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Fertilizer (or fertiliser) is any organic or inorganic material of natural or synthetic origin (other than liming materials) that is applied to soils or to plant tissues (usually leaves) to supply one or more plant nutrients essential to the growth of plants. Conservative estimates report 30 to 50% of crop yields are attributed to natural or synthetic commercial fertilizer. Global market value is likely to rise to more than US$185 billion until 2019. The European fertilizer market will grow to earn revenues of approx. €15.3 billion in 2018.

Fertilizers typically provide, in varying proportions:

• three main macronutrients: nitrogen (N), phosphorus (P), potassium (K);
• three secondary macronutrients: calcium (Ca), magnesium (Mg), and sulfur (S);
• eight micronutrients: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn) and nickel (Ni) (1987).

The macronutrients are consumed in larger quantities and are present in plant tissue in quantities from 0.15% to 6.0% on a dry matter (DM) (0% moisture) basis. Micronutrients are consumed in smaller quantities and are present in plant tissue on the order of parts per million (ppm), ranging from 0.15 to 400 ppm DM, or less than 0.04% DM.

Only three other structural elements are required by all plants: carbon, hydrogen, and oxygen. These nutrients are supplied by water (through rainfall or irrigation) and carbon dioxide in the atmosphere.

Classification and types

Fertilizers are classified in many ways. They are classified according to whether they provide a single nutrient (say, N, P, or K), in which case they are classified as "straight fertilizers." "Multinutrient fertilizers" provide two or more nutrients (N and P, for example). Fertilizers are also classified according to whether they are solid or liquid. About 90% of fertilizers are applied as solids. Liquid fertilizers comprise anhydrous ammonia, aqueous solutions of ammonia, aqueous solutions of ammonium nitrate and or urea.

Single nutrient ("straight") fertizers: phosphorus

Single superphosphate consists of 14–18% P₂O₅ in the form of Ca(H₂PO₄)₂, but also and calcium sulfate (CaSO₄). Triple (or concentrated) superphosphate (TSP) typically consists of 44-48% of P₂O and no gypsum. A mixture of single superphosphate and triple superphosphate is called double superphosphate. More than 90% of a typical superphosphate fertilizer is water-soluble.

Multinutrient fertilizers

These fertilizers are the most common. The consist of more than two or more nutrient components. Major two-component fertilizers provide both nitrogen and phosphorus to the plants. These are called NP fertilizers. The main NP fertilizer are monoammonium phosphate and diammonium phosphate, abbreviated MAP and DAP, respectively. The active ingredient in MAP is NH4H2PO4. The active ingredient is DAP is (NH4)2HPO4. About 85% of MAP and DAP fertilizers are soluble in water. NPK fertilizers are three-component fertilizers. They provide sources of potassium as well as nitrogen and phosphorus. Such NPK fertilizers are 70-75% soluble in water.

Labeling and rating of chemical fertilizer

In Africa, the Americas, Asia, Europe, a popular labeling scheme consisting of three numbers separated by dashes (e.g. 10-10-10 or 16-4-8) is employed to describe the chemical content of fertilizers. The first number represents the percentage of nitrogen in the product; the second number, P₂O₅ consists of 56.4% oxygen and 43.6% elemental phosphorus; the third, K₂O consists of 17% oxygen and 83% elemental potassium. The generalized form is N-P-K. A 50-pound bag of fertilizer labeled 16-4-8 contains 8 pounds of nitrogen (16% of the 50 pounds), 2 pounds of P₂O₅ (4% of 50 pounds), and 4 pounds of K₂O (8% of 50 pounds). Australian convention adds a fourth number for Sulphur.

Although available in various forms, fertilizer is typical solid as granulated or powdered forms. The next most common form is liquid fertilizer; some advantages of liquid fertilizer are its more rapid effect and easier coverage. Slow-release fertilizers (various forms including fertilizer spikes, tabs, etc.) which reduce the problem of "burning" the plants due to excess nitrogen. Polymer coating of fertilizer ingredients gives tablets and spikes a 'true time-release' or 'staged nutrient release' (SNR) of fertilizer nutrients. Organic fertilizer usually contain less nutrients, but offer other advantages as well as appealling to environmentally friendly users.

