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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.
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 [[Rothamsted Experimental Station]].<ref>{{EB1911}}</ref>
[[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 YOLO SWAG LOL [[Rothamsted Experimental Station]].<ref>{{EB1911}}</ref>


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.<ref>{{cite book
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.<ref>{{cite book

Revision as of 07:15, 23 January 2014

Tennessee Valley Authority: "Results of Fertilizer" demonstration 1942
A large, modern fertilizer spreader
A Lite-Trac Agri-Spread lime and fertilizer spreader at an agricultural show

Fertilizer (or fertiliser) is any organic or inorganic material of natural or synthetic origin (other than liming materials) that is added to soil to supply one or more plant nutrients essential to the growth of plants.[1] Conservative estimates report 30 to 50% of crop yields are attributed to natural or synthetic commercial fertilizer.[2] Global market value is likely to rise to more than US$185 billion until 2019.[3] The European fertilizer market will grow to earn revenues of approx. €15.3 billion in 2018.[4]

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.[5]

Fertilizers typically provide, in varying proportions:

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 (0% moisture) basis (DM). 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.[6][7]

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.

Labeling of chemical fertilizer

Main article: Labeling of fertilizer

In the US and Canada, the labeling scheme presents three numbers separated by dashes (e.g. 10-10-10 or 16-4-8).[8][9] The first number represents the percentage of Nitrogen in the product; the second number, Phosphorus; and the third, 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 phosphorus (4% of 50 pounds), and 4 pounds of potassium (8% of 50 pounds). Australian convention adds a fourth number for Sulphur.[10]

History

Founded in 1812, Mirat, producer of manures and fertilizers, is claimed to be the oldest industrial business in Salamanca (Spain).
Main article: History of fertilizer

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 YOLO SWAG LOL Rothamsted Experimental Station.[11]

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.[12] 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.[13]

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.[14]

Forms

Fertilizers come in various forms. The most typical form is solid fertilizer in granulated or powdered forms. The next most common form is liquid fertilizer; some advantages of liquid fertilizer are its immediate effect and wide coverage.

There are also 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.

More recently, organic fertilizer is on the rise as people are resorting to environmental friendly (or 'green') products. Although organic fertilizers usually contain a lower concentration of nutrients, this lower concentration avoids complication of nitrogen burn harming the plants. In addition, organic fertilizers such as compost and worm castings break down slowly into complex organic structures (humus) which build the soil's structure and moisture- and nutrient-retaining capabilities.[citation needed]

Inorganic commercial fertilizer

Fertilizers are broadly divided into organic fertilizers (composed of organic plant or animal matter), or inorganic or commercial fertilizers. Plants can only absorb their required nutrients if they are present in easily dissolved chemical compounds. Both organic and inorganic fertilizers provide the same needed chemical compounds. Organic fertilizers provided other macro and micro plant nutrients and are released as the organic matter decays—this may take months or years. Organic fertilizers nearly always have much lower concentrations of plant nutrients and have the usual problems of economical collection, treatment, transportation and distribution.

Inorganic fertilizers nearly always are readily dissolved and unless added have few other macro and micro plant nutrients nor added any 'bulk' to the soil. Nearly all nitrogen that plants use is in the form of NH3 or NO3 compounds. The usable phosphorus compounds are usually in the form of phosphoric acid (H3PO4) and the potassium (K) is typically in the form of potassium chloride (KCl). In organic fertilizers nitrogen, phosphorus and potassium compounds are released from the complex organic compounds as the animal or plant matter decays. In commercial fertilizers the same required compounds are available in easily dissolved compounds that require no decay—they can be used almost immediately after water is applied. Inorganic fertilizers are usually much more concentrated with up to 64% (18-46-0) of their weight being a given plant nutrient, compared to organic fertilizers that only provide 0.4% or less of their weight as a given plant nutrient.[15]

Nitrogen fertilizers are often made using the Haber-Bosch process (invented about 1915) 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) as the end product. 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.[citation needed]

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 all three N:P:K: plant nutrients 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.[16]

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)

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.[17] Without commercial fertilizers it is estimated that about one-third of the food produced now could not be produced.[18] 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.[19] Yara International is the world's largest producer of nitrogen based fertilizers.[20]

Controlled-release types

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.[21] 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 Center 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.[21] 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.

