Can crop rotations cure dead zones?

It is now fairly well documented that much of the water quality problems leading to the infamous "dead zone" in the Gulf of Mexico (pictured above) come from fertilizer applications on corn. Fertilizer on corn is probably a big part of similar challenges in the Chesapeake Bay and Great Lakes.

This is a tough problem.  The Pigouvian solution---taxing fertilizer runoff, or possibly just fertilizer---would help.  But we can't forget that fertilizer is the main source of large crop productivity gains over the last 75 years, gains that have fed the world.  It's hard to see how even a large fertilizer tax would much reduce fertilizer applications on any given acre of corn.

However, one way to boost crop yields and reduce fertilizer applications is to rotate crops. Corn-soybean rotations are most ubiquitous, as soybean fixes nitrogen in the soil which reduces need for applications on subsequent corn plantings.  Rotation also reduces pest problems.  The yield boost on both crops is remarkable.  More rotation would mean less corn, and less fertilizer applied to remaining corn, at least in comparison to planting corn after corn, which still happens a fair amount.

I've got a new paper (actually, an old but newly revised paper), coauthored with Mike Livingston of USDA and Yue Zhang, a graduate student at NCSU, that might provide a useful take on this issue.  This paper has taken forever.  We've solved a fairly complex stochastic dynamic model that takes the variability of prices, yields and agronomic benefits of rotation into account. It's calibrated using the autoregressive properties of past prices and experimental plot data.  All of these stochastic/dynamics can matter for rotations. John Rust once told me that Bellman always thought crop rotations would be a great application for his recursive method of solving dynamic problems.

Here's the jist of what we found:

Always rotating, regardless of prices, is close to optimal, even though economically optimal planting may rotate much less frequently.  One implication is that reduced corn monoculture and fertilizer application rates might be implemented with modest incentive payments of  \$4 per acre or less, and quite possibly less than \$1 per acre.

In the past I've been skeptical that even a high fertilizer tax could have much influence on fertilizer use. But given low-cost substitutes like rotation, perhaps it wouldn't cost as much as some think to make substantial improvements in water quality.

Nathan Hendricks and coauthors have a somewhat different approach on the same issue (also see this paper).  It's hard to compare our models, but I gather they are saying roughly similar things.


  1. If corn is a problem, then ethanol is a problem. Ethanol doesn't actually reduce greenhouse gas emission, at least not the way the US makes ethanol, but does lead to dead zones. Lugar lost his seat. Time to end subsidies to ethanol.

  2. Hi Anonymous: Yes, ethanol is a factor pushing demand and price. But it's a bit more subtle than you may think. Soybean prices have risen too. If you took a look at the paper you saw that we spent a lot of effort and spaced looking at the effect of the premium on corn that farmers near ethanol plants. The effect is smaller than you may think.

  3. Michael: I am interested in your use of `corn monoculture'. I have been used to using monoculture as`one crop species in one field for one season' - that is, corn is still a monoculture no matter what it is rotated with. Are you using `monoculture' as sequential cropping with the same crop species? If so, then corn-soybean rotation is not a monoculture. In my usage it is a monoculture of corn followed by a monoculture of soybean.

  4. Hi Unknown: Yes, in this context I'm using 'monoculture' to indicate sequential planting of the same crop. I realize some might have different things in mind, and I've replaced a lot of that with plainer statements like 'corn after corn.' Actually, I think I've seen the word used in different ways. Hopefully it's clear enough to my main audience (agricultural economists), but I'll work to reduce use of the word further given its apparent ambiguity.

  5. I would like to post this into a comment on another blog, with your permission. The paper is highly technical and I wonder if you can advise on a less technical summary on the benefits of crop rotation.

    1. Hi am, you're welcome quote from this, even the whole thing, with attribution and a link back here.

    2. I have posted the whole html in a comment on I hope that is sufficient attribution and link to here. It is off topic and the blogger may not wish to post it but that is where it is. Thanks for the permission.

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