Fertilizer

Lierre Keith argues that animals are needed to provide manure to fertilize crops. She writes, "Up until about 1950, agriculture was still limited by the amount of energy that fell from the sun. What that meant practically was that animals had to be integrated into small farms because their manure -- the best source of naturally occurring nitrogen -- was needed there" (104).
 
In terming manure a “source” of nitrogen, she evades a very important question: Where does the nitrogen in manure come from?
 
Keith quite correctly tells us that the nitrogen in the air is not biologically available. She tells us that essentially all of the biologically-available nitrogen has been fixed naturally by nitrogen-fixing bacteria in the roots of leguminous plants or industrially by the Haber-Bosch process. She’s right about that, too.
In particular, farm animals do not fix nitrogen. The nitrogen in manure comes from the foods that these animals eat.
Keith writes about Joel Salatin’s Polyface Farm as a model of sustainable farming, so it will be instructive to consider this example in particular. Keith’s information on Polyface Farm comes from Michael Pollan’s best-selling The Omnivore’s Dilemma. In that book, Michael Pollan credits the farm’s chickens with providing Polyface’s pastures with nitrogen.
 
The nitrogen, then, comes from the food that the chickens eat, which is mostly corn, soy, and oats [1]. And where does the nitrogen in that grain come from? Pollan quotes Salatin as saying that his feed corn might have been sprayed with atrazine [2], so it’s safe to say that this is industrial grain. It is exactly the stuff that Keith believes we shouldn’t be growing. (For a more detailed look at the feed grain at Polyface Farm, see The Polyface Myth).
 
There is another useful way to think about this issue. Of her gardening experience, Keith writes, “No nitrogen-fixing plant could make up for all the nutrients I was taking out” (20). But no animals could make up for these nutrients, either. As has been mentioned, farm animals don’t fix nitrogen.
 
Keith goes on to say of her decision to start using manure as fertilizer, “I closed the nutrient cycle” (21). But what would it really mean to “close the nutrient cycle”? That would mean putting back the nutrients that she took out. If that is her goal, Keith should advocate for composting toilets, rather than animal farming. Animals can complicate the flow of nitrogen, but they don’t make more of it available, nor do they close a cycle in any meaningful sense. (If, instead of closing the cycle, one wanted to increase availability of nitrogen, a good approach would be to eat a diet high in legumes like beans and peas).
 
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[1]Salatin, Joel. Pastured Poultry Profit$, page 52.
 
[2]Pollan, Michael. The Omnivore’s Dilemma, page 132.