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.[21]

Top users of nitrogen-based fertilizer[22]
Country Total N use

(Mt pa)

Amt. used for feed/pasture

(Mt pa)

China 18.7 3.0
U.S. 9.1 4.7
France 2.5 1.3
Germany 2.0 1.2
Brazil 1.7 0.7
Canada 1.6 0.9
Turkey 1.5 0.3
UK 1.3 0.9
Mexico 1.3 0.3
Spain 1.2 0.5
Argentina 0.4 0.1

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.[23]

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 wastes the growers' time and money. To avoid over-application, the nutrient status of crops should be assessed. Nutrient deficiency can be detected by visually assessing the physical symptoms of the crop. 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.

Problems with inorganic fertilizer

See also Negative environmental effects

Water pollution

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.[24] In Europe these problems are being addressed by the European Union's Nitrates Directive.[25] Within Britain farmers are encouraged to manage their land more sustainably in 'catchment-sensitive farming'.[26] In the US, excess fertilizer runoff is classified as non-point source pollutants due to the inability to quantify the amount entering bodies of water and shallow aquifers.[27]

Contamination with impurities

Common agricultural grade phosphate fertilizers 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 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 in U. S. have fertilizer registration programs with on-line databases listing chemical analyses of fertilizers.[28][29]

The most widely used inorganic fertilizer is super-phosphate and its double and triple strengthed derivatives double super and triple super. Super phosphate was first developed by Lawes at the Rothamstead Agricultural Research Institute in England in the early 19th century.[30] Lawes added sulfuric acid to conventional rock phosphate containing the mineral apatite, a calcium fluoro-phosphate. The resulting water soluble phosphorus was able to significantly improve yields on a variety of crops at the Rothamstead Centre and the Superphosphate industry was born. Unfortunately over decades of subsequent usage - it became clear that the solubilisation of fluorine also occurred in the process and this had the same effect as the other halogen sterilants(chlorine, bromine, iodine) over time - soil sterilization.[31]

Fertilizer dependency

Effectively farmers unknowingly became 100% dependent on 'bought in' water soluble, inorganic fertilizers since the sterilization of soil microflora including its mycorrhiza, reduced the availability of other natural and trace minerals within the soil. This to some extent explains the resurgence of interest in organic and particularly 'biodynamic' farming systems since these systems replace the soil organisms essential to converting soil minerals into plant available (but rarely water soluble) nutrients.[32] They do this by a variety of processes including chelation whereby essential minerals become plant available - as measured by weak citric acid extraction techniques. Hence the citric acid solubility of phosphate rocks has emerged as a measure of plant availability and enabled so-called 'reactive' phosphate rocks to be used as fertilizer minerals. These should not be confused with high fluorine apatite rocks in which the fluoride content performs a similar function to its role in hardening teeth enamel, i.e. immobilizing phosphorus. This explains the oceanic origins of many of these high fluorine rocks (Christmas Island, Ocean Island) since the fluorine absorbed from the sea has prevented what were originally massive deposits of bird guano - from being leached from the coral based limestone rocks on which they were originally deposited.

Soil acidification

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.

Trace mineral depletion

Many inorganic fertilizers, particularly those based on superphosphate, may not replace trace mineral elements 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.[33] Explanations for this include the early encouragement of so-called "luxury consumption" of trace elements as a result of their acidulation and subsequent dissolution in soil water, by free sulphuric acid sourced from superphosphate. This mechanism has also been identified as a possible causal agent for take-up of the heavy metal cadmium from superphosphate based fertilizers. 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.[34] Such nutrients are described as 'rate limiting' nutrients. Soils in Western Australia are very old, highly weathered and deficient in many of the major nutrients and trace elements.[34] Since this time these trace elements are routinely added to inorganic fertilizers used in agriculture in this state.[34]

Many soils around the world are deficient in zinc, leading to deficiency in plants and humans.[35]

Overfertilization

See also: Fertilizer burn
Fertilizer burn

Over-fertilization of a vital nutrient can be as detrimental as underfertilization.[36] "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.[37]

