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Fertilizers are soil amendments applied to promote plant growth. They are usually applied directly onto the soil, but can also be applied onto leaves (foliar feeding). Fertilizers can also be applied to aquatic environments for geoengineering, notably ocean fertilization. The main nutrients added in fertilizer are nitrogen, phosphorus, and potassium but other nutrients are added in smaller amounts. Fertilizers can be either organic (e.g. manure) or inorganic (mined or synthesized chemically). Organic fertilizers and some mined inorganic fertilizers have been used for centuries whereas chemically-synthesized inorganic fertilizers were only developed on an industrial scale in the 20th century. Increased understanding and use of fertilizers was an important part of both the pre-industrial British Agricultural Revolution and the industrial green revolution of the 20th century.



Chemical content

Fertilizers typically provide, in varying proportions, the three major plant nutrients: nitrogen, phosphorus, and potassium, known shorthand as N-P-K). They may also provide secondary plant nutrients such as calcium, sulfur, magnesium. Micronutrients may be provided: boron, chlorine, manganese, iron, zinc, copper, molybdenum and selenium.

Macronutrients and micronutrients

Fertilizers can be classified by their macronutrients and micronutrients content (concentrations by dry matter). There are six macronutrients: nitrogen, phosphorus, and potassium, often termed "primary macronutrients" because their availability is usually managed with NPK fertilizers, and the "secondary macronutrients" — calcium, magnesium, and sulfur — which are required in roughly similar quantities but whose availability is often managed as part of liming and manuring practices rather than fertilizers .

The macronutrients are consumed in larger quantities and normally present as a whole number or tenths of percentages in plant tissues (on a dry matter weight basis) . There are many micronutrients, required in concentrations ranging from 5 to 100 parts per million (ppm) by mass . Plant micronutrients include iron (Fe), manganese (Mn), boron (B), copper (Cu), molybdenum (Mo), nickel (Ni), chlorine (Cl), and zinc (Zn).

Tennessee Valley Authority: "Results of Fertilizer" demonstration 1942


Macronutrient fertilizers

Synthesized materials are also called artificial, and may be described as straight, where the product predominantly contains the three primary ingredients of nitrogen (N), phosphorus (P), and potassium (K), (known as N-P-K fertilizers or compound fertilizers when elements are mixed intentionally).

Reporting of N-P-K

Such fertilizers are named according to the content of these three elements. For example, if nitrogen is the main element, the fertilizer is often described as a nitrogen fertilizer.

Regardless of the name, however, they are labeled according to the relative amounts of each of these three elements, by weight (i.e, mass fraction). The percent of nitrogen is reported directly. However, phosphorus is reported as the mass fraction of phosphorus pentoxide (P2O5), the anhydride of phosphoric acid, and potassium is reported as the mass fraction of potassium oxide (K2O), which is the anhydride of potassium hydroxide.

Fertilizer composition is expressed in this fashion for historical reasons in the way it was analyzed (conversion to ash for P and K mass fractions); this practice dates back to Justus von Liebig.

Mass fraction conversion to elemental values

Since the N-P-K reporting basis just described does not give the actual fraction of the respective elements, some packaging also reports the elemental mass fractions. The UK fertilizer-labelling regulations allow for additionally reporting the elemental mass fractions of phosphorous and potassium, rather than phosphoric acid and potassium hydroxide, but these must be listed in parentheses after the standard values. The regulations specify the factors for converting from the P2O5 and K2O values to the respective P and K elemental values as follows:

In phosphorous pentoxide, the element phosphorous constitutes 43.6% of the total mass of the compound. Thus, the official UK mass fraction (percentage) of elemental phosphorus is 43.6%. [P] = 0.436 x [P2O5]

Likewise, the mass fraction (percentage) of elemental potassium is 83%. [K] = 0.83 x [K2O]

Thus an 18−51−20 fertilizer contains, by weight, 18% elemental nitrogen (N), 22% elemental phosphorus (P), and 16% elemental potassium (K).

(Note: The remaining 11% [100 - (18 + 51 + 20)] is known as ballast or filler and may or may not be valuable to the plants, depending on what is used as filler.)

History

While manure, cinder and ironmaking slag have been used to improve crops for centuries, the use of fertilizers is one of the great innovations of the Agricultural Revolution of the 19th Century.

