Most no-tillers use some external inputs like herbicides and synthetic fertilizers, though there are certainly those who have or are on their way to reducing those inputs substantially.

But many messages from the mainstream media and society at large suggest that these herbicides, insecticides and synthetic fertilizers are more harmful than tillage. The entire organic industry is predicated on this idea and I’ve been wondering what science has to say about the comparative effects of the various management practices on the soil.

And as it turns out, Andrew McGuire, an irrigated cropping systems agronomist for the Center for Sustaining Agriculture and Natural Resources at Washington State University covered this topic in 2016 and added some updates in 2018.

In his post, he documents a number of studies that look at the effects on soil of glyphosate, insecticides, fungicides, tillage and synthetic fertilizers.

For example, he references a 2016 meta-analysis of 36 glyphosate studies (Nguyen et al.) that found “‘field application rates [of glyphosate products] had no significant effect on SMR [soil microbial respiration] or SMB [soil microbial biomass].’ They did find effects when applied at higher rates, but that is why we have the EPA and pesticide labels.”

He cites several other studies that make similar statements and says “While not conclusive, this evidence does not raise any red flags about the use of herbicides and their effect on the soil.”

A study by Bunemann et al. that reviewed all agricultural inputs suggests that some insecticides and fungicides “proved to be quite toxic,” yet a review by Imfeld and Vuilleumier (2012) said “the literature on the effects of pesticides on soil micro-organisms suggests that they only have minor or transient effects when they are applied at the recommended doses.”

McGuire also referenced a meta-analysis of 107 data sets from 64 long-term trials that looked at the effects of synthetic fertilizers and concluded that fertilizer applications actually led to increased microbial biomass compared to unfertilized control treatments.

Tillage, on the other hand, as no-tillers know, degrades soil structure, causes erosion and compaction, kills earthworms and destroys the soil ecosystem. As the NRCS says, “Tilling the soil is the equivalent of an earthquake, hurricane, tornado, and forest fire occurring simultaneously to the world of soil organisms. Simply stated, tillage is bad for the soil.”

Granted, these studies are limited to looking at the effects of these products on soil microbes and don’t get into effects on human health, persistence in the environment and the like. Nor do they look at the potential benefits of a no-till organic system.

But in terms of how pesticides and fertilizers compare to tillage and the long-term outlook for soil degradation, McGuire clearly makes the case that “if protecting the soil is the first requirement for sustaining agricultural production, then clearly tillage is not our first choice if other, less damaging tools, like herbicides, are available. The tradeoffs between herbicide use and tillage favor herbicides.”

With the help of cover crops and new weed management tools, many no-tillers are making headway toward reducing these inputs as well, which is fantastic. But as the population continues to grow, it’s comforting to know that no-tillers are on the right path where soil health is concerned and that the judicious use of fertilizers and pesticides can be part of a sustainable agricultural system.