Production

Nitrogen fertilizers are made using the Haber-Bosch process, which uses natural gas (CH₄) for the hydrogen and nitrogen gas (N₂) from the air at an elevated temperature and pressure in the presence of a catalyst to form ammonia (NH₃). This ammonia is used as a feedstock for other nitrogen fertilizers, such as anhydrous ammonium nitrate (NH₄NO₃) and urea (CO(NH₂)₂). These concentrated products may be diluted with water to form a concentrated liquid fertilizer (e.g. UAN). Deposits of sodium nitrate (NaNO₃) (Chilean saltpeter) are also found the Atacama desert in Chile and was one of the original (1830) nitrogen rich inorganic fertilizers used. It is still mined for fertilizer.

In the nitrophosphate process or Odda process (invented in 1927), phosphate rock with up to a 20% phosphorus (P) content is dissolved with nitric acid (HNO₃) to produce a mixture of phosphoric acid (H₃PO₄) and calcium nitrate (Ca(NO₃)₂). This can be combined with a potassium fertilizer to produce a compound fertilizer with the three macronutrients N, P and K in easily dissolved form.

Phosphate rock can also be processed into water-soluble phosphate (P₂O₅) with the addition of sulfuric acid (H₂SO₄) to make the phosphoric acid in phosphate fertilizers. Phosphate can also be reduced in an electric furnace to make high purity phosphorus; however, this is more expensive than the acid process.

Potash can be used to make potassium (K) fertilizers. All commercial potash deposits come originally from marine deposits and are often buried deep in the earth. Potash ores are typically rich in potassium chloride (KCl) and sodium chloride (NaCl) and are obtained by conventional shaft mining with the extracted ore ground into a powder. For deep potash deposits hot water is injected into the potash which is dissolved and then pumped to the surface where it is concentrated by solar induced evaporation. Amine reagents are then added to either the mined or evaporated solutions. The amine coats the KCl but not NaCl. Air bubbles cling to the amine + KCl and float it to the surface while the NaCl and clay sink to the bottom. The surface is skimmed for the amine + KCl which is then dried and packaged for use as a K rich fertilizer—KCl dissolves readily in water and is available quickly for plant nutrition.

Compound fertilizers often combine N, P and K fertilizers into easily dissolved pellets. The N:P:K ratios quoted on fertilizers give the weight percent of the fertilizer in nitrogen (N), phosphate (P₂O₅) and potash (K₂O equivalent)

Controlled-release types

Slow- and controlled-release Involve only 0.15% (562 000 tons) of the fertilizer market (as of 1995). Their utility recognizes that fertilizers are subject to antagonistic processes. In addition to their providing the nutrition to plants, excess fertilizers can be poisonous to the same plant. Competitive with the uptake by plants is the degradation or loss of the fertilizer. Microbes degrade many fertilizers, e.g. by immobilization or oxidation. Furthermore fertilizers are lost by evaporation or leaching. Most slow-release fertilizers are derivatives of urea, a straight fertilizer providing nitrogen. Isobutylidenediurea IBDU) and urea-formaldehyde slowly convert in the soil to free urea, which is rapidly uptaken by plants. IBDU has the formula (CH₃)₂CHCH(NHC(O)NH₂)₂ and the urea-formaldehydes consist of mixtures of the approximate formula (HOCH₂NHC(O)NH)_nCH₂. Other coated products use thermoplastics (and sometimes ethylene-vinyl acetate and surfactants, etc.) to produce diffusion-controlled release of urea or soluble inorganic fertilizers. "Reactive Layer Coating" can produce thinner, hence cheaper, membrane coatings by applying reactive monomers simultaneously to the soluble particles. "Multicote" is a process applying layers of low-cost fatty acid salts with a paraffin topcoat.

Besides being more efficient in the utilization of the applied nutrients, slow-release technologies also reduce the impact on the environment and the contamination of the subsurface water.

Application

Synthetic fertilizers are commonly used for growing all crops, with application rates depending on the soil fertility, usually as measured by a soil test and according to the particular crop. Legumes, for example, fix nitrogen from the atmosphere and generally do not require nitrogen fertilizer.

