Dirt is not dead. It’s actually a thriving community of microbes, some helpful and some harmful.
Janice Thies, associate professor of soil biology and ecology at Cornell University, presented “Soil Biota and Their Impact on Soil Health” at the Soil Health & Climate Resiliency Field Day held recently in Seneca Falls, NY.
Soil biology is related to aggregate formation; water flow, storage and filtration; plant growth enhancement; decomposition and nutrient cycling; plant protection; pathogen protection; influencing atmospheric composition (nitrogen fixation); and for detoxifying pollutants.
Thies said that energy drives the entire soil health system. Light penetration for plants is the energy supply for phototrophs such as plants as well as the chemicals in the substrate.
It’s also “critically important to keep carbon in the soil for feeding the soil microbes,” Thies said. Plants feed soil organisms and soil organisms feed plants.
“It’s a symbiotic cycle between plants and soil microbes,” Thies said. As members of the soil food web, organic matter “drives the whole system. They rely on each other.”
Thies also said that soil biota is much more diverse in the number of unique species than what’s found in humans, marine water or lake water. But she added that they’re very slow to grow in soil.
Soil bacteria decompose organic matter, release nutrients, retain nutrients, control pathogens, can be pathogens, solubilize phosphorus, aggregate soil and form soil organo-mineral associations.
“The nitrogen cycle is mediated by microbes,” Thies said. “We have nitrogen controlled by the bacteria population.”
The original source of most nitrogen is the atmosphere. The majority – more than 90% – is bound in soil organic matter, which is why inorganic nitrogen is not included in routine nutrient analysis.
Janice Thies is an associate professor of soil biology and ecology at Cornell University. Photo by Deborah J. Sergeant
“Most transformations in the nitrogen cycle are mediated by microbes: nitrogen fixation, mineralization, nitrification and denitrification. To predict the state and fate of nitrogen, we need to understand those microbes,” she said.
Planting legumes helps add more nitrogen to the soil, a practice that farmers have embraced for more than a century.
Thies said the drivers of inoculation response are soil nitrogen and native rhizobia. Her research on soil inoculation in soybeans indicates that inoculation should amend soil low in nitrogen.
“We have to think what happens in the soil,” Thies said. “When trying to put inoculation in the soil, think of how many are already in the soil. How many organisms are you putting on? Go with the most specific organisms you can try.”
To manage bacterial populations, approaches can include inoculants like nitrogen-fixing bacteria, phosphorus-solubilizing microbes, hormone analogues, microbial soups and compost teas. Cultural controls also include adding organic matter, reducing toxins, keeping plants in the system, oxygenating the soil, improving drainage and lowering bulk density.
Thies also mentioned the many roles of fungi, including decomposing organic matter, mobilizing phosphorus, controlling pathogens, promoting plant growth, controlling insects and aggregating and stabilizing soil.
To help control fungal populations, Thies suggested inoculants and cultural control, including reducing tillage, adding higher carbon-to-nitrogen organic matter, reducing the use of toxins and not allowing fallow land to remain bare.
“Earthworms help stimulate microbial activity, mix and aggregate soil, increase infiltration and water holding capacity, provide channels for roots and bury and shred plant residue,” Thies said.
With all of these benefits, it pays for farmers to support the humble earthworm. Approaches Thies suggested including cultural control, reduced tillage, reduced use of chemicals and toxins and increased organic matter inputs.
Protists are the primary consumers of bacteria in soil. “They regulate population size and composition, accelerate turnover of soil biomass and organic matter, maintain plant-available nitrogen and prevent pathogen establishment,” Thies explained.
Protists are a food source for fungi, nematodes and more. But they can also cause disease as animal parasites.
Nematodes have a diverse range of feeding strategies – as plant parasites, bacterial feeders, fungal feeders, predators and omnivores. Thies said for managing nematode populations, farmers can use entomopathogens, bacteria, viruses and microsporidia – and encourage a diverse saprophytic food web.
Soil fauna are also part of the picture, as they shed organic material, stimulate microbial activity, mix microbes with their food, mineralize plant nutrients, enhance soil aggregation, burrow and control pests. But they can also be pests.
Soil may not seem alive, but Thies encourages farmers to help enrich it.
“Please don’t starve it,” she said. “We need this healthy soil food web. Soil is a complex and dynamic living system. Soil inputs are needed for energy to drive the system. Soil life is sensitive to tillage, pesticides and other toxins. A healthy soil food web is needed to increase nutrient availability and plant productivity. Soil life is soil health.”
Thies currently chairs the Action Team for Research at the Soil Health Institute in Morrisville, NC. She consults on soil health internationally.
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