Fertilizer is a substance added to soil to improve plants' growth and yield. First used by ancient farmers, fertilizer technology developed significantly as the chemical needs of growing plants were discovered. Modern synthetic fertilizers are composed mainly of nitrogen, phosphorous, and potassium compounds with secondary nutrients added. The use of synthetic fertilizers has significantly improved the quality and quantity of the food available today, although their long-term use is debated by environmentalists.
Like all living organisms, plants are made up of cells. Within these cells occur numerous metabolic chemical reactions that are responsible for growth and reproduction. Since plants do not eat food like animals, they depend on nutrients in the soil to provide the basic chemicals for these metabolic reactions. The supply of these components in soil is limited, however, and as plants are harvested, it dwindles, causing a reduction in the quality and yield of plants.
Fertilizers replace the chemical components that are taken from the soil by growing plants. However, they are also designed to improve the growing potential of soil, and fertilizers can create a better growing environment than natural soil. They can also be tailored to suit the type of crop that is being grown. Typically, fertilizers are composed of nitrogen, phosphorus, and potassium compounds. They also contain trace elements that improve the growth of plants.
The primary components in fertilizers are nutrients which are vital for plant growth. Plants use nitrogen in the synthesis of proteins, nucleic acids, and hormones. When plants are nitrogen deficient, they are marked by reduced growth and yellowing of leaves. Plants also need phosphorus, a component of nucleic acids, phospholipids, and several proteins. It is also necessary to provide the energy to drive metabolic chemical reactions. Without enough phosphorus, plant growth is reduced. Potassium is another major substance that plants get from the soil. It is used in protein synthesis and other key plant processes. Yellowing, spots of dead tissue, and weak stems and roots are all indicative of plants that lack enough potassium.
Calcium, magnesium, and sulfur are also important materials in plant growth. They are only included in fertilizers in small amounts, however, since most soils naturally contain enough of these components. Other materials are needed in relatively small amounts for plant growth. These micronutrients include iron, chlorine, copper, manganese, zinc, molybdenum, and boron, which primarily function as cofactors in enzymatic reactions. While they may be present in small amounts, these compounds are no less important to growth, and without them plants can die.
Many different substances are used to provide the essential nutrients needed for an effective fertilizer. These compounds can be mined or isolated from naturally occurring sources. Examples include sodium nitrate, seaweed, bones, guano, potash, and phosphate rock. Compounds can also be chemically synthesized from basic raw materials. These would include such things as ammonia, urea, nitric acid, and ammonium phosphate. Since these compounds exist in a number of physical states, fertilizers can be sold as solids, liquids, or slurries.
The process of adding substances to soil to improve its growing capacity was developed in the early days of agriculture. Ancient farmers knew that the first yields on a plot of land were much better than those of subsequent years. This caused them to move to new, uncultivated areas, which again showed the same pattern of reduced yields over time. Eventually it was discovered that plant growth on a plot of land could be improved by spreading animal manure throughout the soil.
Over time, fertilizer technology became more refined. New substances that improved the growth of plants were discovered. The Egyptians are known to have added ashes from burned weeds to soil. Ancient Greek and Roman writings indicate that various animal excrements were used, depending on the type of soil or plant grown. It was also known by this time that growing leguminous plants on plots prior to growing wheat was beneficial. Other types of materials added include sea-shells, clay, vegetable waste, waste from different manufacturing processes, and other assorted trash.
Organized research into fertilizer technology began in the early seventeenth century. Early scientists such as Francis Bacon and Johann Glauber describe the beneficial effects of the addition of saltpeter to soil. Glauber developed the first complete mineral fertilizer, which was a mixture of saltpeter, lime, phosphoric acid, nitrogen, and potash. As scientific chemical theories developed, the chemical needs of plants were discovered, which led to improved fertilizer compositions. Organic chemist Justus von Liebig demonstrated that plants need mineral elements such as nitrogen and phosphorous in order to grow. The chemical fertilizer industry could be said to have its beginnings with a patent issued to Sir John Lawes, which outlined a method for producing a form of phosphate that was an effective fertilizer. The synthetic fertilizer industry experienced significant growth after the First World War, when facilities that had produced ammonia and synthetic nitrates for explosives were converted to the production of nitrogen-based fertilizers.
