What are synthetic fertilizers, anyway? Well, as the name suggests, they’re a departure from natural processes. The question is how far they depart from nature, in what ways, and with what consequences. I’ll turn again to Michael Pollan to set the context: as he explains in The Omnivore’s Dilemma,
all life on earth can be viewed as a competition among species for the solar energy captured by green plants and stored in the form of complex carbon molecules. A food chain is a system for passing those calories on to species that lack the plant’s unique ability to synthesize them from sunlight. One of the themes of this book is that the industrial revolution of the food chain, dating to the close of World War II, has actually changed the fundamental rules of the game. Industrial agriculture has supplanted a complete reliance on the sun for our calories with something new under the sun: a food chain that draws much of its energy from fossil fuels instead. (p 7)
Specifically,
a key turning point in the industrialization of our food, can be dated with some precision to the day in 1947 when the huge munitions plant at Muscle Shoals, Alabama, switched over to making chemical fertilizer. After the war the government had found itself with a tremendous surplus of ammonium nitrate, the principal ingredient in the making of explosives. [...] The chemical fertilizer industry (along with that of pesticides, which are based on poison gases developed for the war) is the product of the government’s effort to convert its war machine to peacetime purposes. As the Indian farmer activist Vandana Shiva says in her speeches, “We’re still eating the leftovers of World War II.” (p 41)
And Pollan explains where that route has led us:
What had been a local, sun-driven cycle of fertility, in which the legumes fed the corn which fed the livestock which in turn (with their manure) fed the corn, was now broken. Now [a farmer] could plant corn every year and on as much of his acreage as he chose, since he had no need for the legumes or the animal manure. [...]
Liberated from the old biological constraints, the farm could now be managed on industrial principles, as a factory transforming raw material—chemical fertilizer—into outputs of corn. Since the farm no longer needs to generate and conserve its own fertility by maintaining a diversity of species, synthetic fertilizer opens the way to monoculture, allowing the farmer to bring the factory’s economies of scale and mechanical efficiency to nature. (pp 44-45)
...but so much for efficiency in the end:
When you add together the natural gas in the fertilizer to the fossil fuels it takes to make the pesticides, drive the tractors, and harvest, dry, and transport the corn, you find that every bushel of industrial corn requires the equivalent of between a quarter and a third of a gallon of oil to grow it [....] Put another way, it takes more than a calorie of fossil fuel energy to produce a calorie of food; before the advent of chemical fertilizer [farms] produced more than two calories of food for every calorie of energy invested. From the standpoint of industrial efficiency, it’s too bad we can’t simply drink the petroleum directly. (pp 45-46)
In other words, sure, I could easily drive to Home Depot or the gardening section of the grocery store and pick up a few bags of synthetic fertilizer. But that would mean paying for the product of fossil fuels and industrial processing, trucked in from a distant factory. How could that possibly dovetail with my mission of sustainably growing my own local food in local land?
The fact is that there’s enough plant-nourishing energy pouring in around here—showers of sunlight, piles of dry grass and raked leaves, the peels and cuttings of my own cooking. I’d much rather tend to that natural, abundant, renewable energy and ensure it’s going to good use than passively let it go to waste and then pay for environmentally costly industrial manufacturing to make up the difference.