Fertilizers vary in their tendency to burn roughly in accordance with their salt index.[38]

High energy consumption

In the USA in 2004, 317 billion cubic feet of natural gas was consumed in the industrial production of ammonia, less than 1.5% of total U.S. annual consumption of natural gas.[39] 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.[40]

Ammonia is overwhelmingly produced from natural gas, but other energy sources, together with a hydrogen source such as water (via water splitting or electrolysis),[41] can be used for the production of nitrogen compounds suitable for fertilizers.[42] The cost of natural gas makes up about 90% of the cost of producing ammonia.[43] 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.[44]

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.[45] Nitrogen fertilizer can be converted by soil bacteria to nitrous oxide, a greenhouse gas.

Impacts on mycorrhizas

High levels of fertilizer may cause the breakdown of the symbiotic relationships between plant roots and mycorrhizal fungi.[46]

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.[citation needed] 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.

Organic fertilizer

Main article: Organic fertilizer
Compost bin for small-scale production of organic fertilizer
A large commercial compost operation

Organic fertilizers include naturally occurring organic materials, (e.g. chicken litter, manure, worm castings, compost, seaweed, guano, bone meal) or naturally occurring mineral deposits (e.g. saltpeter). Poultry litter and cattle manure often create environmental and disposal problems, making their use as fertilizer beneficial. Bones can be processed into phosphate-rich bone meal; however, most are simply buried in landfills.

The extent of imbalance in the phosphate and other mineral cycles is such that if all human, animal and plant wastes were recovered to the extent practical and used for fertilizer, mineral fertilizers and synthetic nitrogen would be needed to make up for losses impractical to recover through leaching, atmospheric dispersion and runoff.[citation needed]

Benefits of organic fertilizer

Organic fertilizers have been known to improve biodiversity (soil life) and long-term productivity of soil,[47][48] and may prove a large depository for excess carbon dioxide.[49][50][51]

Organic nutrients increase the abundance of soil organisms by providing organic matter and micronutrients for organisms such as fungal mycorrhiza,[52] (which aid plants in absorbing nutrients), and can drastically reduce external inputs of pesticides, energy and fertilizer, at the cost of decreased yield.[53]

Disadvantages of complex fertilizers

  • Some composted biowastes used as organic fertilizers may support the growth of pathogens and other disease causing organisms if not properly composted.[54]
  • Nutrient contents are variable and their release to available forms that the plant can use may not occur at the right plant growth stage.[55]

Comparison with inorganic fertilizer

Organic fertilizer nutrient content, solubility, and nutrient release rates are typically all lower than inorganic fertilizers.[56][57] One study[which?] 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 the University of California's integrated pest management program, all organic fertilizers are classified as 'slow-release' fertilizers, and therefore cannot cause nitrogen burn.[58]

Organic fertilizers from composts and other sources can be quite variable from one batch to the next.[59] Without batch testing, amounts of applied nutrient cannot be precisely known. Nevertheless, one or more studies have shown they are at least as effective as chemical fertilizers over longer periods of use.[60]

Examples of organic fertilizer

Chicken litter, which consists of chicken manure mixed with sawdust, is an organic fertilizer that has been shown to better condition soil for harvest than synthesized fertilizer. Researchers at the Agricultural Research Service (ARS) studied the effects of using chicken litter, an organic fertilizer, versus synthetic fertilizers on cotton fields, and found that fields fertilized with chicken litter had a 12% increase in cotton yields over fields fertilized with synthetic fertilizer. In addition to higher yields, researchers valued commercially sold chicken litter at a $17/ton premium (to a total valuation of $78/ton) over the traditional valuations of $61/ton due to value added as a soil conditioner.[61]

Other ARS studies have found that algae used to capture nitrogen and phosphorus runoff from agricultural fields can not only prevent water contamination of these nutrients, but also can be used as an organic fertilizer. ARS scientists originally developed the "algal turf scrubber" to reduce nutrient runoff and increase quality of water flowing into streams, rivers, and lakes. They found that this nutrient-rich algae, once dried, can be applied to cucumber and corn seedlings and result in growth comparable to that seen using synthetic fertilizers.[62]