The fertilizers outlined here are compound fertilizers composed of primary fertilizers and secondary nutrients. These represent only one type of fertilizer, and other single nutrient types are also made. The raw materials, in solid form, can be supplied to fertilizer manufacturers in bulk quantities of thousands of tons, drum quantities, or in metal drums and bag containers.
Primary fertilizers include substances derived from nitrogen, phosphorus, and potassium. Various raw materials are used to produce these compounds. When ammonia is used as the nitrogen source in a fertilizer, one method of synthetic production requires the use of natural gas and air. The phosphorus component is made using sulfur, coal, and phosphate rock. The potassium source comes from potassium chloride, a primary component of potash.
Secondary nutrients are added to some fertilizers to help make them more effective. Calcium is obtained from limestone, which contains calcium carbonate, calcium sulphate, and calcium magnesium carbonate. The magnesium source in fertilizers is derived from dolomite. Sulfur is another material that is mined and added to fertilizers. Other mined materials include iron from ferrous sulfate, copper, and molybdenum from molybdenum oxide.
Fully integrated factories have been designed to produce compound fertilizers. Depending on the actual composition of the end product, the production process will differ from manufacturer to manufacturer.
Nitrogen fertilizer component
- 1 Ammonia is one nitrogen fertilizer component that can be synthesized from in-expensive raw materials. Since nitrogen makes up a significant portion of the earth's atmosphere, a process was developed to produce ammonia from air. In this process, natural gas and steam are pumped into a large vessel. Next, air is pumped into the system, and oxygen is removed by the burning of natural gas and steam. This leaves primarily nitrogen, hydrogen, and carbon dioxide. The carbon dioxide is removed and ammonia is produced by introducing an electric current into the system. Catalysts such as magnetite (Fe3O4) have been used to improve the speed and efficiency of ammonia synthesis. Any impurities are removed from the ammonia, and it is stored in tanks until it is further processed.
- 2 While ammonia itself is sometimes used as a fertilizer, it is often converted to other substances for ease of handling. Nitric acid is produced by first mixing ammonia and air in a tank. In the presence of a catalyst, a reaction occurs which converts the ammonia to nitric oxide. The nitric oxide is further reacted in the presence of water to produce nitric acid.
3 Nitric acid and ammonia are used to make ammonium nitrate. This material is a good fertilizer component because it has a high concentration of nitrogen. The two materials are mixed together in a tank and a neutralization reaction occurs, producing ammonium nitrate. This material can then be stored until it is ready to be granulated and blended with the other fertilizer components.
Phosphorous fertilizer component
- 4 To isolate phosphorus from phosphate rock, it is treated with sulfuric acid, producing phosphoric acid. Some of this material is reacted further with sulfuric acid and nitric acid to produce a triple superphosphate, an excellent source of phosphorous in solid form.
- 5 Some of the phosphoric acid is also reacted with ammonia in a separate tank. This reaction results in ammonium phosphate, another good primary fertilizer.
Potassium fertilizer component
- 6 Potassium chloride is typically supplied to fertilizer manufacturers in bulk. The manufacturer converts it into a more usable form by granulating it. This makes it easier to mix with other fertilizer components in the next step.
Granulating and blending
- 7 To produce fertilizer in the most usable form, each of the different compounds, ammonium nitrate, potassium chloride, ammonium phosphate, and triple superphosphate are granulated and blended together. One method of granulation involves putting the solid materials into a rotating drum which has an inclined axis. As the drum rotates, pieces of the solid fertilizer take on small spherical shapes. They are passed through a screen that separates out adequately sized particles. A coating of inert dust is then applied to the particles, keeping each one discrete and inhibiting moisture retention. Finally, the particles are dried, completing the granulation process.
- 8 The different types of particles are blended together in appropriate proportions to produce a composite fertilizer. The blending is done in a large mixing drum that rotates a specific number of turns to produce the best mixture possible. After mixing, the fertilizer is emptied onto a conveyor belt, which transports it to the bagging machine.
- 9 Fertilizers are typically supplied to farmers in large bags. To fill these bags the fertilizer is first delivered into a large hopper. An appropriate amount is released from the hopper into a bag that is held open by a clamping device. The bag is on a vibrating surface, which allows better packing. When filling is complete, the bag is transported upright to a machine that seals it closed. The bag is then conveyored to a palletizer, which stacks multiple bags, readying them for shipment to distributors and eventually to farmers.
