Another look at the nutrient cycle

Last week, I wrote about this claim on Michael Pollan’s new website:

A truly sustainable agriculture will involve animals, in order to complete the nutrient cycle, and those animals are going to be killed and eaten.

I want to revisit this claim because it relates to something I’ve recently read in The Omnivore’s Dilemma.

It seems to me that to say that farm animals “complete the nutrient cycle” is to miss a very important point. Every time we take food — whether it be from plant or animal source — from a farm, we’re taking nutrients away. Unlike wild animals, though, we don’t tend to return all of those nutrients when we’re done with them. Instead, we flush them down the toilet, sending them to septic tanks or sewage treatment plants. (Some nutrients eventually find their way back to the farm, but the process is far less direct than in nature.)

I want to focus on nitrogen in particular, since Pollan writes about it quite a bit. When we take food from a farm, we’re removing nitrogen from the system. In order to continue to produce food on that land, we need to somehow replenish that nitrogen. As it happens, nitrogen is plentiful in the atmosphere. However, atmospheric nitrogen isn’t usable to plants and animals. It needs to be converted to a biologically-available form, which — as Pollan writes on page 42 — is done naturally by nitrogen-fixing bacteria in the roots of leguminous plants or synthetically by the Haber-Bosch process. There are a few other ways that nitrogen can be fixed naturally, such as by lightning or bacteria living symbiotically with termites, but the key fact is that farm animals don’t contribute to nitrogen fixation.

What this means is that when animal manure is used to fertilize plants, the nitrogen it provides was fixed either by the Haber-Bosch process or by nitrogen-fixing bacteria in the roots of plants. Any nitrogen animals leave on the pasture was ingested in their food. This is a point that Pollan seems to miss when he writes,

The chief reason Polyface Farm is completely self-sufficient in nitrogen is that a chicken, defecating copiously, pays a visit to virtually every square foot of it at several points during the season. (210)

Polyface Farm isn’t completely self-sufficient in nitrogen, though. There’s plenty of nitrogen in the eighty percent of the chickens’ diet that Pollan has told us (just a few sentences earlier) comes from  corn and soy that Salatin buys. For the soil to maintain a stable nitrogen level from year to year, it’s necessary for the chicken feed together with the nitrogen-fixing bacteria in the roots of clover  and other legumes to provide enough nitrogen to replenish whatever is removed from the farm in food for humans. Passing through the chickens’ digestive systems doesn’t increase the amount of biologically-available nitrogen in the feed. (In his very next sentence, Pollan describes the chicken feed as Polyface’s “sole off-farm source of fertility,” apparently seeing no contradiction with the claim of self-sufficiency.)

What farm animals might be said to do with nutrients is to take them from grasses we can’t eat (though we can apparently eat clover like that on Salatin’s pasture) and convert them to a form that we can eat. Pollan does also describe at length how intensive grazing increases the productivity and diversity of the pasture. I can’t see how either of these roles would be said to be completing a nutrient cycle, though.

In the comments of my last post on the subject, Andy D and bill suggested using human manure, or humanure to fertilize crops. I don’t know enough about the public health implications, so I’m not going to endorse the idea. I will say, however, that using humanure would return the nutrients that have been removed from the farm. If Pollan wants to “complete the nutrient cycle” in any meaningful sense of the word “cycle,” it seems to me that humanure would do a lot more to this end than farm animals would.



  1. Becci said

    This was fascinating and well-researched–thanks!

  2. […] agriculture without animals to cycle nutrients”  (326), two claims related to issues I’ve addressed before. Pollan hasn’t addressed the fact that the nutrients in manure come from the food that […]

  3. […] awful lot like the proverbially unattainable free lunch” (127). I’ve pointed out before that Polyface does use feed grains from other farms. What I want to look at is how Polyface’s […]

  4. […] hard to tell whether he grasps the fact that the nitrogen in the chickens’ feces comes from the food they eat, eighty percent of which is grain-based feed from off the farm. What is certain, though, is that he […]

  5. petemry said

    Interesting work, Adam. I’ve just begun to work through your blog. I too felt the Omnivore’s Dilemma chapter on ethics was disappointing and I’m glad to see you working through the book in such a rigorous fashion.

    I’ve been thinking about the nitrogen process and your point that animals can transform nutrients in grass, inaccessible to humans, into manure that can help other crops to grow. Now, since grass (and perhaps other grains that humans don’t or can’t eat?) can grow in places that corn and wheat don’t do particularly well, grazing animals can perhaps be a tool to move nitrogen fixed by these plants and other nutrients from poor farmland to better farmland. (The animals can be part of this system, of course, without also being killed for food themselves.)

