The evidence of healthy soil is as much what you can’t see as what you can.
Billions of microorganisms live in the soils that feed the world and they all have a purpose.
Keith Hanson, a microbiology specialist who works in the soil lab at the Semiarid Prairie Agricultural Research Centre in Swift Current, Sask., said there are more microorganisms in a handful of soil than there are people on the Earth.
Those unseen microbes are critical to soil structure, health and fertility.
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They include organisms such as bacteria, fungi, protozoa and nematodes. “Soil biodiversity is good for the environment,” Hanson said.
“It helps the robustness of an ecosystem.”
Terry Tollefson, associate professor of soil sciences at the University of Saskatchewan, said people have to understand how the planet functions to understand the importance of microbes.
The sources of elements and nutrients on the Earth are finite. Only the sun adds energy to what the Earth already has, he said.
Living creatures use the nutrients but then die.
“Those nutrients need to be recycled, need to be reused, and that’s where the microorganisms in the soil come in. They are the great recyclers.”
Different microbes have different jobs and some are better known than others. Rhizobia, for example, are bacteria that form an association with the roots of legumes to be able to access more nitrogen more efficiently.
Hanson said these are important bacteria for agriculture.
“They are little nitrogen factories,” he said.
Farmers generally understand how this works when they plant peas, lentils, chickpeas, alfalfa and clover and see improved nitrogen levels in their soil.
A beneficial fungus, arbuscular mycorrhizal fungi, might also be familiar.
The fungi contribute to overall plant growth and can take in phosphorus from areas where plants can’t go, Hanson said.
In the plant root, arbuscules are sites where the fungi and plant make an exchange -the fungi give the plant nutrients and the plant gives the fungi carbon for energy. The fungi grow from the plant root into the soil, extending hyphae, or thread-like structures, from the main root. These threads mine the soil for nutrients.
“This network can actually connect roots from one plant to the roots from another,” Hanson said.
Researchers are also examining the link between arbuscular mycorrhizal fungi and nitrogen.
Nitrogen is locked when in organic residue, making it unavailable for use. The fungi allow for significant increases in the mineralization of nitrogen from organic to inorganic forms, he said.
“We’re trying to determine how cropping practices impact arbuscular mycorrhizal fungi.
If enough fungi are present, producers may not have to put on as much phosphorus.
Hanson said there are other things in the soil that scientists don’t yet understand.
Actinomycetes are a specific type of bacteria similar to fungi. There are several types, some beneficial and some not. Hanson said they are responsible for that fresh soil smell.
Dark septate endophytic fungi are a diverse group of fungi that somehow help plants in extreme environments.
These fungi could be important in developing drought tolerance.
Jill Clapperton, a long-time rhizosphere ecologist with Agriculture Canada who now has a consulting business in Montana, said she thinks of bacteria and fungi as primary producers.
“Then you have the protozoa and the nematodes and some of the small mites, which we call our microfauna,” she said.
“Then you have your mesofauna, which the enchytraeids (pot worms) are a part of.”
Megafauna include earthworms, ants, termites and beetles.
Clapperton said the nematodes and protozoa require soil structure to function. They need soils that have pores they can move through because they can’t burrow.
“As long as they can move, they can function really well and they’re just voracious eaters and grazers of bacteria and fungi, and they keep the primary producers in their place,” she said.
The protozoa and nematodes excrete waste, which is rich in nitrogen, phosphorus, sulfur and other good things for plants.
Mites and small creatures such as collembola, or springtails, work higher up in the soil profile, helping to decompose litter and plant residue.
The entire system is self-perpetuating and dependent on each organism to do its job. Clapperton, Hanson and Tollefson agree that tillage can cause parts of the system to fail. Bacteria and fungi generally can survive but the protozoa and nematodes require the soil structure to play their parts.
Tollefson said the move to minimum tillage and the effects on soil organisms haven’t been extensively studied, but appear to have been beneficial.
“There’s been very little work done in this part of the world that really has looked at populations of most of the microorganisms, so we don’t really have much to compare it to,” he said.
“I think all we can do is work on the assumption that if you have vibrant growth of plant material above ground and you’re adding a significant amount of residue to the soil every year, and associated with warm temperature and decent moisture, that those kinds of conditions are going to support a relatively vibrant soil microbial population.”
Chemicals pose another threat to soil inhabitants.
Tollefson and Clapperton said most herbicides don’t have much impact. Fungicides are more difficult to assess.
“Fungicides don’t just kill one type of fungus, they kill all fungi,” Clapperton said. “So they tend to mess with things that eat fungal colonized things like collembola and so they have secondary effects, too, on anything that feeds on fungi.”
Insecticides are worst because so many things in the soil are insect-related.