Studies have shown that application of nitrogen fertilizer on off-season cover crops can increase the biomass (and subsequent green manure value) of these crops, while having a beneficial effect on soil nitrogen levels for the main crop planted during the summer season.

Nutrients in soil can be thrown out of balance with high concentrations of fertilizers. The interconnectedness and complexity of this soil ‘food web’ means any appraisal of soil function must necessarily take into account interactions with the living communities that exist within the soil. Stability of the system is reduced by the use of nitrogen-containing fertilizers, which cause soil acidification.

Applying excessive amounts of fertilizer has negative environmental effects, and is expensive. By assessing the physical symptoms of the crop, the appropriate level of fertilization can be determined. Nitrogen deficiency, for example has a distinctive presentation in some species. However, quantitative tests are more reliable for detecting nutrient deficiency before it has significantly affected the crop. Both soil tests and Plant Tissue Tests are used in agriculture to fine-tune nutrient management to the crops needs.

Organic fertilizer

The main "organic fertilizers" are in ranked order: peat, animal wastes (often from slaugherhouses), plant wastes from agriculture, and sewage sludge.

Peat is the main source of organic fertilizer. This immature form of coal confers no nutritional value to the plants, but improves the soil by aeration and absorbing water. Animal sources include the products of the slaughter of animals. Bloodmeal, bone meal, hides, hoofs, and horns are typical components.

Environmental effects

Water

The nitrogen-rich compounds found in fertilizer runoff are the primary cause of serious oxygen depletion in many parts of oceans, especially in coastal zones, lakes and rivers. The resulting lack of dissolved oxygen is greatly reducing the ability of these areas to sustain oceanic fauna. Visually, water may become cloudy and discolored (green, yellow, brown, or red).

About half of all the lakes in the United States are now eutrophic, while the number of oceanic dead zones near inhabited coastlines are increasing. As of 2006, the application of nitrogen fertilizer is being increasingly controlled in northwestern Europe and the United States. If eutrophication can be reversed, it may take decades before the accumulated nitrates in groundwater can be broken down by natural processes.

Nitrate pollution

High application rates of nitrogen-containing fertilizers in order to maximize crop yields, combined with the high solubility of nitrate in soil water leads to increased runoff into surface water as well as leaching into groundwater. The use of excessive rates of nitrogen-containing fertilizers (whether they are commercial inorganic fertilizers or manures) is particularly damaging, as much of the nitrogen that is not taken up by plants is transformed into nitrate which is easily leached.

Nitrate levels above 10 mg/L (10 ppm) in groundwater can cause 'blue baby syndrome' (acquired methemoglobinemia), leading to hypoxia (which can lead to coma and death if not treated). The nutrients, especially nitrates, in fertilizers can cause problems for natural habitats and for human health if they are washed off soil into watercourses or leached through soil into groundwater. In Europe these problems are being addressed by the European Union's Nitrates Directive. Within Britain farmers are encouraged to manage their land more sustainably in 'catchment-sensitive farming'. In the US, high concentrations of nitrate and phosphorus in runoff and drainage water are classified as non-point source pollutants due to their diffuse origin; this pollution is regulated at state level.

Contamination with impurities

Phosphate rocks all contain hazardous elements such as fluorine, heavy metals and radioactive elements. Consequently, common agricultural grade phosphate fertilizers (which are derived from these phosphate rocks) usually contain impurities such as fluorides, cadmium and uranium, although concentrations of the latter two heavy metals are dependent on the source of the phosphate and the fertilizer production process. These potentially harmful impurities can be removed; however, this significantly increases cost. Highly pure fertilizers are widely available and perhaps best known as the highly water soluble fertilizers containing blue dyes used around households. These highly water soluble fertilizers are used in the plant nursery business and are available in larger packages at significantly less cost than retail quantities. There are also some inexpensive retail granular garden fertilizers made with high purity ingredients.

Oregon and Washington, both in the United States, have fertilizer registration programs with on-line databases listing chemical analyses of fertilizers.

The fluoride content of many widely used phosphate fertilizers has increased soil fluoride concentrations, prompting considerable research into the possibility that soil productivity and food quality may be compromised. It has been found that food contamination from fertilizer is of little concern as plants accumulate little fluoride from the soil; of greater concern is the possibility of fluoride toxicity to livestock that ingest contaminated soils. Also of possible concern are the effects of fluoride on soil microorganisms.