    I don’t know if this makes any sense on a large scale: there’d need to be a way to transfer the nutrients, either by moving manure or by moving the animals. Anyway, I’m just thinking that animals might have a useful role to play in a macro-scale farm system (not just a Salatin-style small-scale ecology).

    • Adam Merberg said

      Thanks for your thoughtful comment. Your idea seems plausible, with a couple of caveats:

      1. The poor pasture would have be be grazed relatively lightly. You would have to remove less nitrogen in manure than was fixed by bacteria in the roots of legumes on the pasture.
      2. Nutrients other than nitrogen would need to be replaced on the pasture as they were removed in manure.
      3. The nitrogen might instead be moved without the animals by cutting the grasses by hand and composting it. The cutting part might sound labor-intensive, but Joel Salatin actually already does that in order to feed his cattle in the winter.

      Anyway, I think I’ve written in one or more places that I’m willing to believe that animals might play a useful role on a farm, but I don’t think that “completing the nutrient cycle” is a reasonable way of characterizing those roles.

  6. […] and fertility problems, but then goes on to say that animals make nutrients for plants. As I have mentioned, the nutrients in animal manure come from their […]

  7. Carolyn said

    I’m curious what your references are, specifically with regards to the nitrogen in the chicken manure being directly from the (off-farm) chicken feed. Or in other words, why use manure vs. cover crops to fix nitrogen in the soil?

    • Adam Merberg said

      Hi Carolyn, thanks for your comment. It’s a fair question. Usually I try to provide a lot of references, but here I did not because all I’ve used is some basic science and a bit of critical thinking. If you read about the nitrogen cycle in a high school or college level biology textbook, you’ll find the following facts:

      • The atmosphere is mostly nitrogen, but atmospheric nitrogen is not directly useful to living organisms.
      • Atmospheric nitrogen can be converted to the biologically available form, or fixed, by bacteria living symbiotically in the roots of leguminous plants. There are a few other ways that nitrogen might be fixed naturally (i.e. lightning and bacteria living symbiotically in the guts of termites) but the amount of nitrogen fixed in this way is relatively small. There are no farm animals that fix any significant amount of nitrogen.

      (Actually, I think you could even find both of these points in The Omnivore’s Dilemma, though I would never cite that as a source for reasons that this blog should make clear.) The key point is the second one. Chickens don’t fix nitrogen. The nitrogen in their feces has to come from somewhere. They aren’t going to breathe it in because atmospheric nitrogen isn’t biologically available. Thus, they have to ingest it in the form of feed. Some of their feed is pasture-based, but if they some nitrogen from the pasture and then drop it in the form of feces, that doesn’t increase the amount of nitrogen on the pasture. It just puts back some of what has been removed. Any new biologically-available nitrogen in the manure has to come from off the farm.

  8. Eric B. said

    Where do you get the 80% figure from? I suspect it’s more like 98-99%. (That would only strengthen your point, of course.)

    As for the importance of manure, however, manure is much more concentrated and bio-available than hay. It makes far more sense for me to spread animal manure on garden crops than to try to harvest hay for nitrogen fertilizer. And if I can harvest meat or dairy along the way, all the better. You could think of a cow as a highly efficient compost pile with added benefits (that is also able to harvest most of its own inputs without fossil fuel powered harvesting equipment, the only real alternative being draft animals which likewise can produce meat and dairy and manure.)

    I think it also makes sense to save humanure to fertilize animal pastures and instead use animal manure on fields growing food crops.

    • Adam Merberg said

      The 80% figure is from The Omnivore’s Dilemma, as I recall. As for manure, the point there is not that we should use the grain crops as fertilizer, but that if we grew those crops, it must have been in fertile soil, which might instead be used to grow human-edible crops.

      Anyway, I can’t claim to know the extent to which it’s good to have animals on a farm. However, I do think that when we should talk about these issues, we should avoid using phrases like “complete the nutrient cycle” because that obscures what’s really going on. When you talk about nutrient concentration and bio-availability, you sound like you know what you’re talking about. When Pollan talks about completing the nutrient cycle and says that Polyface is completely self-sufficient in nitrogen, he sounds like he doesn’t understand the laws of chemistry.

  9. […] for those in the “sustainable meat camp” that I could say I more or less belong to. This blog did a really great job pointing out the inconsistency in Michael Pollan’s argument that Polyface is closed loop. The […]

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