How Does Tilling Soil Affect Microbes?

Soil microbes are the hidden workhorses of a grower’s operation. They provide crops with access to the plant-available forms of nutrients like phosphorus and nitrogen, increase roots’ ability to access soil nutrients and water, and create habitats for themselves that helps stabilize the soil and increase moisture retention. 

One way to support a healthy soil microbiome is to consider the benefits of reducing or eliminating tillage. Tilling can provide growers with significant benefits, but it also impacts those beneficial soil organisms and their habitat; switching to no-till or even reduced tillage may relieve some of the pressure on the microbiome. To make the best decisions for their operation, growers must understand both the advantages of tilling and the toll it takes on soil microbes.

A good soil is rich in aggregates — clumps of soil made up of sand, silt and clay arranged around organic matter. These aggregates ensure that soil is neither too loose nor too compact. Healthy soil should be porous enough that there’s plenty of oxygen for the beneficial soil microorganisms and so that roots and water can penetrate the soil, but not so porous that it is unable to hold on to water and nutrients. 

Aggregates are held together by various mechanisms, including the physical properties of the soil particles and organic compounds in the soil that include root exudates, biofilms produced by bacteria and glomalin, a sticky protein created by hair-like fungal hyphae. Soil gathered together into aggregates is less prone to erosion and nutrient loss and has an improved water-holding capacity. 

The mechanical action of tilling soil breaks up existing aggregates in the soil, increasing erosion and harming both soil organisms and plants. Aggregates provide important habitat to many soil microorganisms and help maintain the oxygenated structure that beneficial soil organisms need to survive. In an unfortunate feedback loop, the loss of aggregates makes the soil less hospitable to the organisms that help create soil aggregates, like earthworms, fungi, and bacteria. 

Without aggregates, soil becomes more prone to compaction, says Jeff Divan, Sound’s Director of Sales Agronomy. ​“If you have done a lot of tillage, soil can become like a handful of flour — with no aggregates there’s no structure, so running over it with heavy equipment will just pack it together. There’s no room for air or water to penetrate the soil,” Divan explains.

If it becomes compacted enough, the crops roots may not be able to penetrate the soil, limiting the plants’ access to water and nutrients and potentially impacting overall plant growth.

Soil aggregates also help hold topsoil in place; imagine that handful of flour again and how it would react to a stiff breeze or some water. Small particles like flour or soil without any aggregation are more prone to being blown away in the wind or washed away by water. Aggregates are not only less prone to erosion, but they also hold water better than loose soil. 

Tilling damages beneficial soil microbes’ habitats, and it can also harm the microbes themselves, either directly or by reducing their access to the food they need to survive. Soil organic matter is a crucial indicator of soil health and a key building block of soil aggregates. One of the primary components of organic matter is soil carbon; it’s not only a major food source for many soil microbes, but it’s also easily lost as a result of tilling. 

There are ways to measure the impact of tillage on soil carbon, says Divan. The Haney Soil Test estimates the amount of nutrients available for microbes to consume, mainly carbon and nitrogen. ​“The carbon component is a lot lower where tillage has been done,” he says. ​“Those carbon molecules are a major food source for microbes. They are made up of dying and decaying roots and other organic matter and they are released to the atmosphere when the soil is opened and exposed to air.”

Some other microbes rely on living plant roots for nutrition. Nitrogen-fixing bacteria convert atmospheric nitrogen into forms that plants can absorb and then trade that nitrogen with crops for sugars the plants produce through photosynthesis. But the loss of soil aggregates from tilling reduces the oxygen available to these bacteria, and the mechanical action can damage the bacteria themselves and their habitats. 

Larger soil organisms feed on bacteria and smaller microbes, storing important nutrients in their bodies to be broken down when they die or are themselves consumed, or excreting plant-available nutrients in their waste. For this soil ecosystem to function, the soil must provide an appropriate habitat for these organisms; they need an environment with the right amount of oxygen and water, and most of them need to be within the root zone.

“Soil microorganisms’ proximity to roots is what matters,” says Divan. ​“Especially with aggressive tillage, they can end up all over the place.”

Plants are the primary source of sugars and carbohydrates in the soil ecosystem. Hence, organisms like nitrogen-fixing bacteria and mycorrhizal fungi that have particularly close relationships with crops need to be near the roots to exchange nutrients for sugars. Larger organisms that feed on bacteria and fungi need to be near their prey and so on up the food chain. 

Tilling can not only disrupt the organisms’ habitats by destroying soil structure, scattering the organisms throughout the soil profile, but it can also destroy many of the organisms themselves. Larger organisms like earthworms are an obvious example. Still, microscopic fungal hyphae — the thread-like structures that transport water and nutrients throughout the fungal body — are also broken up by tillage. This is a critical loss because mycorrhizal fungi are one of the biggest contributors to in-season phosphate.

Organisms that survive the mechanical disruption tillage have to regrow, repopulate, and reestablish themselves in the root zone to begin benefitting the crops again. 

“Those microbes have a place in the soil profile where they thrive. Especially with deep or intensive tillage, we’re digging deep into the soil and pulling some of that material up to the surface. When we start throwing soil and material around, that’s a serious disruption of how that environment would be naturally.”