Soil

Also regular use of acidulated fertilizers generally contribute to the accumulation of soil acidity in soils which progressively increases aluminium availability and hence toxicity. The use of such acidulated fertilizers in the tropical and semi-tropical regions of Indonesia and Malaysia has contributed to soil degradation on a large scale from aluminium toxicity, which can only be countered by applications of limestone or preferably magnesian dolomite, which neutralises acid soil pH and also provides essential magnesium.

Nitrogen-containing inorganic and organic fertilizers can cause soil acidification when added. This may lead to decreases in nutrient availability which may be offset by liming.

High levels of fertilizer may cause the breakdown of the symbiotic relationships between plant roots and mycorrhizal fungi.

Trace mineral depletion

Scientific investigations have indicated a trend of decreasing concentrations of minerals (such as iron, zinc, copper and magnesium) in many foods over the last 50-60 years. Intensive farming practices, including the use of inorganic fertilizers are frequently suggested as reasons for these declines and organic farming is often suggested as a solution. Although improved crop yields resulting from inorganic NPK fertilizers are known to dilute the concentrations of other nutrients in plants, much of the measured decline can be attributed to the use of progressively higher-yielding crop varieties which produce foods with lower mineral concentrations than their less productive ancestors. It is, therefore, unlikely that organic farming or reduced use of inorganic fertilizers will solve the problem; foods with high nutrient density are more likely to be achieved using older, lower-yielding varieties or the development of new high-yield, nutrient-dense varieties.

Inorganic fertilizers are, in fact, more likely to solve trace mineral deficiency problems than cause them: In Western Australia deficiencies of zinc, copper, manganese, iron and molybdenum were identified as limiting the growth of broad-acre crops and pastures in the 1940s and 1950s. Soils in Western Australia are very old, highly weathered and deficient in many of the major nutrients and trace elements. Since this time these trace elements are routinely added to inorganic fertilizers used in agriculture in this state. Many other soils around the world are deficient in zinc, leading to deficiency in both plants and humans, and inorganic zinc fertilizers are widely used to solve this problem.

Overfertilization

Over-fertilization of a vital nutrient can be as detrimental as underfertilization. "Fertilizer burn" can occur when too much fertilizer is applied, resulting in drying out of the leaves and damage or even death of the plant.

Fertilizers vary in their tendency to burn roughly in accordance with their salt index.

Energy consumption and sustainability

In the USA in 2004, 317 billion cubic feet of natural gas were consumed in the industrial production of ammonia, less than 1.5% of total U.S. annual consumption of natural gas. A 2002 report suggested that the production of ammonia consumes about 5% of global natural gas consumption, which is somewhat under 2% of world energy production.

Ammonia is overwhelmingly produced from natural gas, but other energy sources, together with a hydrogen source such as water (via water splitting or electrolysis), can be used for the production of nitrogen compounds suitable for fertilizers. The cost of natural gas makes up about 90% of the cost of producing ammonia. The increase in price of natural gases over the past decade, along with other factors such as increasing demand, have contributed to an increase in fertilizer price.

Contribution to climate change

The greenhouse gases carbon dioxide, methane and nitrous oxide are produced during the manufacture of nitrogen fertilizer. The effects can be combined into an equivalent amount of carbon dioxide. The amount varies according to the efficiency of the process. The figure for the United Kingdom is over 2 kilogrammes of carbon dioxide equivalent for each kilogramme of ammonium nitrate. Nitrogen fertilizer can be converted by soil bacteria to nitrous oxide, a greenhouse gas.

Lack of long-term sustainability

Inorganic fertilizers are now produced in ways which theoretically cannot be continued indefinitely by definition as the resources used in their production are non-renewable. Potassium and phosphorus come from mines (or saline lakes such as the Dead Sea) and such resources are limited. However, more effective fertilizer utilization practices may decrease present usage from mines. Improved knowledge of crop production practices can potentially decrease fertilizer usage of P and K without reducing the critical need to improve and increase crop yields. Atmospheric (unfixed) nitrogen is effectively unlimited (forming over 70% of the atmospheric gases), but this is not in a form useful to plants. To make nitrogen accessible to plants requires nitrogen fixation (conversion of atmospheric nitrogen to a plant-accessible form).