To ensure the quality of the fertilizer that is produced, manufacturers monitor the product at each stage of production. The raw materials and the finished products are all subjected to a battery of physical and chemical tests to show that they meet the specifications previously developed. Some of the characteristics that are tested include pH, appearance, density, and melting point. Since fertilizer production is governmentally regulated, composition analysis tests are run on samples to determine total nitrogen content, phosphate content, and other elements affecting the chemical composition. Various other tests are also performed, depending on the specific nature of the fertilizer composition.
A relatively small amount of the nitrogen contained in fertilizers applied to the soil is actually assimilated into the plants. Much is washed into surrounding bodies of water or filters into the groundwater. This has added significant amounts of nitrates to the water that is consumed by the public. Some medical studies have suggested that certain disorders of the urinary and kidney systems are a result of excessive nitrates in drinking water. It is also thought that this is particularly harmful for babies and could even be potentially carcinogenic.
The nitrates that are contained in fertilizers are not thought to be harmful in themselves. However, certain bacteria in the soil convert nitrates into nitrite ions. Research has shown that when nitrite ions are ingested, they can get into the bloodstream. There, they bond with hemoglobin, a protein that is responsible for storing oxygen. When a nitrite ion binds with hemoglobin, it loses its ability to store oxygen, resulting in serious health problems.
Nitrosamines are another potential byproduct of the nitrates in fertilizer. They are the result of a natural chemical reaction of nitrates. Nitrosamines have been shown to cause tumors in laboratory animals, feeding the fear that the same could happen in humans. There has, however, been no study that shows a link between fertilizer use and human tumors.
Fertilizer research is currently focusing on reducing the harnful environmental impacts of fertilizer use and finding new, less expensive sources of fertilizers. Such things that are being investigated to make fertilizers more environmentally friendly are improved methods of application, supplying fertilizer in a form which is less susceptible to runoff, and making more concentrated mixtures. New sources of fertilizers are also being investigated. It has been found that sewage sludge contains many of the nutrients that are needed for a good fertilizer. Unfortunately, it also contains certain substances such as lead, cadmium, and mercury in concentrations which would be harmful to plants. Efforts are underway to remove the unwanted elements, making this material a viable fertilizer. Another source that is being developed is manures. The first fertilizers were manures, however, they are not utilized on a large scale because their handling has proven too expensive. When technology improves and costs are reduced, this material will be a viable new fertilizer.
Where to Learn More
Rao, N. S. Biofertilizers in Agriculture & Forestry. IBH, 1993.
Stocchi, E. Industrial Chemistry. Ellis Horwood, 1990.
Lowrison, George. Fertilizer Technology. John Wiley and Sons, 1989.
Kirschner, Elisabeth. "Fertilizer Makers Gear up to Grow." Chemical & Engineering News, March, 31 1997, p. 13-15.
A fertilizer is a plant nutrient added to a soil to increase its yield. Plants need nutrients to grow and produce fruits and vegetables. Two categories of nutrients have been identified in fertilization: macronutrients and micronutrients. There are only six macronutrients and they are required in large amounts by plants: nitrogen, phosphorus, potassium, sulfur, magnesium, and calcium. However, a larger number of micronutrients are required but in trace amounts: iron, manganese, boron, zinc, copper, molybdenum, chlorine, cobalt, nickel, sodium, and silicon. Eliminate any of these elements, and plants will display abnormal growth and deficiency, or they may not reproduce.
The most popular fertilizers contain the three major nutrients: nitrogen, phosphorus, and potassium, and they are therefore referred to as NPK fertilizers. To illustrate their importance in any economy, in 2000, the world consumption of the total fertilizer nutrient (N + P2O5 + K2O) was 140
million tons, representing 52 million tons for developed countries and 88 million tons for developing countries.
Nitrogen forms part of proteins, hormones, chlorophyll , vitamins , and enzymes, and promotes stem and leaf growth. Too much nitrogen can delay fruiting, while a deficiency of it can reduce yields and induce yellowing of leaves and stunted growth. Nitrogen fertilizers are applied in organic and/or inorganic forms. Organic nitrogen fertilizers are farmyard manure, guano (excreta and remains of seabirds), dried blood, hoof, and horn. However, organic nitrogen sources must undergo microbial processes that produce nitrate nitrogen.
