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.