Artificial nitrogen fertilizers are typically synthesized using fossil fuels such as natural gas and coal, which are limited resources. In lieu of converting natural gas to syngas for use in the Haber process, it is also possible to convert renewable biomass to syngas (or wood gas) to supply the necessary energy for the process, though the amount of land and resources (ironically often including fertilizer) necessary for such a project may be prohibitive.

Atmosphere

Methane emissions from crop fields (notably rice paddy fields) are increased by the application of ammonium-based fertilizers; these emissions contribute greatly to global climate change as methane is a potent greenhouse gas.

Through the increasing use of nitrogen fertilizer, which is was used at a rate of about 110 million tons (of N) per year in 2012 to the already existing amount of reactive nitrogen, nitrous oxide (N₂O) has become the third most important greenhouse gas after carbon dioxide and methane. It has a global warming potential 296 times larger than an equal mass of carbon dioxide and it also contributes to stratospheric ozone depletion.

The use of fertilizers on a global scale emits significant quantities of greenhouse gas into the atmosphere. Emissions come about through the use of:

• animal manures and urea, which release methane, nitrous oxide, and carbon dioxide in varying quantities depending on their form (solid or liquid) and management (collection, storage, spreading)
• fertilizers that use nitric acid or ammonium bicarbonate, the production and application of which results in emissions of nitrogen oxides, nitrous oxide and carbon dioxide into the atmosphere.

By changing processes and procedures, it is possible to mitigate some, but not all, of these effects on anthropogenic climate change.

Other

Persistent organic pollutants

Toxic persistent organic pollutants ("POPs"), such as Dioxins, polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) have been detected in agricultural fertilizers and soil amendments as documented in a 1999 EPA report. However, that report stated that "generally, the potential risk from certain liming agents and micronutrient fertilizers can be attributed to a single product sample with a single high constituent concentration that far exceeds contaminant levels found in other similar fertilizer products. With these few exceptions, the contaminant levels found in the fertilizer products analyzed for this report are not expected to cause risks of concern, either through contamination of food products or through incidental ingestion of either the fertilizer product or of soil amended with fertilizer."

Heavy metal accumulation

The concentration of cadmium in phosphorus-containing fertilizers varies considerably; for example, mono-ammonium phosphate fertilizer may have a cadmium content of as low as 0.14 mg/kg or as high as 50.9 mg/kg. This is because the phosphate rock used in their manufacture can contain as much as 188 mg/kg cadmium (examples are deposits on Nauru and the Christmas islands). Continuous use of high-cadmium fertilizer can contaminate soil (as shown in New Zealand) and plants. A proposal to limit the cadmium content of phosphate fertilizers is being considered by the European Commission.

Steel industry wastes, recycled into fertilizers for their high levels of zinc (essential to plant growth), wastes can include the following toxic metals: lead arsenic, cadmium, chromium, and nickel. The most common toxic elements in this type of fertilizer are mercury, lead, and arsenic.

Radioactive element accumulation

The radioactive content of the fertilizers varies considerably and depends both on their concentrations in the parent mineral and on the fertilizer production process. Uranium-238 concentrations range can range from 7 to 100 pCi/g in phosphate rock and from 1 to 67 pCi/g in phosphate fertilizers. Where high annual rates of phosphorus fertilizer are used, this can result in uranium-238 concentrations in soils and drainange waters that are several times greater than are normally present. However, the impact of these increases on the risk to human health from radinuclide contamination of foods is very small (less than 0.05 mSv/y).

Increased pest fitness

Excessive nitrogen fertilizer applications can also lead to pest problems by increasing the birth rate, longevity and overall fitness of certain agricultural pests, such as aphids (plant lice).

History

Management of soil fertility has been the pre-occupation of farmers for thousands of years. The start of the modern science of plant nutrition dates to the 19th century and the work of German chemist Justus von Liebig, among others.

John Bennet Lawes, an English entrepreneur, began to experiment 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 sulphuric acid, and thus was the first to create the artificial manure industry. In the succeeding year he enlisted the services of Joseph Henry Gilbert, with whom he carried on for more than half a century on experiments in raising crops at the Institute of Arable Crops Research.

