Soldiers of the Soil

By Emily Unglesbee, DTN Staff Reporter

Most soil ecosystems are a friendlier place than we realize.

"I think it's safe to say that -- just like with the human microbiome -- most of the microbes in the soil are not pathogenic," says M. Soledad Benitez Ponce, who studies phytobacteriology at Ohio State University. "Especially in more natural ecosystems, pathogens are usually a minority."

From plant-loving mycorrhizae to nitrogen-fixing bacteria, your crop's roots are living in a diverse subsoil world of helpful microbes. Here's a look at some of what's working.

FUNGAL FAVORITES

Mycorrhizae are a diverse group of soil-dwelling fungi that form a "symbiotic" relationship with many plants, meaning they have a "mutualistic" or mutually beneficial relationship.

"This is a very old relationship between this group of fungi and plants," Benitez Ponce explains. "One of the hypotheses out there is that plants were able to initially establish on land because of their association with this type of fungi."

The best-known type of these fungi for agricultural crops is called arbuscular mycorrhizal fungi. Just like a fungal pathogen, these fungi infect and colonize a crop's plant tissue. Once inside, they form tiny, multibranched structures called arbuscules.

Here, the fungi can take in the carbon nutrients they need to survive. In return, they help the plant uptake key nutrients from the soil, she says.

"They are most famous for helping with phosphorus uptake. But, there are other ways they can benefit plant growth -- some research has shown that they help prime a plant's immunity. The plant has a better response to stresses like pathogens and drought," Benitez Ponce says.

These soil fungi also form long, white threads called hyphae. These threads, as thin as a hair, wind their way through their earthen surroundings to create delicate, soil-hugging webs.

"They can actually help with soil health by creating good soil structure -- these hyphae help keep the soil together and help with soil aggregation," Benitez Ponce explains.

Another group of beneficial fungi is called Trichoderma. Research shows that some strains of these fungi can help with photosynthesis, improve nutrient uptake and even help fight soil-dwelling pathogens like Fusarium.

"There is a very specific activity in some species of Trichoderma that can kill other fungal pathogens -- they are mycoparasites of those fungal pathogens," Benitez Ponce says.

BACTERIAL BLESSINGS

Rhizobia bacteria are probably the most famous of their kin for their legume-loving ways, Benitez Ponce notes. Like beneficial fungi, these bacteria engage in a courtship-like ritual with the roots of a potential host plant -- sniffing out a chemical signal from the plant, sending out their own signal and awaiting the plant's response before latching onto its roots.

Once accepted by the plant, rhizobia bacteria form large, round nodules on the roots of the legume that are visible to the human eye. Each nodule, teeming with bacteria, fixes nitrogen gas from its surroundings and makes it available to the plant.

This process is so beneficial for legumes such as soybeans that many companies offer soybean inoculants that introduce rhizobia into a field to encourage nitrogen fixation.

Other bacteria helpers belong to the genus Pseudomonas. While some species within this genus are harmful pathogens, many others have valuable biocontrol abilities to protect crops from pathogens, Benitez Ponce says. Some even produce their own type of antibiotics that target other bacteria in the soil.

Another group of bacteria, Actinobacteria, are also valuable disease-fighters (though some can also be pathogenic). One type, Streptomyces, are known to be prolific antibiotic producers. This group of bacteria can produce a range of other biocontrol agents, as well, such as antifungals, antivirals, herbicides, fungicides and growth-promoting substances.

In general, these bacteria hang out in the soil near the root zone of plants and are free-floating organisms, Benitez Ponce explains. Because of their beneficial properties, many companies have commercialized species from these bacterial groups and sell them as biological products to farmers.

HOSPITABLE SOILS

How can farmers encourage these plant-loving microbes?

Because soil ecosystems are so complex, there is no simple answer, Benitez Ponce says. But, a good start is increasing the diversity of your cropping system, which can supply microbes with a greater variety of food.

Given that tillage can destroy the soil-aggregating hyphae structures, no-till systems are likely better for fungal species like mycorrhizae, as well as overall soil health, she adds.

Finally, overfertilization can also mute the activity of nutrient-uptaking fungi like mycorrhizae, she cautions.

"That symbiotic system sort of shuts off in heavily fertilized situations," Benitez Ponce explains.

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