Microbial communities in agricultural soil are receiving increasing attention from both the scientific community and crop input companies.
When it comes to growing a crop, the importance of a soil’s micro-organism cannot be understated: they repel or outcompete pathogens, manipulate the hormonal signals of plants and increase the bioavailability of soil-borne nutrients.
But how closely should growers pay attention to the microbes in their soil?
There are up to 10 billion bacterial cells in each gram of soil in and around plant roots, and bacteria is only one of the five micro-organism groups in the soil microbiome.
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Fortunately, growers can help foster healthy microbial communities in their soil without figuring out the relationship between the thousands of microbial species present in any spoonful of their soil.
This is because most growers already know how to help out the micros in their soil.
A balanced nutrient management program and growing techniques that increase soil organic carbon create the environment that healthy microbial communities require, said Bobbie Helgason, associate professor at the University of Saskatchewan.
“If you think about the soil organic matter as a really valuable habitat and food source for the micro-organisms, then anytime that you’re building or judicially managing the soil organic matter, you ought to be having the soil microbial community,” she said.
Helgason said growers should manage crop residue as a valuable resource rather than thinking of it as trash that gets in the way of seed bed preparation.
“That is one of the things that is really great about cover cropping because it means that there are plants growing in the soil for most of the year, and those plants are depositing carbon, or food, for the micro-organisms that are there,” Helgason said.
Providing a diverse carbon food source also helps micro-organisms.
So again, recommended best practices, in this instance a diverse rotation, will foster a healthy microbial community.
“Residues or plant carbon quality can also impact the microbial community, so have a diverse rotations of crops,” Helgason said.
If growers want to start to learn more about their soil microbial community, a good place to start is with arbuscular mycorrhizal fungi (AMF) because they are associated with almost all crops grown on the Prairies, aside from brassica species.
“There should be AMF in the soil, although there are inoculants out there to try to enhance the specific populations of mycorrhizal that enhance various crops species,” Helgason said.
The biomass of arbuscular mycorrhizal fungi is a network of mycelium and hyphae, so Helgason uses the analogy of bread mould when describing how to promote the fungus in soil.
“If you think about mould growing across the surface of a piece of bread and you dragged a fork through it, you would break up the connected network of biomass and they wouldn’t like that very much. Tillage is very similar to that sort of model for the soil,” Helgason said.
“So arbuscular mycorrhizal really don’t like tillage because it breaks apart their biomass and reduces their growth.”
Growing host crops is also important for AMF. For example, continuous canola would not support a healthy mycorrhiza population because canola is not a host for mycorrhiza.
“So that population (AMF) would be greatly under stress under conditions where there was no host crop for it year after year,” Helgason said.
It’s difficult to monitor AMF and the microbial community because they are not visible to the naked eye, and no user friendly test is available that can distil the results into a simple measurement.
“I’m not sure there is a simple interpretation that relates directly soil microbial community composition to soil health,” she said.
“There is a lot of other aspects outside of who is there and what the relative abundances are that factor into what that means for soil health and productivity.”
However, growers who are implementing a practice to specifically manage the microbial biomass should monitor the effects they are causing.
“If you are intending to implement a practice specifically to manipulate the soil microbial community, then I think it’s really important that you pay attention to who is there, but I’m not sure that every producer needs to understand the proportion of different microbial taxi in their soil, as an example,” Helgason said.
She said it’s important to recognize that there is an enormous and diverse microbial community in the soil and that farmers need to support those organisms by fostering good soil health.
That being said, the soil microbial community is incredibly resilient to stress.
“Every time it’s not a natural ecosystem, we’re sort of working against the factors that developed a particular soil and its inherent microbial community, but it’s really complex,” Helgason said.
“There is something called functional resilience. What that term means is that there are more than one population or more than one type of organism capable of performing the same function when conditions change.”
For instance, there are more than one type of organism that can perform nitrification. So, for example, if the soil changes from wet soil to dry soil, different organisms will take over the nitrification function in that soil.
