Inexact Change

Thoughts on science, politics, and social progress.

Are Longer Crop Rotations the Answer for Subsistence Farmers?

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Twice in the last week, Tom Philpott has mentioned what he considers to be “the most important ag research released in 2012.” That research is a comparison of three different crop rotations conducted by scientists at the Leopold Center for Sustainable Agriculture at the Marsden Farm at Iowa State University. The researchers compared a typical two-year rotation of corn and soy with a three-year rotation that incorporated a grain crop and a cover crop and a four-year rotation which also incorporated an alfalfa crop.

The results? As Philpott puts it, “The more diverse systems reduced the need for synthetic nitrogen fertilizer—an energy-intensive, water-polluting substance—by a stunning 86 percent, while maintaining yields.” Based on this finding, Philpott argues that the government should pay farmers to plant cover crops.

The study itself seems interesting enough, but Philpott’s interpretation of the results omits an important point. The diversified systems weren’t different from the two-year rotations only in that they included additional crops. The longer rotations also received applications of manure. In other words, the diversified systems didn’t require less synthetic nitrogen merely because they were more diverse and incorporated nitrogen-fixing cover crops but also because they received added “natural” nitrogen. In fact, according to Table S3 (DOCX) in the Supporting Information, the diversified systems actually had somewhat more total nitrogen (manure and synthetic) applied over the course of the study than the two-year rotation.

That’s not to say that we should conclude that cover crops increase a system’s nitrogen requirement. After all, the different systems produced different crops, and the researchers also observed that the diversified systems saw larger reductions in their synthetic nitrogen requirement as the study progressed. But it is nonetheless misleading to attribute the entire reduction in synthetic fertilizer inputs to the cover cropping and the diversified crop rotation system.

That might seem like a trivial point. It might seem to simply suggest that the way to achieve those “stunning” reductions in synthetic nitrogen application is to apply manure in addition to increasing crop diversity. Indeed, to the extent that manure is available, using it as fertilizer will reduce the need for other fertilizers. However, manure is not an unlimited resource, and this should raise questions about the extent to which the reduction in synthetic fertilizer is scalable.

The researchers’ intention was to provide a model for an agricultural system in farm animals were raised in close proximity to the plot on which their feed was grown. In the article’s comments, the corresponding author pointed me to another paper (paywalled) in which the authors estimated that the amount of nitrogen in manure applied was about 70% of the amount that would be made available by feeding the corn and forage grown on the plots to cattle. Based on that estimate, it does seem reasonable to hope that following this model of integrated farming would provide comparable reductions in synthetic nitrogen application for growing animal feed, though it should be noted that those reductions are relative to a relatively wasteful baseline.

However, not all crops are grown for animal feed, and this presents a serious problem for Philpott’s second mention of this study. That came in the context of his response to the British environmentalist Mark Lynas’s recent lecture to the Oxford Farming Conference describing his conversion from a staunch opponent of genetically modified organisms (GMOs) to a believer in the idea that GMOs can play an important role in feeding the world sustainably. It’s that shift from the United States to the world that is the problem.

Philpott, apparently countering Lynas’s mention of a project at the John Innes Centre to genetically engineer cereal grains to fix nitrogen, points to that same paper about the Marsden Farm, this time claiming that “diverse crop rotations along with nitrogen-fixing cover crops maintain crop yields while drastically reducing the need for synthetic fertilizers.” If all it took to drastically cut the use of synthetic fertilizers were longer crop rotations and more cover crops, it might be fair to ask whether trying to produce nitrogen-fixing grains was a poor investment of resources. But since the Marsden study also used cattle manure, much of the reduction in the dependence on synthetic nitrogen came by recycling a waste product in a system that was creating lots of it.

That might be plausible in the United States, but it’s unlikely that comparable reductions could be achieved on a global scale, where a less animal-centric diet is the norm. Indeed, Americans eat an unusually animal-heavy diet, with the FAO estimating (PDF, see the table beginning on page 140) that we consumed 900.0 kilocalories from livestock foods per person per day in 2005, more than twice the global figure of 388.2 kilocalories from livestock foods per person per day. And that research into nitrogen fixation in cereal grains aims to benefit farmers in sub-Saharan Africa, where people consumed only 128.8 kilocalories per person per day of livestock foods in 2005. Suffice it to say that the farmers of sub-Saharan Africa are not feeding most of their crop to animals and letting the manure go to waste. It is beyond absurd to suggest that the methods of the Marsden study would alleviate the need for nitrogen fertilizer there.

One could conceivably make the argument that the research at the John Innes Centre is unlikely to succeed and that resources would be better directed elsewhere. However, it is incredibly sloppy to suggest that the Marsden study offers an existing solution to the same problem. That study offers insights on removing some inefficiency in nitrogen use in the United States, but it has less to offer to the subsistence farmers who don’t have that inefficiency upon which to improve.