If You Build It, They Will Come: Boosting Your Soil’s Microbial Workforce

Growers don’t need to fret about soil microbe depletion; instead, focus on creating work for these beneficial organisms. Maintaining a balanced microbial community and minimizing soil disturbance are crucial for a healthy soil microbiome. By fostering the right environment with organic matter and smart farming practices, farmers can enhance soil health, leading to better nutrient availability and higher crop yields.

Written by Rachel Sim, with expertise provided by Erin Doran, Technical Agronomist, and Celeste Artiga, Research Associate.

The good news about soil microbe levels is that, in general, growers don’t need to worry about depleting their soil completely.

“Microbes exist where there’s work to do and a need for that work to be done,” says Celeste Artiga, Research Associate at Sound.

“It’s hard for beneficial microbes like phosphorus-solubilizers to ​‘decide’ to actively mobilize nutrients locked away in the soil if they have a more accessible option available,” she explains. ​“Everyone is going to be able to feed on those freely-available nutrients so P‑solubilizers and N‑fixers aren’t going to have a real edge over other microbes in the soil and may be out-competed by ​‘lazier’ microbes. Relying more on the ​‘unavailable’ forms of these nutrients already in the environment will feed and recruit those P‑solubilizers and N‑fixers growers would like to keep around.”

The more work available for these beneficial microbes, the more biologically active the soil. Growers worried about soil microbial levels should focus on creating work for microbes — if you build it, they will come. Understanding a soil’s microbial makeup is more difficult.

“Testing microbe levels efficiently is really challenging because they change from day to day based on factors like weather and temperature,” says Erin Dornan, Technical Agronomist at Sound. ​“Instead, the more important thing to look at is the balance of microbes in the soil.”

In soils, really high levels of a single kind of microbe like fungi or bacteria increase the risk of potential nutrient tie-up, disease pressure, and other yield-restricting factors. With a diverse population of microbes that includes bacteria, fungi, protozoa and even nematodes, beneficial microbes compete with pathogenic ones for food and habitat and are able to limit the impact of the potentially harmful microbes.

What specific steps growers should take to create a hospitable environment for soil microbes will differ somewhat based on factors like location, climate, and on-farm practices. Each soil and farm is different, and growers are the experts on their own operation.

“Growers in parts of the United States might have very bioactive soil — that’s soil with characteristics that create a favorable environment for biological processes like nutrient solubilization to take place,” says Celeste. ​“There are other parts of the US where soil types may not be as amenable to being bioactive.”

There are some guidelines for the environments microbes prefer, however. Like all living things, microbes need both nutrition and water, although both too much and too little water can make survival difficult. Nutrition comes in the form of organic matter, so sufficient levels are important.

“If there are really low organic matter levels in the soil, there’s not enough food for all the microbes to feed on, and then they’ll start fighting over it,” says Erin. ​“Some of the bad microbes will win out over the good.”

In general, microbes also dislike soil disturbance, in part because of where in the soil they do their work. Moving from the top of the soil, through the root zone and even below, there are microclimates where different microbes thrive and do their work.

“Microbes’ biological processes happen in very specific types of conditions; the microbes in the top layer of soil can be completely different from those found closer to plant roots, and those are different from the ones below the root system,” explains Celeste. ​“Disturbing the soil also disturbs these microclimates, creating a constant reshuffling of the microbial community across the soil. Sometimes that redistribution isn’t favorable for what we want the microbes to do for the plant.”

Take arbuscular mycorrhizal fungi (AMF), which create fine, thread-like networks of hyphae that help transport nutrients and water between plants and other microbes. These beneficial fungal networks are easily broken up by soil-disturbing practices like tillage. AMF are also closely associated with the bacteria that solubilizes phosphorus for plants, so shaking up the soil can also impact that relationship.

“When we run tillage across that field or anhydrous — anything that knifes into the ground — it’s going to impact those good microbial levels,” says Erin. ​“Practices like no-till and strip tilling not only minimize soil disturbance, they also leave organic matter on the soil.”

Stalks and other organic matter can act as blankets to protect soil and its microbe populations over the winter. For growers who are concerned the organic matter isn’t breaking down fast enough, Erin says there are digesters that can be applied.

“When we add digesters to the system that help break down organic matter quicker without tillage, you’re actually feeding the microbes rather than disturbing them,” she adds.

In bioactive soil, plants and microbes communicate constantly with each other. In exchange for carbohydrates that plants create through photosynthesis, microbes provide the crop with plant-available forms of a variety of nutrients, including nitrogen, phosphorus, potassium, and even micronutrients. Microbes also interact with each other; AMF, for example, act as bridges between micro-pockets of microbial activity in the soil, providing water and transporting nutrients.

“Microbial communities create a sort of hum. Close to plant roots, the hum is louder, but it persists beyond that,” says Celeste. ​“When microbes find pockets of soil with favorable conditions where there are either freely-available nutrients or nutrient-processing to be done, they’re vocal about it. They’re constantly sending and receiving signals, so when one kind of microbe finds a good spot, everyone around will know.”

Nitrogen-fixing and phosphorus-solubilizing microbes are closely associated with roots, which either trade carbohydrates or are otherwise creating an environment where nitrogen fixation is possible. Other microbes that don’t fix nitrogen but may help those that do or provide other benefits will also hang out nearby.

“One microbe can attract a different type of microbe until they daisy-chain themselves into a microbial community. They’re all working together and competing against each other in order to survive and thrive,” says Celeste.

For many growers, nitrogen and phosphorus are abundant in the environment, but without access to the phosphorus tied up in the soil or atmospheric nitrogen, growers rely on the fertilizers they purchase each year. When all of a crop’s nutrition needs are met through fertilizer, the plants stop signaling to soil microbes and the microbes stop providing nutrition to the plant.

“The root zone is often where the most disturbance occurs and the most synthetic fertilizer is placed, both of which will slow down microbial activity,” says Erin.

A healthy soil microbiome offers free access to many of the nutrients that are commonly applied in synthetic form.

“If you give plants synthetic fertilizers, they’ll soak some of it up, but those fertilizers take money and resources to produce, transport and apply. In some cases, there can be plenty of nitrogen, phosphorus and potassium in the soil and the environment, but they’re just not freely available yet,” says Celeste. ​“A microbial community will process those unavailable nutrients into their more biologically-available forms, and growers may not have to apply that costly nutrition to the same degree.”

One way to support a more balanced microbial community is with SOURCEⓇ, Sound’s microbiome activator. SOURCE is a molecule that mimics the plant-to-microbe signal to attract microbes already present in the soil and increase crops’ access to key soil nutrients.

“SOURCE helps to bring balance back to the microbiome and to plant nutrition,” says Erin. ​“When we look at in-season tissue and soil samples, we see nutrients balancing out, rather than huge spikes or lows in various nutrients throughout the season. That’s because SOURCE is activating beneficial microbes that were depleted.”

For Celeste, it’s ultimately about creating an environment where microbes can do their work, and actively want to. ​“If you have low microbe activity, there’s a reason. You could apply a microbial product, but if the environment itself isn’t conducive to supporting an active microbial community, they won’t survive long-term,” she explains. It’s essential to start with an understanding of what you have.

“You can’t improve something you haven’t measured,” says Celeste. ​“We like to make decisions based on our gut, but keeping a detailed record of practices gives us data to back those decisions up.”