NPK: The Secret Life of Fertilizer
With man’s transition from hunter-gatherer to farmer, a never-ending search for fertilizers began. That search for a constant supply of vital crop nutrients has astonishingly, and at times disturbingly, taken man where it’s hard to believe he’d go. It all began back in a time when global tensions arose not from terrorism, global warming or nuclear war, but from bird poop, plaster and human bones.
Once humans began to live in settled communities and cultivate the soil to grow food crops some 10,000 years ago, the finding, keeping and recycling of these essential nutrients on the farm required constant management. Once the virgin topsoils were exhausted, the fertilizer battle cry was sounded.
Since ancient times, agriculture has evolved from trial and error (the farmer’s observant eye) to modern agricultural science (chemical analysis) eventually determining and explaining the values of a multitude of manures and minerals. But until recent times it has been simple accidents that yielded new discoveries in plant nutrition.
Gypsum is one good example. It is high in calcium, which, among other things such as improving soil structure, is essential for most nutrients to be absorbed by plants. As the story goes, the practice of using gypsum as a fertilizer was begun by German farmers who learned of its value from the experiences of a mortar mixer who had been working with gypsum. In going back and forth to his cottage nearby, this workman made a path across the field along which the crops appeared to grow better than they did back farther in the field. Noticing this, he surmised that the dust on his clothing and shoes must have been responsible for it. On experimenting with gypsum in his garden he became convinced of its value. As a result the practice spread. It later became so highly regarded as a miraculous fertilizer to American farmers as to invoke its smuggling from Nova Scotia for use on wheat fields and causing the so-named Plaster War of 1820.
It has been little more than 200 years since the calculated results of field experiments have begun to identify plant nutrients and rank them in order of importance. The NPK label on fertilizer bags is a good start to indicating the most important, or fundamental, elements required to produce the food we eat. Many other factors of course, such as water, micronutrients and weather are also important, as is the balance of these nutrients, but NPK are the important measures of our soil fertility and since the discovery of their importance in agriculture people have vied for possession and control of their sources.
The N in NPK stands for nitrogen. Nitrogen was not such a worry at first, as it could be captured from the atmosphere in nitrogen-fixing cover crops. Some plant species, legumes and clovers developed a symbiosis with microorganisms in nodules on their roots. These microbes have enzymes that can fix atmospheric nitrogen, which are then available to the host plant. When these plants die and decay, a substantial part of the nitrogen becomes available for uptake from the soil by other plants. The nitrogen cycle between organisms and the environment is an endless one of growth and decay and represents one of the most important processes in nutrient recycling.
Nitrogen is one of the most widely distributed elements in nature and it is more associated with life than any other. Where there is nitrogen in nature, there is life. But without the early discovery of a method of fixing nitrogen from the atmosphere, there certainly would have been widespread starvation worldwide as world human population expanded.
During the eighteenth century, nitrogen fixation was improved by growing leguminous fodder crops or clover-grass mixtures on fallow land and allowing livestock to graze on it, which resulted in more farmyard manure, which also recycled nitrogen. Crop takes up nutrients, livestock eat crop, livestock manure spread on fields, crop takes up nutrients, we eat crop and round and round. When you feed your plants you really are feeding yourself. But as the world population continued to grow, other sources of nitrogen were needed.
The production of sodium nitrate (a mined, nitrate salt) initially for use in gunpowder (also called Peru saltpeter) started in 1804 in Chile, Bolivia and Peru, and helped to ease the insatiable hunger for nitrogen. For more than a century, the world supply of saltpeter was mined almost exclusively from the Atacama Desert in northern Chile and was used extensively as a fertilizer and a raw material for the making of gunpowder.
Around the same time another nitrogen fertilizer also began to be supplied by the discovery of guano deposits found in the same region. Composed almost entirely of nitrogen, phosphate and potassium, guano suddenly became the new “wonder fertilizer” for the western world, although it was long recognized as an excellent fertilizer in its native land since the time of the Inca. Seemingly endless heaps of this “white gold” were piled atop small islands scattered off the coast rising some 25 feet high with excrements of seabirds, cormorant, boobies and pelican. It is estimated that 100,000 workers were eventually shipped in to harvest it, spade by spade, and load it on to ships bound for distant shores.
