Back in 2020, I wrote a post about why we should expect a short-term temperature increase as a direct result of phasing out fossil fuel use. The TL:DR is that it would reduce air pollution that currently makes our atmosphere more reflective. The long-term effect will be cooling, but that’s about a gradual decrease in insulation, vs a sudden decrease in shade. Well, it turns out there’s been another “downside” to the successes in reducing air pollution over the last few decades, this time in the field of agriculture. I have to say, would not have guessed this, but seeing it all laid out makes sense: farmers had to increase their use of sulfur fertilizer as air quality improved.

Sulfur, an essential nutrient for plants, was as free as air back in the 1980s, drifting down onto farmer’s fields from the polluted sky. The nutrient also caused acid rain, however, and it triggered chemistry that meant more mercury in fish. Regulations led to less sulfur in the air, but in the Midwest, where sulfur-hungry corn and soybean fields were proliferating, crops still needed the nutrient.

“We find a clear increase in sulfur fertilizer use commensurate with a decline in atmospheric deposition,” said Eve-Lyn Hinckley, a CIRES Fellow, CU Boulder ecologist, and lead author of a new assessment of sulfur fertilizer use. “We have compiled the first time-series of sulfur fertilizer data spanning decades, from 1985-2015.”

As sulfur stopped dropping from the sky, farmers began applying it directly, Hinckley and her colleague,  Charles Driscoll from Syracuse University, reported in late December in the journal Communications Earth & Environment. Moreover, sulfur fertilization accelerated quickly, they found, far outpacing the growth in use of other nutrients such as nitrogen and phosphorus.

I’m a big advocate of moving our farming indoors, but most of that is also pretty fertilizer-dependent. I don’t know a lot about the mechanics of hydroponic and aeroponic agriculture, but I believe they’re generally more dependent on fertilizers, because they don’t have the support of a soil ecosystem. For that farming which remains outdoors, for for efforts to do soil-based indoor farming, it seems as though this is yet another reason for us to move away from the current monoculture model. Our farms do still get a lot of benefits from the ecosystems around them, but in a lot of ways they’re sort of like ecosystem parasites, giving little in return beyond pollution and vulnerability to disease.

This is not how it has to be. Biodynamic farming, for example, puts soil health front and center, and while I certainly have my philosophical disagreements with Rudolph Steiner, it seems to be a much more sustainable approach to food production. I was lucky enough to be part of a biodynamic CSA in New Hampshire growing up, and while the farmers put a lot of work in, they got reliable results, and we had in-season vegetables, fresh milk and eggs, and a number of other food items year round. It’s doubly impressive when you consider how rocky and uneven the land is in NH. I think one of the changes we may need to make is to go back to farming based on what the land can reliably support, rather than depending on the endless importing of additives, at least where we’re not shifting to more of a managed/edible ecosystem model. See, sulfur has some unfortunate downstream effects:

[…] sulfur’s impacts can be serious: the chemical can essentially make heavy metals, including toxic mercury, more “mobile” and more likely to make their way into fish, for example.

Hinckley said it’s not yet clear how extensively sulfur fertilization impacts the mercury cycle. “It’s the same form of sulfur as was going on with acid rain. However, that was diffuse, widespread atmospheric deposition, and this is intense, targeted applications in much larger amounts.” She and her colleagues are already digging into the connection, “looking at the potential interaction between agricultural sulfur runoff and stimulation of methylmercury formation downstream.”

  I feel like every time there’s news like this, there’s a part of me that just has to note that this is a description of the world as it exists. There are ways in which it has improved – I think reducing air pollution was an unquestionable good, but we’ve still got a very long way to go. That’s part of why, as much as I harp on about systemic change, I’m glad that there are people looking at how things could be improved without that. See, these folks aren’t the only ones looking at sulfur in agriculture, and a team at the University of Colorado, Boulder, has been working on ways to differentiate and track agricultural vs. atmospheric sulfur:

To pull apart the atmospheric and human-applied sulfur, the researchers worked like forensic detectives. All over the Napa Valley, throughout its beautiful hills and valleys, they took samples to measure the concentrations of sulfur from its path through soil to surface water. As they went, they analyzed the chemistry of the sulfur and found a unique chemical signature of the agricultural sulfur, identifiable at the atomic level.

Even as the agricultural sulfur undergoes several chemical transformations, from interactions with microbes and other chemicals in the environment, a unique signature stays with the applied sulfur that allows it to be traced, said Hinckley.

“It’s very different from the signature that we see in atmospheric deposition or geologic weathering, which are the other background sources of sulfur,” said Hinckley.

Yet the objective of her work is not to shut down the use of sulfur in agriculture—which has been used since the time of the Egyptians—but to strategically fine-tune its use and application to both sustain the wine industry and minimize unintended environmental impacts.

“This work could help inform the development of technologies that help farmers to choose when and how much they apply, rather than just applying the same amount preventatively all the time,” said Hinckley.

That team’s focus was on sulfur used as an antifungal treatment for grape vines, so maybe there’s some way in which this approach wouldn’t work for sulfur spread as a fertilizer, but to my layman’s eyes it seems like progress is already being made on tracking the stuff. Heck, given that both of the articles I’ve quoted today come from UC Boulder, and the lead author of the first paper also worked on the second, I’m honestly expecting more on this topic from this particular group of authors before too long.

This is not one of those things that demands immediate action. I think it demands action, and it’s certainly a part of the larger problem of chemical pollution, but as the second article points out, we’ve been using sulfur in farming for literally thousands of years. I suppose it might be possible that changes to farming practices would eliminate the need for stuff like sulfur, but that seems unlikely to me, and far less important than the numerous other reasons for plotting a new course. I find this interesting mainly because I hadn’t thought of air pollution as a fertilizer before, and because coupled with the tracking project, it seems like we might be on the verge of a much more detailed understanding of how our use of sulfur affects the world around us.

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