Inorganic nitrogen sources are directly available to plants and include the following: sodium nitrate, calcium nitrate, ammonium sulfate, ammonium nitrate, urea, calcium cyanamide, and ammonia. In addition, atmospheric nitrogen may be used as a source of plant nitrogen by the process called "nitrogen fixation." Legumes and a few other plants, in association with cyanobacteria (microscopic aquatic bacteria, for example, Anabaena azollae ), convert nitrogen to biologically useful ammonia. This process occurs in small growths on the roots called "nodules." Ammonia is subsequently available for many biological molecules, such as amino acids, proteins, vitamins, and nucleic acids.
Phosphorus plays an important role in seed germination , photosynthesis , protein formation, overall growth and metabolism , and flower and fruit formation. Phosphorus deficiency induces purple stems and leaves, poor flowering and fruiting. Low soil pH (<4) ties up phosphates by favoring the formation of insoluble aluminum and iron phosphates. Phosphorus fertilizers come from different sources: bones, rock phosphate, superphosphate (a mixture of calcium dihydrogen phosphate and calcium sulfate), nitrophosphate, ammonium phosphate, basic slag (by-product in steel manufacture), etc.
Potassium contributes to the formation of sugars, carbohydrates, proteins and to cell division; adjusts water balance; enhances the flavor, color, and oil content of fruits; and is very important for leafy crops. Potassium deficiency produces a spotted, curled, or burned appearance to leaves and lowers crop yields. Potassium fertilizers are applied in the following forms: potassium chloride, potassium sulfate, potassium nitrate, and wood ash.
Other macronutrients are supplied as part of NPK fertilizers. First, sulfur is available from the sulfate of fertilizers. It contributes to the formation of amino acids, proteins, and enzymes, and is essential to chlorophyll. It also affects flavor in many vegetables. Second, magnesium is naturally present in the soil and is generally associated with potassium sulfate and calcium phosphate, used as NPK. It is a critical part of chlorophyll, and contributes to the functioning of enzymes for carbohydrates, fruit and nut formation, and the germination of seeds. Magnesium deficiency induces yellowing between the veins of older leaves, and leaves droop (hang down) as a result. Finally, calcium is also present in the soil and is available from calcium phosphate and nitrate, and lime. It activates enzymes, contributes to the structural part of cell walls, and influences water movement, cell growth, and division.
Micronutrients are not specifically applied to soil since they are naturally found in soils. However, there are some extreme cases where they must be supplied. For example, animal disorders have been linked to a lack of trace amounts of elements, not necessary for plant growth but essential for some species of animals. In some parts of Great Britain, for example, sheep and cattle suffered from "pining disease" that resulted in severe weight loss and general debilitation. The disease was found to result from a shortage of cobalt in the herbage. It has also been established that selenium deficiencies in some soils cause muscular dystrophy , while selenium excesses induce selenium toxicity in livestock.
see also Calcium; Carbohydrates; Enzymes; Magnesium; Nitrogen; Nucleic Acids; Phosphorus; Potassium; Proteins; Silicon; Sulfur.
Green Air Products. "Wonder of Plants." Available from <http://www.greenair.com/plantlnk.htm>.
International Fertilizer Industry Association. "Total Fertilizer Consumption Statistics by Region from 1970/71 to 2000/01." Available from <http://www.fertilizer.org/ifa/statistics.asp>.
Any substance that is applied to land to encourage plant growth and produce higher crop yield. Fertilizers may be made from organic material—such as recycled waste, animal manure, compost, etc.—or chemically manufactured. Most fertilizers contain varying amounts of nitrogen , phosphorus , and potassium, inorganic nutrients that plants need to grow.
Since the 1950s crop production worldwide has increased dramatically because of the use of fertilizers. In combination with the use of pesticides and insecticides, fertilizers have vastly improved the quality and yield of such crops as corn, rice, wheat, and cotton. However overuse and improper use of fertilizers have also damaged the environment and affected the health of humans, animals, and plants.