The first economic boom of modern Peruvian history, owing to its vast natural resources, came to a devastating end in the War of the Pacific (1879-1883) in which Peru and Bolivia were defeated by Chile for the rights to guano and nitrate deposits in the disputed territories. Believed to be an inexhaustible, regenerating, natural resource, they all were soon proved wrong and as world population grew through fertilizer-generated crop production, even the strict restrictions put on the harvest of guano did not help. Within 35 years the seemingly endless supply of guano, which had taken thousands of years to accumulate, was completely exhausted. It never could recover, partially owing to the depletion of fish in the area, which had reduced the seabird populations.
The world was again desperate for a new source of nitrogen. In 1898 English chemist Sir William Crookes called on all chemists to find a way to fix nitrogen chemically from the unlimited reserves in the air. He prophesied that, without a new source of nitrogen fertilizer, famine would be inevitable within two to three decades.
At the turn of the twentieth century, German chemists Fritz Haber and Carl Bosch developed a process for producing ammonia from the atmosphere on an industrial scale and this discovery resulted in a dramatic decline in demand for sodium nitrate from natural sources.
The K in NPK stands for potassium (also referred to as potash), whose earliest source was found in wood ashes. The chemical symbol for the element, K, derives from kalium, the Latin version of the Arabic word for alkali. The term potash comes from the Middle Dutch word potaschen (pot ashes). It is important for agriculture because it improves water retention, crop yield, nutritive value and disease resistance, not to mention color, taste and texture of food crops.
The old method of procuring it was by making wood ash, leaching the ashes and then evaporating the water from them in large iron pots. Plants take up considerable quantities of potassium from the soil. When the plants are burned, about ten percent of the weight of the ashes becomes potassium carbonate.
Although it wasn’t until around 1813 that it was chemically proven that the ashes of plants contain the mineral constituents which accumulate in plants from the soil, surely the value of ashes as fertilizer may have been known by farmers in very early times through the burning of camp fires and the increased growth of surrounding vegetation left behind.
In the late eighteenth and early nineteenth centuries, potash production provided American settlers a way to obtain money and credit as they cleared wooded land for crops. Any wood not needed for fuel or construction could be burned and boiled down to be used to make lye, which could then be used to make soap.
In Europe potash pits were used to produce potash for wool washing. Potash pits were kiln sites dug and lined with drystone. The pits were a feature found in sheep farming districts with extensive woodlands throughout England. The scouring or degreasing of the lanolin from wool requires the use of soft soap, produced using fat and an alkaline potash solution that contains water-soluble potassium salts.
In keeping a tight rein on a farm’s recycling of nutrients, the sheep themselves served multiple purposes. Although potash was principally obtained by leaching the ashes of land and sea plants, other inventive methods of recapturing potash were also used. As the sheep across the country removed thousands of tons of potassium from the land every year, it had to be replaced by potassium fertilizer. As such, not only was their manure recycled, even the water from washing sheep before shearing was saved and was an important source of crude potash found in the “yolk” or “suint” of wool, a water-soluble substance found in the fleece of sheep originating from its perspiration. In parts of England in the nineteenth century, environmental pollution from scouring mills was so bad that it is said that one of the local pastimes was to set fire to the wool wax floating on polluted waterways.
As crazy as this all sounds, modern field tests conducted by the Victorian Department of Agriculture State Chemical Laboratory in Australia have recently shown that the potassium concentrate from sheep suint is as good if not better than processed muriate of potash in improving pasture crop yields, and systems are being devised to capture and process it for use as a modern day fertilizer.
Potassium is a common element, about seventh in order of abundance in the Earth’s crust and total resources of potassium today are estimated to be about 150 billion tonnes, which will last many millennia.
But it is without doubt that the search for the P (phosphorus) in NPK has been the most unusual, or better said, macabre, odyssey of all the elements. Due to its use in explosives, poisons and nerve agents, as well as the sometimes-repulsive sources through which it was attained, is has been referred to as “the Devil’s element”.
