In 1898, the chemist and physicist William Crookes devoted his presidential address to the British Association for the Advancement of Science to “a life and death question for generations to come.” At existing rates, he argued, the world’s wheat crop would cease to be able to feed the world’s wheat-eating people within a few decades. Crookes based his forecast on an estimate of the amount of nitrogen available for the growth of wheat crops. For centuries, farmers had observed that land would become less productive when it was planted year after year. With the advent of modern chemistry, scientists came to understand that these declining yields resulted from dwindling levels of nutrients in the soil as these substances were taken up by crops and removed from the land at harvest.

Rhizobia fix nitrogen in nodules of the roots of legumes (soybean pictured). Photo copyright International Institute of Tropical Agriculture. Used under Creative Commons License

All life requires nitrogen, which is an important component of many vital molecules, including DNA and proteins. Elemental nitrogen makes up nearly four-fifths of the earth’s atmosphere, but plants and animals can’t use nitrogen in that form. Before atmospheric nitrogen can be used by plants or animals, it must be converted to the biologically available form, or fixed. In Crookes’ day, virtually all of the biologically available nitrogen had been fixed by microorganisms known as diazotrophs, such as the bacteria of genus Rhizobia in the roots of legumes like beans, clover, and alfalfa. European and North American farmers had begun augmenting yields by importing nitrogen-rich bat guano from Peru and sodium nitrate from Chilean mines. However, these were not unlimited resources, and Crookes reasoned that unless something were to change, the lands available for wheat cultivation would cease to be able to produce enough wheat to meet world demand by the 1930s.

But Crookes emphasized that he intended his remarks as “a warning rather than of a prophecy”; ultimately he predicted that “the Chemist will step in and postpone the day of famine” by discovering a method for fixing atmospheric nitrogen in the lab. Just eleven years later, the German chemist Fritz Haber answered that call, successfully synthesizing ammonia from nitrogen and hydrogen in his laboratory at Karlsruhe. Haber worked with Carl Bosch of BASF to refine the method in the years that followed, and 1913 saw the deployment of the first industrial application of what came to be known as the Haber-Bosch process.

1 Global consumption of nitrogen fertilizer from Haber-Bosch synthesis.

Manufacture of nitrogen fertilizers remained limited through the end of World War II. Crookes, it turned out, had failed to account for an increase in arable land resulting from mechanization of agriculture and had underestimated the reserves of sodium nitrate in Chile. However, his basic premise–that a growing population needs a growing supply of nitrogen to feed itself–was sound, and the second half of the twentieth century saw global application of synthetic nitrogen fertilizers increase by a factor of more than 20, and the interdisciplinary researcher Vaclav Smil has estimated that synthetic fertilizers now provide more than half the nitrogen received by the world’s crops.

Unfortunately, this abundance of nitrogen brought a new set of problems. According to a report released last month by the United Nations Environment Programme and several other organizations, excessive fertilizer use contributes to “a growing pollution web requiring urgent action.” As the report explains, nitrogen not taken up by crops contributes to a host of environmental problems, including contamination of drinking water, algal blooms, coastal and freshwater dead zones, air pollution, climate change, and loss of biodiversity.

Concerns about synthetic fertilizers are not new. In his 1940 text, An Agricultural Testament, the British botanist Sir Albert Howard argued that using synthetic fertilizers instead of composts was detrimental to soil health, which he hypothesized was “the real basis of public health.” Howard’s work inspired the American J.I. Rodale to found Organic Farming and Gardening magazine, which promoted farming without synthetic fertilizers and pesticides, and Rodale’s publication gave its name to the movement for a more natural agriculture. Though evidence for Howard’s belief that synthetic fertilizers lead to poor public health remains scant, the organic movement remains alive and well today, and leading organic advocate Michael Pollan now argues that “the whole point of organic food is that it’s more environmentally sustainable.”

