You are here
Soil Microbes Secure the Future
Pick up just a pinch of healthy soil, and you could be holding as many as 2,500 different species of microbes in your hand. These are the stuff of life.
Without the microbes, plant growth stymies, relative to its potential. Even our most ingeniously designed soil-fertilization efforts cannot totally make up for their loss.
“Soil ecosystems have many members, each filling a necessary niche to ensure proper operation,” says David C. Johnson, a New Mexico State University molecular biologist. “All members have to be present and functioning for the system to work efficiently and correctly.”
Johnson’s research indicates that when the soil is highly populated with diverse microbes coexisting in balanced communities, plant growth explodes, and soil carbon starts to increase. These processes trigger the building of soil organic matter and sequestration of carbon from the atmosphere.
Two Microbial Keys
Two key players driving the system are the microbes fungi and bacteria.
“Fungi and bacteria provide the foundation for soil biological ecosystems, and estimates of their population and structure appear to provide an accurate measure of ecosystem health and productivity,” says Johnson.
Proper balance between the two groups of microbes is key, with fungal domination of the microbial community most healthful for soils.
This was dramatically borne out when Johnson developed a low-cost composting system for manure. He didn’t turn the compost and found that a static composting process yielded a compost highly populated with fungi.
“The product from the unturned compost had four times the microbial biodiversity and four times the fungal biomass of the turned compost,” he says. “Turning the compost destroys the development of the fungal community. Without the fungi, the compost is not as beneficial to the soil.”
Perhaps most surprising was the vigorous growth of chili peppers in response to the high-fungi compost in a greenhouse trial. Compared with chilies grown in eight other types of composts – with lower populations of fungi relative to bacteria – chilies growing in soil treated with fungal-dominated compost had twice the growth volume.
This dramatic plant response made Johnson wonder if he could produce similar results in field plots by growing cover crops in soil inoculated with small amounts of the compost. He hoped to jump-start a balanced repopulation of microbes in
He started by inoculating the soil with a dusting of the compost – about 400 pounds per acre. He then planted a winter cover crop mix of three types of vetch, bell beans, winter peas, Dunsdale peas, and Cayuse oats.
In spring, after mowing and disking this cover crop into the soil, he replanted a multispecies cover crop including sunflower, forage soybean, radishes, and millet. He followed that with another winter cover crop.
After several seasons of this practice, the treated soil produced five times the plant biomass as the untreated control plot.
Similar yield increases in cotton and chili peppers occurred on plots previously growing cover crops. “I saw nearly a doubling of cotton production and a doubling of the production of chili peppers, all without using any fertilizers,” says Johnson.
Hand in hand with these yield increases, Johnson’s more detailed measurements of soil life and its processes shed more light on the relationships between microbes, soil, and plant growth. Surprisingly, his measurements indicated that plant growth is poorly correlated with soil levels of nitrogen, phosphorus, potassium, and organic matter. On the other hand, he found a good correlation between plant growth and fungal-bacteria ratios in fungal-dominated soils.
“Fungal-bacteria ratio was highly predictive of both plant biomass growth and subsequent increases in soil organic matter,” he says. “The system is self-reinforcing. Once soil carbon reaches 1.72% – or organic matter approaches 3% – both soil fertility and plant growth appear to increase.”
Johnson’s finding inspired him to envision an agricultural system he calls Biologically Enhanced Agricultural Management (BEAM), which, at its heart, is the growing of plants to feed the microbes. The system’s goal is the shifting of fungal-bacteria ratios away from the bacteria-dominated soils of many cropping systems toward a plant-based system where fungi dominate the soils.
Season-long production of diverse plants like cover crops and perennial forages is key so that fungi can colonize living roots. “Both fungi and bacteria derive their energy requirements directly from plants – as exudates from plant roots – or indirectly through consumption of plant-originated organic matter,” says Johnson.
“The plants provide carbon, and carbon is the energy vehicle of the system,” he says. “With increases in this flow of carbon, the population of microbes increases. Then they start to specialize and then cooperate. As all the players in the system work together, the multispecies community creates a synergy, with effective communication and trading of products. But there needs to be a continual flow of carbon – energy – through the system.”
The synergy occurring in a fungal-dominated microbial community provides the nutrients needed by plants. Johnson measured changes in soil nutrients over a 20-month, BEAM-application period. He found increases of about 50% in the plant availability of nitrogen, phosphorus, and potassium.
“The increases in plant-available micronutrients ranged from a 65% increase in copper to an 1,100% increase in iron and manganese, respectively,” he says.
“In field tests, using soil inoculated with fungal-dominated compost, I demonstrated a potential for biomass production of one and one-half times the biomass production of tropical rain forests,” says Johnson.
With farmers’ production systems moving toward such a goal, he sees a bright future for agriculture.
“As you improve soil fertility, BEAM soils have about seven times the soil carbon as more conventionally managed soils, and yet they only require twice the amount of carbon dioxide,” he says. “Thus, these soils will be more efficient for reducing atmospheric carbon dioxide. They’re also potentially more profitable to farmers and ranchers because yields increase, and the need to apply purchased inputs is reduced. Growing fungi and bacteria in the soil in a balanced community helps take care of soil and crops.”
Empowering natural processes to do their best work is at the heart of Biologically Enhanced Agricultural Management, says David C. Johnson, New Mexico State University molecular biologist. His visionary system focuses on building balanced communities of microbes in soil.
Livestock are potential partners in the process.
“I’m just trying to mimic a natural system,” he says. “The grazing of livestock can be critical. When managed in an adaptive grazing system, livestock graze about a third of the plant material and trample the rest into the ground. Their saliva and dung inoculate the soil with microbes. Dung beetles place this manure into the soil to compost in place. It’s this system we are trying to mimic: the microbes, the insects, the birds. All parts working together as a system make the process most effective.
“By figuring out ways to align ourselves with nature, we can develop agricultural systems that are self-sustaining,” he adds.