Inorganic fertilizers (synthetic fertilizer)

Fertilizers are broadly divided into organic fertilizers (composed of enriched organic matter—plant or animal), or inorganic fertilizers (composed of synthetic chemicals and/or minerals). Inorganic fertilizer is often synthesized using the Haber-Bosch process, which produces ammonia. This ammonia is used as a feedstock for other nitrogen fertilizers (e.g. anhydrous ammonium nitrate and urea). These concentrated products may be diluted with water to form a concentrated liquid fertilizer, UAN. Ammonia can also be used in the Odda Process in combination with rock phosphate and potassium fertilizer to produce compound fertilizers.

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Application

Synthetic fertilizers are commonly used to treat fields used for growing maize, followed by barley, sorghum, rapeseed, soyand sunflower. One study has shown that application of nitrogen fertilizer on off-season cover cropscan increase the biomass (and subsequent green manurevalue) of these crops, while having a beneficial effect on soil nitrogen levels for the main crop planted during the summer season.

Over-fertilization

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

Trace mineral depletion

Many inorganic fertilizers do not replace trace mineralelements in the soil which become gradually depleted by crops. This depletion has been linked to studies which have shown a marked fall (up to 75%) in the quantities of such minerals present in fruit and vegetables. However, a recent review of 55 scientific studies concluded "there is no evidence of a difference in nutrient quality between organically and conventionally produced foodstuffs"

In Western Australiamarker 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 .

Energy consumption

The production of synthetic ammonia currently consumes about 5% of global natural gasconsumption, which is somewhat under 2% of world energy production.

Natural gas is overwhelmingly used for the production of ammonia, but other energy sources, together with a hydrogen source, 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 gas over the past decade, along with other factors such as increasing demand, have contributed to an increase in fertilizer price .

Long-Term Sustainability

Inorganic fertilizers are now produced in ways which cannot be continued indefinitely. Potassium and phosphorus come from mines (or saline lakes such as the Dead Seamarker) and such resources are limited.While atmospheric nitrogen is effectively unlimited (forming over 70% of atmospheric gases), relatively few plants engage in nitrogen fixation (conversion of atmospheric nitrogen to a plant-accessible form). To make nitrogen accessible to plants, nitrogen fertilizers are synthesized using fossil fuelssuch as natural gasand coal, which are limited.

Organic fertilizers

A compost bin

Organic fertilizers include naturally-occurring organic materials, such as manure, worm castings, compost, seaweed, guanoand peat moss, or naturally occurring mineraldeposits (e.g. saltpeter). In addition to increasing yield and fertilizing plants directly, organic fertilizers can improve the health and long-term productivity of soil. Organic nutrients increase the abundance of soil organisms by providing organic matter and micronutrientsfor organisms such as fungal mycorrhiza, which aid plants in absorbing nutrients. It is believed by some that 'organic' agricultural methods are more environmentally friendly and better maintain soil organic matter (SOM)levels.

Comparison with inorganic fertilizer

Organic fertilizer nutrient content, solubility, and nutrient release rates are typically all lower than inorganic fertilizers.One study found that over a 140-day period, after 7 leachings:
  • Organic fertilizers had released between 25% and 60% of their nitrogen content
  • Controlled release fertilizers (CRFs) had a relatively constant rate of release
  • Soluble fertilizer released most of its nitrogen content at the first leaching


In general, the nutrients in organic fertilizer are both more dilute and also much less readily available to plants. According to UC IPM, all organic fertilizersare classified as 'slow-release' fertilizers, and therefore cannot cause nitrogen burn.

Non-concentrated organic fertilizers with dilute concentrations of nutrients have greater transport and application costs.

Organic fertilizers from treated sewage, composts and other sources can be quite variable from one batch to the next. Without batch testing the amounts of applied nutrient cannot be precisely known.

Sources

Animal

Animal-sourced Ureaand Urea-Formaldehyde(from urine), are suitable for application organic agriculture, while pure synthetic forms of urea are not. The common thread that can be seen through these examples is that organicagriculture attempts to define itself through minimal processing (e.g. via chemical energy such as petroleum—see Haber process), as well as being naturally-occurring or via natural biological processes such as composting.

Sewage sludge use in organic agricultural operations in the U.S. has been extremely limited and rare due to USDAmarker prohibition of the practice (due to toxic metal accumulation, among other factors).The USDA now requires 3rd-party certification of high-nitrogen liquid organic fertilizers sold in the U.S.

Plant

Cover cropsare also grown to enrich soil as a green manurethrough nitrogen fixationfrom the atmosphere; as well as phosphorus (through nutrient mobilization) content of soils.Minerals such as mined rock phosphate, sulfate of potashand limestoneare considered organic fertilizers, though by a contain no (carbon) molecules (inorganic chemicals in an organic chemistrysense).