In the United States, it is estimated that as much as 25% of fertilizer is carried away as runoff . Fertilizer runoff has contaminated groundwater and polluted bodies of water near and around farmlands. High and unsafe nitrate concentrations in drinking water have been reported in countries that practice intense farming, including the United States. Accumulation of nitrogen and phosphorus in waterways from chemical fertilizers has also contributed to the eutrophication of lakes and ponds. Ammonia, released from the decay of fertilizers, causes minor irritation to the respiratory system.
While very few advocate the complete eradication of chemical fertilizers, many environmentalists and scientists urge more efficient ways of using them. For example, some farmers use up to 40% more fertilizer than they need. Frugal applications—in small doses and on an as-needed-basis on specific crops—helps reduce fertilizer waste and runoff. The use of organic fertilizers, including animal waste , crop residues, or grass clippings, is also encouraged as an alternative to chemical fertilizers.
Adding nutrients to agricultural systems is essential to enhance crop yield, crop quality, and economic returns. Commercial fertilizers are typically used to supply needed nutrients to crops. Nitrogen (N), phosphorus (P), and potassium (K) fertilizers are used extensively. Other secondary and micronutrient fertilizers are generally required in small quantities to correct plant nutrient deficiencies.
Commercial fertilizers contain a guaranteed quantity of nutrients, expressed as fertilizer grade on a label showing the weight percentage of available N, P2 O5, and K2 O equivalent (N-P-K) in the fertilizer. Additional nutrients in fertilizer formulations are listed at the end of the fertilizer grade with the nutrient identified. Commonly used commercial fertilizers include ammonium nitrate (fertilizer grade 33-0-0), urea (45-0-0), urea-ammonium nitrate (28-0-0), anhydrous ammonia (82-0-0), diammonium phosphate (18-46-0), monoammonium phosphate (10-52-0), ammonium polyphosphate (10-34-0), ammonium thiosulfate (12-0-0-26S), potassium chloride (0-0-60-45Cl), potassium sulfate (0-0-50-18S), and potassium-magnesium sulfate (0-0-22-22S-11Mg). The secondary plant nutrients sulfur (S) and magnesium (Mg) are often contained in the nitrogen, phosphorus, and potassium fertilizers as shown.
Fertilizers are available in several forms (solids, fluids, and gases), which makes their handling and precise application very compatible with planting and fertilizer application equipment. Fertilizers are applied in several ways; they can be broadcast over the soil surface or in narrow bands on or in the soil, as foliar applications to plants, or through irrigation systems. For more efficient use, fertilizer should normally be applied just prior to the time of greatest plant nutrient uptake. In contrast, organic sources, such as animal manures, need to be applied and incorporated into the soil prior to planting the crop to be most effective.
Management of crop nutrient requirements is easier with commercial fertilizers than with organic fertilizers such as animal manures, bio-solids, byproducts, and other organic waste products. Release of many of the plant nutrients from these sources requires the breakdown of organic material by soil microbes and release of plant nutrients through a process called mineralization. Many of the nutrients from organic sources are not available to plants until this process has occurred. Release of plant nutrients from organic sources may not correspond with the period of greatest crop need.
Organic fertilizers and legumes are good sources of nutrients for crop production. Balancing the quantity of nutrient application with organic sources to match crop need is more difficult than with commercial fertilizers. Application of sufficient animal manure to meet crop nitrogen needs will likely result in an overapplication of phosphorus. Conversely, application of sufficient manure to meet the phosphorus needs of crops could result in the under application of nitrogen. Nutrient content of most organic sources is highly variable and needs to be determined before application to soils to avoid overapplication of some nutrients.
Balancing crop nutrient needs using both inorganic commercial fertilizer and organic sources is an excellent way to avoid overapplication of plant nutrients. Soil and/or plant tissue testing should be used to determine crop nutrient needs before applying nutrients from any source. This will ensure efficient use of plant nutrients while maintaining high crop yields, crop quality and profitability, and preserving or enhancing environmental quality.
see also Agriculture, Modern; Biochemical Cycles; Compost; Nutrients; Organic Agriculture; Soil, Chemistry of; Soil, Physical Characteristics of.
Ardell D. Halvorson
California Fertilizer Association. Soil Improvement Committee. Western Fertilizer Handbook. Danville, IL: Interstate Publishers, 1995.
United Nations Industrial Development Organization. Fertilizer Manual. Norwell, MA: Kluwer Academic, 1998.