Results of a three-year soil health study focusing on arbuscular mycorrhizal (AM) fungi proves there is still much to learn about soil microbes.

“This is the purpose of research,” explains Mike Lehman, soil microbiologist for the U.S. Department of Agriculture - Agriculture Research Station (USDA-ARS) in Brookings, S.D.

The study focused on answering two questions.

1. Would extending the amount of time plants were growing in the soil, through cover crops, increase AM fungal numbers, biomass, or root colonization?

2. If there is an increased amount of AM present on corn roots (in other words, increased root colonization), will this lead to bigger plants with greater nutrient content, and increased yields?

So, what are the answers based on research? Yes and no.

“We learned cover crops can increase the numbers of native AM in the soil. But (the data also showed) we should not expect this increase to translate into an observable positive impact on the crop within one season,” Lehman explains.

These research results align with what farmers implementing soil health practices have also observed, explains Shannon Osborne, Research Agronomist for USDA-ARS.

“We think cover crops are a long-term investment. And we think producers would agree. In order to see benefits of cover crops and other soil health practices, like no-till, it takes about five to seven years,” Osborne says.

Osborne co-authored the study with Lehman and three others: Wendy Taheri, Jeffrey Buyer and Bee Khim Chim.

Research details

The study added fall-seeded cover crops to extend a corn field’s growing season or the amount of time living roots were in the soil, from the traditional four to six months to eight to 12 months.

Researchers utilized several measurement practices to determine soil AM fungal numbers, amount of soil AM fungal biomass, and extent of root colonization. Both soil AM fungal numbers and biomass responded to the cover crops. An alternative lipid marker of AM fungal biomass was found to be more reliable than the conventionally used lipid marker.

Of the three measurements, the study shows that although root colonization is a commonly utilized measurement, “it may not be the best indicator of plant benefits provided by AM fungi,” Lehman says, adding other research has shown similar results.

This USDA-ARS study looked at the effects of one growing season, but the study was replicated over three growing seasons.

Why do AM numbers matter?

As Lehman explains it, for half a billion years, plants and soil microbes have had a successful and dependent relationship.

He compares the enduring relationship between plants and microbes to that of a marriage or best friends.

And like all healthy relationships, this is one of give and take. Lehman explains plants need microbes, like AM fungi, because they make soil nutrients soluble, so that plant roots can easily absorb them. In fact, microbes will actually connect themselves to a plant’s roots through filaments and transport soil nutrients directly to them. In addition to nutrients, in drought conditions these filaments can also provide the plant with water.

In return, plants feed microbes. “Bare soil is like a desert when it comes to providing food to microbes. What microbes like, what makes them grow, is having plenty of living plant roots around,” Lehman says.

He explains that plant roots excrete exudates or starches that fuel or feed microbes.

“It is a barter system. Beyond that, it is an obligatory partnership,” Lehman says.

What’s our take-away?

Based on this and other soil health research they have been involved in, Lehman and Osborne encourage farmers to take a holistic approach to soil health.

“Don’t just focus on soil microbes,” Osborne says. “Extending the growing season through cover crops has many other benefits, including the ability to clean up our water and keep weed and pest cycles down.” 

Lehman adds. “The same management practices you implement to improve soil structure and water holding capacity will also benefit microbes.”