Fertilizers are human-made and natural products most often applied to soil to increase and maintain agricultural production. The practice of using natural fertilizers, particularly the feces and urine of humans and livestock, is probably as old as agriculture itself and may have developed as prehistoric humans practicing hunting and gathering noticed that not only did food plants grow where they had previously left food remains, but that those areas enriched with excrement produced more lush vegetation and a greater abundance of fruits and seeds.See also Green Revolution.
Natural fertilizers include manure from livestock, post-harvest crop residues, "green manure" from surrounding fields and forests, composted plant and animal remains, and even human excrement. One of the major benefits of using these natural fertilizers is that often nutrients and organic matter are returned to the same fields from which they were previously harvested as food or fodder. Another major benefit of using natural fertilizers is to convert what would often be a noxious agricultural by-product into a relatively inexpensive agricultural input. The economic return can therefore be two-fold; reduced costs of waste disposal and reduced costs of agricultural inputs. In rural areas which lack proper waste treatment and disposal facilities, the use of wastes as fertilizers is an elegant solution to both problems.
For example, the use of human wastes as fertilizer is still widely practiced in China where human "night soil" is collected and spread on local fields along with animal wastes. The use of human wastes as fertilizer however has a significant potential for transmitting human parasites and disease. Therefore, the historical use of human wastes as fertilizer greatly accounts for the culturally-derived practice in Chinese cuisine of cooking almost all vegetables.
Other Sources of Natural Fertilizer
A more common source of fertilizer is manure from livestock; primarily cattle, sheep, horses and other animals that feed on grasses and grains. In addition to manure from livestock, farmers throughout the world use "green manure" which is derived from vegetation cut from trees and other plants offsite, and then spread on fields. Often, green manure is obtained from various tree species and is considered a form of agroforestry. Post-harvest crop residues are also commonly plowed back into soils returning nutrients and organic matter to the soil. Other organic sources, most notably food wastes composed of plant and animal material may be converted into fertilizer by composting. Composting uses natural processes of decay and consumption by microorganisms and other creatures to break down the organic matter in food scraps creating a nutrient-rich mulch that can be added to soil or spread on fields and around plants.
In general, the use of natural fertilizers creates more tightly-linked and localized nutrient cycles. In contrast, the use of human-made fertilizers has steadily changed both the source of nutrients and the way that they cycle in (and out) of agricultural systems.
Natural, But Imported
To the list of all the other previously mentioned sources of natural fertilizers, must be added bird guano, which can be "mined" from places, such as islands, where seabirds congregate. Unlike locally derived natural fertilizers, however, bird guano has historically been mined at some distance from where it was used as fertilizer. So, although bird guano is a natural fertilizer, it is the first example of a fertilizer which links agriculture with an imported, highly concentrated form of nutrients, presaging the use of human-made chemically-derived fertilizers.
Certainly the greatest innovation in agriculture since the domestication of plants has been the invention of chemically-derived fertilizers and the highly productive breeds of plants which can take advantage of increased applications of nutrients from these fertilizers. Inseparably, the invention of mass-produced fertilizers and these new breeds of crop plants have come to be known as the "Green Revolution". And, certainly the greatest innovation in the creation of human-made fertilizers has been the so-called "Haber process" Invented in 1908 by German chemist Fritz Haber, the Haber process uses high heat and pressure to combine hydrogen (usually obtained from methane) with diatomic nitrogen from the atmosphere in the presence of an iron catalyst to create ammonia, ammonia nitrate, and urea. The Haber process is energy-intensive and is estimated to consume approximately 1% of the energy used worldwide each year. The link between energy and fertilizer production is very important because the Haber process is used to produce about 500 million tons of nitrogen fertilizer each year which in turn supports about 40% of the world's population. Any decrease in the availability of energy, particularly from a decline in fossil fuel production (used as energy itself and as a feedstock) could cause a significant interruption in the production of fertilizer. In addition, significant amounts of energy are required to transport human-made fertilizers from where they are manufactured to where they are used.
- Smith, Barry E., 2002. Structure: Nitrogenase Reveals Its Inner Secrets. Science, 297(5587):1654-1655.