The Birkeland–Eyde process was one of the competing industrial processes in the beginning of nitrogen based fertilizer production. It was developed by Norwegian industrialist and scientist Kristian Birkeland along with his business partner Sam Eyde in 1903, based on a method used by Henry Cavendish in 1784. This process was used to fix atmospheric nitrogen (N₂) into nitric acid (HNO₃), one of several chemical processes generally referred to as nitrogen fixation. The resultant nitric acid was then used as a source of nitrate (NO₃) in the reaction

HNO₃ → H + NO₃

which may take place in the presence of water or another proton acceptor. Nitrate is an ion which plants can absorb.

A factory based on the process was built in Rjukan and Notodden in Norway, combined with the building of large hydroelectric power facilities.

The Birkeland-Eyde process is relatively inefficient in terms of energy consumption. Therefore, in the 1910s and 1920s, it was gradually replaced in Norway by a combination of the Haber process and the Ostwald process. The Haber process produces ammonia (NH₃) from methane (CH₄) gas and molecular nitrogen (N₂). The ammonia from the Haber process is then converted into nitric acid (HNO₃) in the Ostwald process, or added to phosphoric acid to produce monoammonium phosphate (NH₄H₂PO₄) or diammonium phosphate.

Mined inorganic fertilizers have been used for many centuries, whereas chemically synthesized inorganic fertilizers were only widely developed during the industrial revolution. Increased understanding and use of fertilizers were important parts of the pre-industrial British Agricultural Revolution and the industrial Green Revolution of the 20th century.

Inorganic fertilizer use has also significantly supported global population growth — it has been estimated that almost half the people on the Earth are currently fed as a result of synthetic nitrogen fertilizer use.

The use of commercial inorganic fertilizers has increased steadily in the last 50 years, rising almost 20-fold to the current rate of 100 million tonnes of nitrogen per year. Without commercial fertilizers it is estimated that about one-third of the food produced now could not be produced. The use of phosphate fertilizers has also increased from 9 million tonnes per year in 1960 to 40 million tonnes per year in 2000. A maize crop yielding 6–9 tonnes of grain per hectare requires 31–50 kg of phosphate fertilizer to be applied, soybean requires 20–25 kg per hectare. Yara International is the world's largest producer of nitrogen based fertilizers.

Urea and formaldehyde, reacted together to produce sparingly soluble polymers of various molecular weights, is one of the oldest controlled-nitrogen-release technologies, having been first produced in 1936 and commercialized in 1955. The early product had 60 percent of the total nitrogen cold-water-insoluble, and the unreacted (quick release) less than 15%. Methylene ureas were commercialized in the 1960s and 1970s, having 25 and 60% of the nitrogen cold-water-insoluble, and unreacted urea nitrogen in the range of 15 to 30%. Isobutylidene diurea, unlike the methylurea polymers, is a single crystalline solid of relatively uniform properties, with about 90% of the nitrogen water-insoluble.

In the 1960s, the National Fertilizer Development Centre began developing Sulfur-coated urea; sulfur was used as the principle coating material because of its low cost and its value as a secondary nutrient. Usually there is another wax or polymer which seals the sulfur; the slow release properties depend on the degradation of the secondary sealant by soil microbes as well as mechanical imperfections (cracks, etc.) in the sulfur. They typically provide 6 to 16 weeks of delayed release in turf applications. When a hard polymer is used as the secondary coating, the properties are a cross between diffusion-controlled particles and traditional sulfur-coated.

See also

• Fertigation
• History of organic farming
• Milorganite
• Phosphogypsum
• Soil defertilisation
• History of fertilizer

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External links

• Nitrogen for Feeding Our Food, Its Earthly Origin, Haber Process
• The Fertilizer Institute (TFI) US Fertilizer Industry Association
• International Fertilizer Industry Association (IFA)
• European Fertiliser Manufacturers Association
• How to read fertilizer tags article
• Agriculture Guide, Complete Guide to Fertilizers and Fertilization
• 4R's Nutrient Stewardship program from The Fertilizer Institute

• Fertilizers