Examples

Organic fertilizer sources

Animal

See also: Night soil
Decomposing animal manure, an organic fertilizer source

Animal-sourced and human urea are suitable for application organic agriculture, while pure synthetic forms of urea are not.[67] The common thread that can be seen through these examples is that organic agriculture attempts to define itself through minimal processing (in contrast to the man-made Haber process), as well as being naturally occurring or via natural biological processes such as composting.[citation needed]

Besides immediate application of urea to the soil, urine can also be improved by converting it to struvite already done with human urine by a Dutch firm.[68] The conversion is performed by adding magnesium to the urine. An added economical advantage of using urine as fertilizer is that it contains a large amount of phosphorus.

Recycled sewage sludge (aka biosolids) as soil amendment is only available to less than 1% of US agricultural land. Industrial pollutants in sewage sludge prevents recycling it as fertilizer. The USDA prohibits use of sewage sludge in organic agricultural operations in the U.S. due to industrial pollution, pharmaceuticals, hormones, heavy metals, and other factors.[69][70][71] The USDA now requires 3rd-party certification of high-nitrogen liquid organic fertilizers sold in the U.S.[72]

Plant

Leguminous cover crops are also grown to enrich soil as a green manure through nitrogen fixation from the atmosphere;[73] as well as phosphorus (through nutrient mobilization)[74] content of soils.

Mineral

Mined powdered limestone,[75] rock phosphate and sodium nitrate, are inorganic (not of biologic origins) compounds which are energetically intensive to harvest and are approved for usage in organic agriculture in minimal amounts.[75][76][77]

Negative environmental effects

See also: Environmental impact of agriculture, Human impact on the nitrogen cycle, and Nitrogen fertilizer § Problems with inorganic fertilizer
Runoff of soil and fertilizer during a rain storm
An algal bloom caused by eutrophication

Water quality

Eutrophication

Main article: Eutrophication

The nitrogen-rich compounds found in fertilizer runoff are the primary cause of serious oxygen depletion 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.[78] 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.[79] As of 2006, the application of nitrogen fertilizer is being increasingly controlled in Britain and the United States[citation needed]. If eutrophication can be reversed, it may take decades[citation needed] before the accumulated nitrates in groundwater can be broken down by natural processes.

Blue baby syndrome

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 leaching into groundwater.[80][81][82] The use of ammonium nitrate in inorganic fertilizers 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.[83]

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).[84]

Soil acidification

See also: Soil pH

Nitrogen-containing inorganic and organic fertilizers can cause soil acidification when added.[85][86] This may lead to decreases in nutrient availability which may be offset by liming.

Persistent organic pollutants

Main article: Persistent organic pollutant

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[87]

Heavy metal accumulation

The concentration of up to 100 mg/kg of cadmium in phosphate minerals (for example, minerals from Nauru[88] and the Christmas islands[89]) increases the contamination of soil with cadmium, for example in New Zealand.[90]

Steel industry wastes, recycled into fertilizers for their high levels of zinc (essential to plant growth), wastes can include the following toxic metals: lead[91] arsenic, cadmium,[91] chromium, and nickel. The most common toxic elements in this type of fertilizer are mercury, lead, and arsenic.[92][93]

Radioactive element accumulation

Uranium is another example of a contaminant often found in phosphate fertilizers (at levels from 7 to 100 pCi/g).[94] Eventually these heavy metals can build up to unacceptable levels and build up in vegetable produce.[90] Average annual intake of uranium by adults is estimated to be about 0.5 mg (500 μg) from ingestion of food and water and 0.6 μg from breathing air.[95]

Also, highly radioactive Polonium-210 contained 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 radiation estimated to cause about 11,700 lung cancer deaths each year worldwide.[96][97][98][99][100][101]

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

Global methane concentrations (surface and atmospheric) for 2005; note distinct plumes

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.[102]

Through the increasing use of nitrogen fertilizer, which is added at a rate of 1 billion tons per year presently[103] 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.[104]

The use of fertilizers on a global scale emits significant quantities of greenhouse gas into the atmosphere[citation needed]. 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.[citation needed]

Other problems

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).[105][106][107][108][109][110]

See also

References

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