What happens to soil microbes when you till?


Let’s start here: not all tillage is unwarranted.

In some soils and circumstances, tillage can help warm, dry, or prepare a seedbed before planting and can help bury trash that is otherwise tough for a seeder to get through. At times, burial of residues can provide a larger surface area for microbes within different soil profiles to break down that trash.

Now, with that out of the way, I’m going to say something that you’re not going to like if you’re a conventional tiller:

The benefits of tilling do not outweigh the benefits of not tilling.

Understanding the full reasons why might require getting past a few unfamiliar words, but stick with me.

Tillage disrupts the environments where so many beneficial microbes reside in your soil. I say environments, plural, because within a soil pedon (a basic unit of soil), there are different places where microbes inhabit, interact with one another, and perform their functions.

First off, though, we have to understand that there are multiple types of soil microbes that live in a soil: gram positive bacteria, gram negative bacteria, actinobacteria, arbuscular mycorrhizal fungi (AMF), and other types of fungi — not to mention nematodes, earthworms and others. Those are the major groups you will find, primarily in the top 10 cm (4 inches) of a soil, depending on your A horizon depth. What affects that A horizon depth? Organic matter. So the less organic matter you have, generally the less microbes you have.

When you disturb soil through tillage, you’re destroying soil aggregates, and within those soil aggregates are macroaggregates and microaggregates.

Macroaggregates, are critical places where microbes turnover soil organic matter. Microaggregates, form within macroaggregates and are where that soil organic matter is protected. Within these two spaces, different species of soil microbes exist. Surrounding the macroaggregate are fungal hyphae and roots of plants that release “glues,”  known as root exudates. This whole system is described as the “sticky mesh bag” theory.

When you disturb the soil, you’ve just ripped through and destroyed that sticky mesh bag, so now all of the AMF and other types of fungi have to rebuild their networks, and the bacteria have to scavenge for new sources of food and possibly compete with other bacteria outside the macroaggregate for nutrients.

Macroaggregate and microaggregate microbial communities are different from one another, use different types of plant materials for resources, and have different “specialties.” There are many microbes that can fill a niche quickly, though they can serve more than one purpose in a soil.

You also end up disturbing pore structure within the soil, which will affect water infiltration — by either making water flow too fast through a soil, or water saturates the soil due to a hardpan layer created by tillage — which will also affect microbes, as most are aerobic and need oxygen to perform their functions.

By tilling you’re also redistributing the organic matter that microbes “feed on” or breakdown. This can affect nutrient cycling, because a major function of soil microbes is to break down plant residues and release those nutrients, such as nitrogen, carbon — even phosphorus — back into the soil. This can be helpful in certain situations, e.g. for heavy residue breakdown, but it also changes the soil microbial diversity — the different species or functional groups that reside where you’ve redistributed those residues.

Why should you care? Because there are very real financial benefits to feeding and caring for these soil systems.

Microbes cycle nutrients; they regulate soil carbon and nitrogen cycling. That carbon credit you’re seeking? Microbes regulate, sequester, and assist in keeping that carbon in your soil. That elemental sulphur you applied because it’s less expensive than other forms? It has to be oxidized by a microbe to become plant available.  That product you apply for improved phosphorus uptake? Arbuscular mycorrhizal fungi can perform the same function.

These are just some of the benefits of having a healthy soil microbial community. Keeping your topsoil in place assists in soil microbial habitat which in turn can increase yields and reduce fertilizer costs, the effects trickle.

Maintaining soil aggregates and the microbial colonies that live in them are a key component in improving soil health. Committing to reducing or eliminating tillage from your fields is a long-term investment in the health and resiliency of your land.

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