peery says he’s seen an improvement in his soil.

Peery plans to host a field day this year to demonstrate the benefits he’s seen from annual ryegrass, and Murdock will be there to share his latest research. The field day was originally scheduled to take place in spring of 2020 but had to be postponed due to the coronavirus pandemic.