Diversity or volume? Which is best when it comes to soil microbes


Interest in building soil resiliency or measuring soil health is growing. But before jumping to complicated soil health tests, we first need to understand how the basic physical, chemical, and biological properties of a soil in combination with management practices will affect soil microbial populations.

“You can’t take the genetic base, essentially, like the DNA of that soil, the base that you have to work with, and then discount that, because that’s the home for the microbes and that’s the home for the earthworms,” says Marla Riekman on a recent episode of The Agronomists. “We can’t discount all the other factors, just to favour the biology.”

Microbial abundance in a soil depends on that soil’s texture and aggregate structures, as I’ve previously written about.

Microbial diversity can result from the different crop species, or plant species grown on the soil, which begs the existential question that I think about a lot: is it better to have microbial abundance (lots of microbes) or microbial diversity (different types of microbes).

Microbial activity depends on soil temperature, moisture, and organic matter inputs.

“I’m not sure that we have to understand all the diversity, because there’s a thing called functional redundancy — we’ve got so many different microbes in the soil and they will express themselves when the conditions are right — and by functional redundancy, I mean that they’ll do similar jobs,” says Anne Verhallen in the same Agronomists episode. “We do have some resilience, just in the soil biology by itself, but the biggest thing is we have to support it, so we have to feed them, we have to create a habitat, so that means… avoiding compaction because we need pore space. Many of these live in the rhizopsphere, so along the roots, or in actual pore space [of the soil] and in the water films within pore space.”

Verhallen also mentions that microbes can be supported with good crop rotations. The more recent understanding of how carbon is stabilized in the soil is exciting — having to be processed through a microbe, supported by living roots, root exudates, and complex sugars and carbohydrates from that system. “If we want to cycle more organic matter, it means more living roots, longer,” Verhallen says.

So, if you ask an agronomist or soil scientist how to increase soil resiliency, soil health, or soil organic carbon, the answer should be “it depends,” followed by a series of questions about soil texture, structure, field history, crop rotation, and general management.

Catch the full episode of The Agronomists featuring Verhallen and Riekman on building soil resiliency!

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