This shift in rationales underscores a key feature of the organic movement. Namely, organic does not establish its objectives (say, environmental sustainability and human health) and commit to supporting the agricultural practices which best achieve those goals, updating its methods as science comes to better understand how these goals might be achieved. Instead, it starts from the axiom that natural is best and adapts to new scientific evidence not by rethinking that heuristic but by updating the measure of its success.

According to organic advocates Tom Philpott and Barry Estabrook, we don’t need synthetic fertilizers at all. Both argue that organic agriculture can produce enough food for the world’s population based on comparisons of yields of organic and conventional farms. In cooking terms, comparing the yields (the amounts of crops produced on an acre) of organic and conventional farms is like comparing the end product of different recipes. That’s important if you’re trying to decide what to make for dinner, but even the best-tested recipe is useless without the required ingredients. Likewise, no agricultural system works without nitrogen. Rejecting synthetic fertilizer, as organic does, would leave us reliant on biological nitrogen fixation by diazotrophs (primarily in the roots of legumes on farmland) for nitrogen. Only so much biological nitrogen fixation can take place on a plot of land in a year, so it’s important to ask whether we can get enough nitrogen this way. Arguing for all-organic agriculture without being able to answer that question in the affirmative is a bit like promising your dinner guests a wild mushroom risotto without making sure that you can find enough chanterelles.

Christos Vasilikiotis also falls short in arguing that organic agriculture can feed the world. He acknowledges the need to demonstrate the availability of organic-approved nitrogen, but he does not estimate the potential for biological nitrogen by diazotrophs, instead pointing out that “EPA estimates indicate that US livestock operations generate one billion tons of manure per year; most of this is not utilized in agriculture.” While it seems unlikely that manure from US livestock could be utilized by farmers in China (which now uses a third of the world’s synthetic nitrogen fertilizer) or sub-Saharan Africa (where the UN report identifies environmental damage from too little nitrogen rather than too much), Vasilikiotis’s analysis also suffers from a deeper flaw which arises from a loophole in the organic philosophy.

Organic agriculture does not prohibit the application of nitrogen that has been fixed by the Haber-Bosch process. Instead, it forbids the use of synthetic fertilizers and allows the use of animal manures–including those produced by conventionally-farmed animals. This means that taboo synthetic fertilizers taken up by feed grains become all-natural–and certifiably organic–manure as they pass through an animal’s digestive tract.

By identifying manure as a source of nitrogen, Vasilikiotis dodges the issue of nitrogen fixation entirely. However much nitrogen exists in manure today, much of it has been fixed industrially before being taken up by corn plants and laundered through the guts of conventionally-farmed animals. Vasilikiotis does not explain how that manure might come to be in an organic world. To do so would require demonstrating the potential for sufficient biological nitrogen fixation.

A further source of confusion is the misconception that animals make nutrients. Animals do not fix nitrogen or (aside from one known exception, the shipworm ) acquire significant amounts of nitrogen directly from nitrogen-fixing bacteria. Nor do they make other nutrients, like phosphorous and potassium, so their manure contains no more nutrients than their feed.

This is a point which Dan Barber, the influential New York eco-chef, seems to miss when he urges environmentally-conscious New Englanders to eat “a lot of meat” in part to avoid dependence on synthetic fertilizers. According to Barber, it’s important to raise a lot of animals so that we’ll have enough manure, which he terms a “free ecological resource,” to fertilize our vegetable crops. This advice stands in stark contrast to the recommendations of the UN fertilizer report, which explains that “inclusion of livestock in the food chain substantially reduces overall nutrient use efficiency, leading to large pollution releases to the environment” and identifies as a “Key Action” the reduction of animal protein consumption in affluent regions.

The discrepancy is explained by considering the Stone Barns Center for Food & Agriculture, the small non-profit farm which raises meats for Barber’s Blue Hill restaurants and is, in Barber’s words, “a replicable model for the future of good food.” At Stone Barns, the manure of pigs, chickens, geese and turkeys is largely derived from feed of corn, soy, sunflower, and flax. While sheep, the farm’s ruminants, may take in some “free” nutrients like nitrogen fixed in the roots of clover on the pasture, that pasture is fertilized in part with the manure of grain-fed animals, so even their manure is made possible by the grain inputs.