Mineral

Naturally mined powdered limestone, mined rock phosphateand sodium nitrate, are inorganic(in a chemicalsense), and are energetically-intensive to harvest, yet are still approved for usage in organic agriculture in minimalamounts.This is a contradictory stance however, because high-concentrate plant nutrients (in the form of salts) obtained from dry lake beds by farmers for centuries in a very minimal fashion are excluded from consideration by most organicenthusiasts and many governmental definitions of organic agriculture. No such dichotomy between organic and chemical exists .

Environmental effects of fertilizer use

Water

Eutrophication

The nitrogen-rich compounds found in fertilizer run-off is the primary cause of a serious depletion of oxygen in many parts of the ocean, especially in coastal zones; the resulting lack of dissolved oxygen is greatly reducing the ability of these areas to sustain oceanic fauna. Anoxic respiration by bacteria in the eutrophicated marine zones also releases nitrous oxide to the atmosphere.

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

High application rates of inorganic nitrogen fertilizers in order to maximize crop yields, combined with the high solubilities of these fertilizers leads to increased runoff into surface water as well as leachinginto groundwater.

The use of ammonium nitratein inorganicfertilizers is particularly damaging, as plants absorb ammonium ions preferentially over nitrate ions, while excess nitrate ions which are not absorbed dissolve (by rain or irrigation) into runoff or groundwater.

Blue Baby Syndrome

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).

Soil

Acidification

Nitrogen-containing inorganic fertilizers in the form of nitrate and ammonium cause soil acidification.

Toxic persistent organic compounds

Dioxins, polychlorinated dibenzo-p-dioxins(PCDDs), and polychlorinated dibenzofurans(PCDFs) have been detected in fertilizers and soil amendments

Heavy metal accumulation

The concentration of up to 100 mg/kg of cadmiumin phosphate minerals(for example, minerals from Nauruand the Christmas islandsmarker) increases the contamination of soil with cadmium, for example in New Zealandmarker.Uraniumis another example of a contaminant often found in phosphate fertilizers . Eventually these heavy metals can build up to unacceptable levels and build up in produce. (See cadmium poisoning)

Steel industry wastes, recycled into fertilizers for their high levels of zinc(essential to plant growth), wastes can include the following toxic metals: leadarsenic, cadmium, chromium, and nickel. The most common toxic elements in this type of fertilizer are mercury, lead, and arsenic. Concerns have been raised concerning fish mealmercury content by at least one source in Spain

Also, highly-radioactivePolonium-210contained in phosphate fertilizers is absorbed by the roots of plants and stored in its tissues . Tobacco derived from plants fertilized by rock phosphates contains Polonium-210 which emits alpha radiationestimated to cause about 11,700 lung cancer deaths each year worldwide.

For these reasons, it is recommended that nutrient budgeting, through careful observation and monitoring of crops, take place to mitigate the effects of excess fertilizer application.

Atmosphere

Through the increasing use of nitrogen fertilizer, which is added at a rate of 120 million tons per year presently to the already existing amount of reactive nitrogen, nitrous oxide(N2O) has become the third most important greenhouse gasafter carbon dioxide and methane, with a global warming potential 296 times larger than an equal mass of carbon dioxide, while it also contributes to stratospheric ozone depletion.

Storage and application of some nitrogen fertilizers in some weather or soil conditions can cause emissions of the potent greenhouse gas—nitrous oxide. Ammoniagas (NH3) may be emitted following application of 'inorganic' fertilizers and/or manures and slurries.

The use of fertilizers on a global scale emits significant quantitiesof greenhouse gasinto the atmosphere. Emissions come about through the use of:

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

Increased pest problems

Excessive nitrogen fertilizer applications can also lead to pest problems by increasing the birth rate, longevity and overall fitness of certain agricultural pests.

References

External links



Major users of nitrogen-based fertilizer
Country
Total N consumption(Mt pa)
Amount usedfor feed & pasture
Chinamarker
18.7
3.0
USAmarker
9.1
4.7
Francemarker
2.5
1.3
Germanymarker
2.0
1.2
Brazilmarker
1.7
0.7
Canadamarker
1.6
0.9
Turkeymarker
1.5
0.3
UKmarker
1.3
0.9
Mexicomarker
1.3
0.3
Spainmarker
1.2
0.5
Argentinamarker
0.4
0.1

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