A seed bursting into a plant is a marvelous process. Where does all the matter in the plant come from? Carbon, hydrogen, and oxygen, the basic building blocks of the carbohydrates, proteins, and fats that make up a plant, are available from water and from carbon dioxide in the air. But nitrogen, phosphorus, potassium, and some sixteen other elements needed for plant growth must be furnished by the soil. If the soil runs out, and is not replenished, plant growth suffers. Long before scientists provided detailed accounts of plant nutrition, ancient farmers learned through experience that crop yields decreased from year to year. Eventually they concluded that, like humans and animals, crops had to be fed.
Over 2,000 years ago, Chinese rice growers were already applying burnt animal bones to their fields, and nobody really knows when North American Indians began burying dead fish between rows of corn. We certainly do know that they taught the practice to the Plymouth settlers. We also know that George Washington fertilized more than American minds. America’s first president took great interest in farming and concluded that the criteria for better crop growth were loose earth and soil “amendments.” He experimented with manure, creek mud, plaster of Paris, lime, “green manure” (plowing buckwheat, clover, and peas into the soil), and fish heads! The president was certainly on the right track; each of these “amendments” was capable of contributing some nutrition to the soil.
The most significant advance in fertilizer development, however, came about in an accidental fashion. And for this we can thank a Spanish missionary to Chile whose name has been lost to history. It seems some native Indians had extinguished a fire by throwing hard, dry earth onto the hot coals, and were stunned by the acrid purple vapors that were suddenly released. Some sort of evil spirits, they probably thought, and grabbed a few chunks of the dry earth to show the priest, whom they assumed would have an explanation. He didn’t, and threw the samples into his garden. A few months later, the alert missionary noted an increased growth of vegetation where the chunks had landed. “Chile saltpeter” had made a triumphant entry into the world of agriculture. As chemists later learned, it was mostly sodium nitrate, an excellent substance for introducing soluble nitrogen into the soil. Nitrogen is a component of all proteins, including the enzymes that are instrumental in every phase of plant growth. Many of the vitamins that plants produce, as well as the chemicals with which they protect themselves against insects and fungi, contain nitrogen. But what caused the irritating, purplish vapors? Saltpeter is commonly contaminated with sodium iodate, which, when heated, releases a variety of iodine compounds that can cause a choking sensation.
By the seventeenth century, Chilean saltpeter was widely imported into Europe, along with another South American commodity that had been found to increase soil fertility. This was guano, or bird excreta. Like saltpeter, it was an excellent source of nitrogen and furnished phosphorus to boot. Peru was the first recognized source of extensive deposits of bird poop, but by the 1800s, many South Pacific islands were also found to be overflowing with seabird guano. To this day, guano is an important source of fertilizer, helping to feed the world. But it has also helped to overfeed the natives of the guano-producing islands. Contrary to what many may think, Americans are not the fattest people in the world. That dubious honor goes to the inhabitants of the Pacific island of Nauru, who boast one of the highest per-capita incomes in the world. And that thanks to bird droppings! Fertilizer companies pay high prices for the chance to harvest the guano, allowing natives to trade in their plowshares for easy chairs. As their level of activity decreased, imports of beer, meat, and chips increased. The result is that about 70 percent of the natives of Nauru are obese, and a third suffer from diabetes.
Exactly why saltpeter and guano increased yields interested the great German chemist Justus von Liebig. He decided to solve the mystery by burning plant material and analyzing the residue. In 1840, he published his classic, Organic Chemistry in Its Applications to Agriculture and Physiology, which clearly established him as the father of modern soil science. Liebig’s analysis revealed that the major minerals present in plant residue were nitrogen, phosphorus, and potassium, and that the reason saltpeter and guano enhanced plant growth was because they were rich sources of nitrogen and phosphorus. Potassium, he said, could be supplied by potash. This was originally obtained by soaking the ashes of wood to dissolve the potassium salts, then filtering the suspension and boiling off the water in a pot to leave a white ash, “potash,” which was mostly potassium carbonate.
Liebig’s theories launched research into the systematic development of mineral fertilizers, without which we could not possibly feed the population of the world. And without these fertilizers, we would not have the abundance of fruits and vegetables with which we are blessed today. Still, some look on “chemical” fertilizers with a wary eye and favor crops grown with the aid of “organic” fertilizers, such as manure. Now, I have nothing against animal or human dung, but the idea that it can feed the world is, well, romanticized bull manure.
