Fertilizer
A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from liming materials or other non-nutrient soil amendments. Many sources of fertilizer exist, both natural and industrially produced. For most modern agricultural practices, fertilization focuses on three main macro nutrients: nitrogen, phosphorus, and potassium with occasional addition of supplements like rock flour for micronutrients. Farmers apply these fertilizers in a variety of ways: through dry or pelletized or liquid application processes, using large agricultural equipment, or hand-tool methods.
Historically, fertilization came from natural or organic sources: compost, animal manure, human manure, harvested minerals, crop rotations, and byproducts of human-nature industries. However, starting in the 19th century, after innovations in plant nutrition following Justus von Liebig's discoveries, an agricultural industry developed around synthetically created agrochemical fertilizers. This transition was important in transforming the global food system towards larger-scale industrial agriculture with large crop yields in monocultures.
The invention of Haber process for producing ammonia for nitrogen in the 20th century combined with amplified chemical production capacity created during World War II led to a boom in using nitrogen fertilizers. In the latter half of the 20th century, increased use of nitrogen fertilizers has been a crucial component of the increased productivity of conventional food systems as part of the so-called "Green Revolution".
Fertilizers, especially when applied excessively, can have environmental consequences such as water pollution and eutrophication due to nutrient runoff. Additionally, the chemical process for creating the fertilizers results in byproducts, including carbon and other emissions. In some cases contamination and pollution of soil result by accumulation of heavy metals contained in some fertilizers collected through mining. Sustainable agricultural practices, such as reduced tillage and planting buffer strips, can minimize these adverse environmental effects.
History
Management of soil fertility has preoccupied farmers since the beginning of agriculture. Middle Eastern, Chinese, Mesoamerican, and Cultures of the Central Andes were all early adopters of agriculture. This is thought to have led to their cultures growing faster in population which allowed an exportation of culture to neighboring hunter-gatherer groups. Fertilizer use along with agriculture allowed some of these early societies a critical advantage over their neighbors, leading them to become dominant cultures in their respective regions. Egyptians, Romans, Babylonians, and early Germans are all recorded as using minerals or manure to enhance the productivity of their farms. The scientific research of plant nutrition started well before the work of German chemist Justus von Liebig although his name is most mentioned as the "father of the fertilizer industry". Nicolas Théodore de Saussure and scientific colleagues at the time were quick to disprove the simplifications of von Liebig. Prominent scientists whom von Liebig drew were Carl Ludwig Sprenger and Hermann Hellriegel. In this field, a 'knowledge erosion' took place, partly driven by an intermingling of economics and research. John Bennet Lawes, an English entrepreneur, began experimenting on the effects of various manures on plants growing in pots in 1837, and a year or two later the experiments were extended to crops in the field. One immediate consequence was that in 1842 he patented a manure formed by treating phosphates with sulfuric acid, and thus was the first to create the artificial manure industry. In the succeeding year, he enlisted the services of Joseph Henry Gilbert; together they performed crop experiments at the Institute of Arable Crops Research.The Birkeland–Eyde process was one of the competing industrial processes at the beginning of nitrogen-based fertilizer production. This process was used to fix atmospheric nitrogen into nitric acid, one of several chemical processes called nitrogen fixation. The resultant nitric acid was then used as a source of nitrate. A factory based on the process was built in Rjukan and Notodden in Norway and large hydroelectric power facilities were built.
The 1910s and 1920s witnessed the rise of the Haber process and the Ostwald process. The Haber process produces ammonia from methane and molecular nitrogen from the air. The ammonia from the Haber process is then partially converted into nitric acid in the Ostwald process. It is estimated that a third of annual global food production uses ammonia from the Haber–Bosch process and that this supports nearly half the world's population. After World War II, nitrogen production plants that had ramped up for wartime bomb manufacturing were pivoted towards agricultural uses. The use of synthetic nitrogen fertilizers has increased steadily over the last 50 years of the 20th century, rising almost 20-fold to the current rate of 100 million tonnes of nitrogen per year in 2003.
The development of synthetic nitrogen fertilizers has significantly supported global population growth. It has been estimated that almost half the people on the Earth are currently fed due to synthetic nitrogen fertilizer use. The use of phosphate fertilizers has also increased from 9 million tonnes per year in 1960 to 40 million tonnes per year in 2000, but future phosphorus fertilizer availability is now a critical issue.
Agricultural use of inorganic fertilizers in 2021 was 195 million tonnes of nutrients, of which 56% was nitrogen. Asia represented 53% of the world's total agricultural use of inorganic fertilizers in 2021, followed by the Americas, Europe, Africa and Oceania. This ranking of the regions is the same for all nutrients. The main users of inorganic fertilizers are, in descending order, China, India, Brazil, and the United States of America, with China the largest user of each nutrient.
A maize crop yielding 6–9 tonnes of grain per hectare requires of phosphate fertilizer to be applied; soybean crops require about half, 20–25 kg per hectare. Yara International is the world's largest producer of nitrogen-based fertilizers.
Mechanism
Fertilizers enhance the growth of plants. This goal is met in two ways, the traditional one being additives that provide nutrients. The second mode by which some fertilizers act is to enhance the effectiveness of the soil by modifying its water-holding capacity and aeration. This article, like many on fertilizers, emphasizes the nutritional aspect.Fertilizers typically provide, in varying proportions:
- Three main macronutrients :
- * Nitrogen : leaf growth and stems;
- * Phosphorus : development of roots, flowers, seeds and fruit;
- * Potassium : strong stem growth, movement of water in plants, promotion of flowering and fruiting;
- Three secondary macronutrients: calcium, magnesium, and sulfur ;
- Micronutrients: copper, iron, manganese, molybdenum, zinc, and boron. Of occasional significance are silicon, cobalt, and vanadium.
The nutrients required for healthy plant life are classified according to the elements, but the elements are not used as fertilizers. Instead, compounds containing these elements are the basis of fertilizers. Macro-nutrients are present in plant tissue in quantities from 0.15% to 6.0% on a dry matter basis but they are sometimes consumed in larger quantities than required, in particular when fertilizers are used in excess of plant requirements. Plants are made up of four main elements: hydrogen, oxygen, carbon, and nitrogen. Carbon, hydrogen, and oxygen are widely available respectively in carbon dioxide and in water. Although nitrogen makes up most of the atmosphere, it is in a form that is unavailable to plants. Nitrogen is the most important fertilizer since nitrogen is present in proteins, DNA, and other components, but is absent from the parent rock and thus cannot be obtained from mineral weathering. To be nutritious to plants, nitrogen must be made available in a "fixed" form. Only some free-living and symbiotic bacteria living in root systems of host plants can fix atmospheric nitrogen by converting it to ammonia. Phosphate is required for the production of DNA and ATP, the main energy carrier in cells, as well as certain lipids.
Microbiological considerations
Two sets of enzymatic reactions are highly relevant to the efficiency of nitrogen-based fertilizers.;Urease
The first is the hydrolysis of urea. Many soil bacteria possess the enzyme urease, which catalyzes the conversion of urea to ammonium ion and bicarbonate ion.
;Ammonia oxidation
Ammonia-oxidizing bacteria, such as species of Nitrosomonas, oxidize ammonia to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria, especially Nitrobacter, oxidize nitrite to nitrate, which is extremely soluble and mobile, being leached easily to groundwater then to rivers then to the sea, and is a major cause of eutrophication and algal blooms in rivers, lakes and sea shores.