That’s important because the nutrients in grains need to come from somewhere. Manure from grain-fed animals doesn’t solve the problem of soil fertility so much as transfer that problem to the grain farm. And although animal manures are natural, they aren’t entirely benign. Like synthetic fertilizers, nutrients in manure can run off in groundwater or escape into the atmosphere. This means that growing grains for animals to produce manure for vegetables tends to increase pollution by adding another opportunity (on the grain farm) for nutrients to escape. While recycling nutrients in manure is more efficient than discarding them, the possibility of doing this does not amount to a strong argument against reducing nutrient inputs, an end that would be achieved by eating grains and legumes in place of grain-fed meats.

Barber’s advice highlights the perils of the organic movement’s attitude towards natural and synthetic materials. To organic, industrial fertilizers should be avoided because they are synthetic, and manure is permitted because it is natural. This preempts any discussion of the actual environmental issues surrounding fertilizer use, so it’s unsurprising that we hear pronouncements like Barber’s which stand to exacerbate those problems.

The misconception that animals make nutrients also tends to obscure a basic fact about the problem of fertility in agriculture. Pollan says in the documentary Fresh that by fertilizing crops with manure, farmers “close the nutrient cycle,” but this terminology can be misleading. Farms lose nutrients with the harvest of crops, even if all animal manure is recycled . This stands in contrast to natural ecosystems, where nutrients removed from land in food return to the land when the animal defecates.

Natural processes, like atmospheric nitrogen deposition , can help replenish some nutrients, but the fact remains that the nutrient cycle remains open. Maintaining modern yields generally requires inputs of some kind to replace nutrients removed in crops. For instance, Joel Salatin’s Polyface Farm–which Pollan’s The Omnivore’s Dilemma identifies as a model for sustainable agriculture and describes as “completely self-sufficient in nitrogen”–actually brings in nitrogen in conventionally-grown grain , which is fed to chickens whose manure fertilizes the pasture.

For centuries, farmers in China strived to close nutrient cycles by fertilizing their crops with human excrement. In the early twentieth century, the American agricultural scientist F.H. King argued that this practice had been critical to the perennial fertility of Chinese farmland and identified the Western method of sewage disposal as a major reason for dependence on fertilizers. Over the years, the idea of reusing human waste in agriculture has earned the support of an unlikely alliance of agricultural figures, including not only Albert Howard, but also food movement hero Wendell Berry and Justus von Liebig, the German chemist vilified by organic advocates for his discovery that plants need only nitrogen, phosphorous, and potassium from soil.

In recent years, the US government has begun allowing the recycling of human waste by authorizing the use of treated sewage sludge, called biosolids, as fertilizers. However, in 1998 organic advocates successfully protested proposed guidelines which would have allowed application of biosolids in organic agriculture. Whatever the merits of their objections , it is ironic that the movement for a more “natural” agriculture now opposes ending the waste of nutrients that Liebig once decried as “a sinful violation of the divine laws of Nature.”

All of this is to say that the problem of fertility in agriculture is deeply unnatural. Nature has inspired many good ideas, like crop rotations and recycling of nutrients. The UN report makes clear that these practices, along with reductions in food waste and meat consumption, will be imporant parts of efforts to mitigate the “growing pollution web.” That said, nature does not take an interest in the well-being of humankind, so it seems unwise to expect it to solve the problems that we have created for ourselves. Too much synthetic nitrogen fertilizer causes problems, but that doesn’t mean the solution is to use none at all.

Meanwhile, emerging technologies, such as wheat genetically engineered to use nitrogen more efficiently and chemical treatments to reduce nutrient pollution from manure , aim to lessen the environmental impact of agriculture. Organic agriculture won’t allow these, and, to be fair, it remains to be seen whether these technologies can succeed in that end. However, we ought to make that determination based on evidence, instead of dismissing these tools for being unnatural.