Technology generally arises in response to a need. And that was the case with chemical fertilizers. They were born out of necessity, simply because the traditional methods of fertilizing the soil via crop rotation or the application of compost or manure were not getting the job done. By the middle of the nineteenth century, agricultural yields in Europe had declined so dramatically that famine was in the offing. Large-scale tragedy was averted with the discovery that food production was limited by the soil’s nitrogen content. Unfortunately, fertilization techniques being used at the time were just not replenishing the nitrogen absorbed by crops. Enter sodium nitrate, or “Chile saltpeter,” an excellent nitrogen source. It’s hard to see why the application of this “chemical” substance to fields should be regarded as any less natural than the application of manure; after all, saltpeter is the end result of the decomposition of animal waste, and is mined from the earth. What could be more natural?
Chilean saltpeter deposits were not enough to satisfy the needs of a rapidly growing population, and the problem of supplying the soil with enough nitrogen wasn’t solved until Fritz Haber found a way to make ammonia from nitrogen and hydrogen. Ammonia could then be converted to ammonium nitrate, an ideal nitrogen fertilizer. What an irony that plants are surrounded by a vast amount of nitrogen (the gas makes up 80 percent of air), but are unable to use it!
Plants can only absorb soluble nitrogen compounds from the soil. Some, the legumes, harbor specialized bacteria in nodules on their roots that are able to “fix” nitrogen from the air. In other words, they can convert nitrogen gas into nitrates, which the plant can use directly for growth. That’s why legumes can be plowed into the earth as “green manure,” and why they serve a vital role in crop rotation. All other plants have to rely on soil microbes to convert nitrogen containing organic compounds into soluble nitrates. Such compounds, urea being an example, can be found in manure. Or nitrates can be supplied directly by fertilizer. Basically, as far as a plant is concerned, whether the nitrates it needs are supplied by manure or by the addition of a chemical fertilizer is irrelevant. It is not irrelevant, however, as far as the soil is concerned. Manure is better for soil structure; it contains substances such as humic acids, which limit nitrate leaching, help retain water, and reduce erosion. Chemical fertilizers are more likely to allow nitrate to be leached out into water systems where they can fertilize aquatic plants, which eventually die, decompose, and use up some of the dissolved oxygen needed by fish for their survival.
There are other concerns with the application of mineral nitrates. Overuse leads to their buildup in the soil, where bacteria can convert nitrates to nitrous oxide—a “greenhouse gas” hundreds of times more potent than carbon dioxide. Fertilizer producers have addressed these issues and have developed a number of “slow release” fertilizers. For example, synthetic urea can be converted to urea-formaldehyde, which releases nitrogen gradually through microbial activity in the soil. There are also granular fertilizers, coated with semi-permeable membranes for slow release, or encapsulated in microcrystalline wax. Fertilizers today can be formulated with virtually any ratio of nitrogen, potassium, and phosphorus, the main nutrients plants require, and can be matched to a particular soil’s needs. Manure has a more random composition and is much lower in nutrients, particularly phosphate. Applying it to the land is more difficult and more expensive. It’s not that organic agriculture based on manure cannot work; it can. There is no doubt that in test plots, or on specific farms, organic farming can be very effective. Indeed, experiments in test plots have shown that using manure as fertilizer can match the yields produced by mineral fertilizers, and that the soil is less prone to nitrate leaching. And without a doubt, there are even some large and successful organic farms.
The Pavich family in California farms about 1,800 acres and produces 12,000 tons of table grapes a year using only composted steer manure as fertilizer. But this success could not be duplicated by rice growers in India, or by wheat farmers in Africa. There isn’t enough manure available locally, and transportation costs would be prohibitive. Even when sufficient manure is available, problems still crop up. Numerous cases of food poisoning caused by Salmonella or E. coli bacteria have been related to the use of manure as fertilizer. E. coli O157:H7, the bacterium in contaminated tap water that sickened over 2,000 people and killed seven in Walkerton, Ontario, in 2000, can survive in bovine feces for seventy days. In fact, it probably entered the water system from manure. But even if manure is properly composted to eliminate bacteria, the bottom line is that it is not going to solve the global hunger problem. Without the use of nitrogen fertilizers, we could perhaps feed about half the world’s population. Indeed, it is hard to think of a scientific development that has had a greater beneficial impact on human life than the oftenmaligned “chemical fertilizers.”