Phosphorus is a major nutrient for all living organisms on Earth and has no known substitute. It was first identified as being present in human urine and bone ash and in this form was an important early phosphate source for agriculture. The discovery of phosphorus is credited to the German alchemist Hennig Brand in 1669. Brand experimented with urine and attempted to create the fabled philosopher’s stone (a legendary alchemical substance capable of turning base metals into gold through the distillation of some salts by evaporating urine and for many centuries it was the most sought-after goal in alchemy) and in the process produced a white material that glowed in the dark and burned brilliantly. Not the gold he was hoping for but instead Brand had discovered phosphorus.
In 1769 Johan Gottieb Gahn and Carl Wilhelm Scheele (both Swedish chemists) showed that calcium phosphate could be retrieved from bone ash. Recognition of the value of bones for fertilizer purposes (for the preparation of phosphorus) seems to have first come from England, its value being mentioned in English agricultural literature of the middle of the seventeenth century. By early in the nineteenth century English land owners had used up all of the local supply of bones (from charnel houses and common graves), so they began collecting bones from all over the world and shipping them back to England.
Justus von Liebig, (1803-1873) considered the founder of organic chemistry and the “father of the fertilizer industry”, became alarmed at the rate at which continental Europe was being robbed of bones by the English. In a newspaper article from the Nautical Register in 1822 Liebig writes, “It is estimated that more than a million of bushels of human and inhuman bones were imported last year from the continent of Europe into the port of Hull. The neighborhood of Leipsic, Austerlitz, Waterloo, and of all the places, where during the late bloody war, the principal battles were fought, have been swept alike the bones of the hero, and the horse which he rode. Thus collected from every quarter they have been shipped to the port of Hull thence forwarded to the Yorkshire bone grinders.” The article goes on to say that they are then sold at the Doncaster agricultural market where they are bought by farmers to spread on their fields, and mentions the fact that human bones in particular make the best manure.
Aware that bones may not compensate for the amount of phosphorus needed in the future and lamenting over the fact that the dead were buried so deeply that the phosphorus was wasted completely, other sources were considered, such as this excerpt from an early nineteenth century book suggests. “There are many other substances, as horns, the shells of sea-fish, coral, and shellmarl, which may answer the same purpose, and the produce of which is perfectly inexhaustible. As bones are likely to become rather a scarce article, it may be difficult to supply them in quantities adequate to the demand: it is a most fortunate circumstance therefore, that the shells of oysters and other sea-fish, when properly reduced in size, have been found equally useful as a manure.” In America the native population had been using oyster shells for agricultural purposes for centuries and the colonist made note of this and adopted the practice.
Eventually, following the discoveries of vast deposits of phosphate rock (which took millions of years to form), and also of the superphosphate process, bones came to occupy a much less important place as a source of phosphorus for fertilizer. Today phosphate rock is one of the most highly traded commodities in the world.
Phosphorus is a limited natural resource that is non-renewable in whatever form it takes. The phosphorus cycle is from the land to living organisms and back to the land. Its use in farming contributed to the growth of civilizations in the past and will in the future. According to some phosphorus alarmists, the world will run out of viable phosphate reserves in the very near future.
Until the twentieth century, farms were self-sufficient, tilling the same land for years and recycling and reusing waste, making sure that everything that came out of the soil was put back in. Phosphate would have been able to be reused approximately 46 times as food, fuel, fertilizer and food again. Amazing. But with Big Ag farming practices today, with their annual application of phosphate-enriched chemical mixtures on top of nutrient-starved soil, phosphorus is used exactly once, then swept out to sea, simply unsustainable.
But what goes around comes around, especially in agriculture. Despite the general increase in the use of chemical fertilizers since the 1940s, the original fertilizer of old-time farming—manure—is coming full circle and looks likely to continue into the future with the help of modern technology. The primary means by which phosphorus is reintroduced to the environment is post-consumption through animal and humans.
Livestock and municipal waste provides numerous additional soil conditioning benefits that inorganic fertilizers do not, and new technology for retrieving phosphorus from these sources may become the best solution to solve the future phosphorus dilemma.
The existence of that delectable plate of leafy greens and succulent tomatoes waiting for the bone-in ribeye cooking over apple wood is dependent on the fertility of the land it grew on. Plants are the primary source of all nutrients required by humans, whether eaten directly or via meat, milk or eggs from animals, and the quality and quantity of plant nutrients (fertilizers) have not only had a direct impact on taste, abundance and even the look of the foods we eat, but also the health and growth of civilizations.
