Why You Need to Know Arbuscular Mycorrhizal Fungi

If you're focused on soil fertility, you may want to add a new piece of equipment to your operation: a microscope. With a little training, you'll be able to inspect plant roots to see if arbuscular mycorrhizal fungi (AMF) are flourishing.

You may not know what AMF are, but you will. Wendy Taheri, a USDA research microbiologist and co-owner of TerraNimbus based in Pelham, Georgia, can recite 30 benefits of AMF, and she plans to share them along with microscope training at a first-of-its-kind workshop with the Pennsylvania No-Till Alliance (panotill.org) in March. Participants will take home microscopes and – she hopes – a better understanding of farming practices that build AMF.

"Conventional practices taught us that more fertilizer means more yield," she says. "However, as we learn more about how plants and microbes interact, we have come to learn that the more we add, the less we get."

That knowledge is crucial, she says, especially because of an anticipated phosphorus (P) shortage and seed varieties that are not AMF-compatible.

It takes a village
The concept of "it takes a village" applies to growing healthy roots that sustain healthy plants.

"It's the community of AMF that conveys multiple benefits to plants," Taheri says. That's why the goal is to increase the diversity of the miniscule fungi on plant roots that can only be seen under a microscope.

The fungi colonize roots and create a network that does the following:

• Increases soil fertility.
• Boosts soils' water-holding capacity.
• Improves drought tolerance.
• Increases salinity resistance.
• Adds biomass for increased yields.
• Sequesters soil carbon.
• Makes plants taste better to pollinators.

"Once a root is colonized, pathogens can't get in there," Taheri adds.

She witnessed AMF at work about 15 years ago while doing coal mine-restoration research. Soil was retained where it was covered by plants, which survived growing in shale due to AMF colonization.

"It shows how effective covering soil is," Taheri says. "That's what plants are doing for you, protecting soil and nourishing microbes."

Something covering the soil makes all the difference, whether it's a cover crop, prairie, or woodlot. She compared AMF spores from conventionally tilled and fertilized fields with prairie soils and found 800% more AMF activity in prairie soils.

P shortage
Because conventional farming adds inputs in order to net good yields, the lack of AMF may not seem to be that important to you. However, the impending shortage of P makes building AMF crucial, she says.

It's estimated the U.S. supply of P (only 2% of the world's supply) will run out by 2040, with the majority of P in Morocco (that's near an unstable part of the world). Thus, a shortage of this important nutrient is very real, she emphasizes.

U.S. producers got a tiny glimpse of the potential impact when prices quadrupled when biofuels came on line last decade. This paled compared with the 800% cost increases that occurred in much of the world.

Reducing P applications and choosing practices that build soil nourished by AMF is part of the solution, Taheri says.

"High levels of fertilizer suppress the very microbes that plants evolved with to provide their nutrients," she says. "Without the help of microbes, we are forced to flood our fields with nutrients, as we attempt to get those nutrients out of the soil and into the plant. Yet, we are lucky if even 20% of that very expensive phosphorus actually winds up in the plant," she explains.

"Instead of nourishing plants, much P in erodible tilled systems goes into streams and lakes, ends up in oceans, and creates dead zones," she says.

One solution is to build AMF to reduce the amount of P needed. "Arbuscular mycorrhizal fungi make organic phosphorus available. AMF are more nutrient efficient, and they double phosphorus intake," Taheri says.

Seed issues, too
In addition to the practice of applying too much P, the microbiologist was surprised when she found no colonization of AMF on a corn root. At first, she thought it was just one variety, but later she discovered that other varieties and crops also lacked these important microbes. For example, it's common for soybean roots to have just 3% to 5% colonization, compared with the desired 50% to 90%.

"Heavily colonized roots are heavily protected roots," Taheri says. "However, we have been finding that our high-input plant-breeding systems may be inadvertently selecting for plants incapable of forming a mycorrhizal association. This would be a devastating blow to sustainability, since our domestic phosphorus supply is going to run out, AMF are essential to plant nutrient-use efficiency, and they play an important role in making the so-called unavailable phosphorus available."

Because of that concern, TerraNimbus recently began a new service that tests plant varieties for mycorrhizal competency.

The fix
Her studies of microfungi have led Taheri to agree with other soil scientists and pioneering farmers' recommended production practices that include:

• Using less fertilizer
• Switching to no-till
• Growing cover crops

The more plant diversity, the higher the resulting AMF levels. This can double or triple P available to plants, she says.

Taheri plans to do a study to find which cover crops most benefit that subsequent cash crop. No-till is a no-brainer, she says. Though the first couple of years are difficult, studies of conventional farming transitioning to organic farming have shown increased benefits and soil quality for up to 15 years.

"The benefits are from rebuilding that AMF network," she says.

If you till again, though, it's over. "One tillage event sets it back to zero," she says.

Incorporating AMF inoculants can also boost colonization, but it's important to choose the right microbes from that region that will work with indigenous microbial communities.

"I honestly think we are on the cusp of a paradigm change from high-input systems to a microbial revolution. We need to get there; our food security depends on it," she says.

"Change is hard," Taheri continues, "but the smartest among us are adapting now by learning no-till, religiously soil-testing and embracing soil health and restoration, and building up the soil carbon. They are ahead of the game. When we run out of phosphorus, those who have not prepared themselves in advance are going to flounder and suffer under the smothering costs per acre they will have to spend to maintain a high-input system."