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Fertilizers are physical compounds given to plants to promote growth; they are usually applied either through the soil, for uptake by plant roots, or by foliar feeding, for uptake through leaves. Fertilizers can be divided into organic (composed of plant or animal matter), or inorganic (made of simple, non-carbonaceous chemicals or minerals).

'Organic' fertilizers are composed of 'naturally' occurring compounds such as peat manufactured through natural processes (such as composting) or naturally occuring mineral deposits; or in the case of 'inorganic' fertilizers, manufactured through chemical processes (such as the Haber process) or from naturally occuring deposits that have been chemically altered (concentrated Triple superphosphate[1].

Properly applied, these fertilizers can improve the health, productivity, and appearance of plants as they provide different essential nutrients intended to encourage plant growth.

Fertilizers typically provide, in varying proportions, the three major plant nutrients: nitrogen, phosphorus, potassium known shorthand as N-P-K); the secondary plant nutrients (calcium, sulfur, magnesium) and sometimes trace elements (or micronutrients) with a role in plant or animal nutrition: boron, chlorine, manganese, iron, zinc, copper, molybdenum and (in some countries[which?]) selenium.

Both organic and inorganic fertilizers were called "manure" derived from the French expression for manual (of or belonging to the the hand[2]) tillage, however, this term is currently restricted to organic manure. Though nitrogen is plentiful in the earth's atmosphere, relatively few plants engage in nitrogen fixation[citation needed] (conversion of atmospheric nitrogen to a plant-accessible form).

It is believed by some that 'organic' agricultural methods are more environmentally friendly and better maintain soil organic matter (SOM) levels. There are some scientific studies that support this position.[3]

Regardless of the source, fertilization results in increased unharvested plant biomass left on the soil surface and crop residues remaining in the soil[citation needed].

Over-fertilization of a vital nutrient can be as detrimental as underfertilization.[4] "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.[5]

Organic fertilizers are as likely[citation needed] to cause plant burn as inorganic fertilizers. According to UC IPM, all 'organic' fertilizers, and some specially-formulated inorganic fertilizers are classified as 'slow-release' fertilizers, and cannot cause nitrogen burn[6]

If excess nitrogen is present plants will begin to exude nitrogen from the leafy areas in a process called guttation[citation needed].


[edit] History

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

[edit] Key people

In the 1730s, Viscount Charles Townshend (1674–1738) first studied the improving effects of the four crop rotation system that he had observed in use in Flanders. For this he gained the nickname of Turnip Townshend.

Chemist Justus von Liebig (1803–1883) contributed greatly to the advancement in the understanding of plant nutrition. His influential works first denounced the vitalist theory of humus, arguing first the importance of ammonia, and later the importance of inorganic minerals. Primarily his work succeeded in setting out questions for agricultural science to address over the next 50 years. In England he attempted to implement his theories commercially through a fertilizer created by treating phosphate of lime in bone meal with sulfuric acid. Although it was much less expensive than the guano that was used at the time, it failed because it was not able to be properly absorbed by crops.

At that time in England, Sir John Bennet Lawes (1814–1900) was experimenting with crops and manures at his farm at Harpenden and was able to produce a practical superphosphate in 1842 from the phosphates in rock and coprolites. Encouraged, he employed Sir Joseph Henry Gilbert, who had studied under Liebig at the University of Giessen, as director of research. To this day, the Rothamsted research station that they founded still investigates the impact of inorganic and organic fertilizers on crop yields.

In France, Jean Baptiste Boussingault (1802–1887) pointed out that the amount of nitrogen in various kinds of fertilizers is important.

Metallurgists Percy Gilchrist (1851–1935) and Sidney Gilchrist Thomas (1850–1885) invented the Thomas-Gilchrist converter, which enabled the use of high phosphorus acidic Continental ores for steelmaking. The dolomite lime lining of the converter turned in time into calcium phosphate, which could be used as fertilizer known as Thomas-phosphate.

In the early decades of the 20th Century, the Nobel prize-winning chemists Carl Bosch of IG Farben and Fritz Haber developed the process[7] that enabled nitrogen to be cheaply synthesised into ammonia, for subsequent oxidisation into nitrates and nitrites.

In 1927 Erling Johnson developed an industrial method for producing nitrophosphate, also known as the Odda process after his Odda Smelteverk of Norway. The process involved acidifying phosphate rock (from Nauru and Banaba Islands in the southern Pacific Ocean) with nitric acid to produce phosphoric acid and calcium nitrate which, once neutralized, could be used as a nitrogen fertilizer.

[edit] Industry

The Englishmen James Fison, Edward Packard, Thomas Hadfield and the Prentice brothers each founded companies in the early 19th century to create fertilizers from bonemeal. The developing sciences of chemistry and Paleontology, combined with the discovery of coprolites in commercial quantities in East Anglia, led Fisons and Packard to develop sulfuric acid and fertilizer plants at Bramford, and Snape, Suffolk in the 1850s to create superphosphates, which were shipped around the world from the port at Ipswich. By 1871 there were about 80 factories making superphosphate.[8] After World War I these businesses came under financial pressure through new competition from guano, primarily found on the Pacific islands, as their extraction and distribution had become economically attractive.

The interwar period[9] saw innovative competition from Imperial Chemical Industries who developed synthetic ammonium sulfate in 1923, Nitro-chalk in 1927, and a more concentrated and economical fertilizer called CCF based on ammonium phosphate in 1931. Competition was limited as ICI ensured it controlled most of the world's ammonium sulfate supplies. Other European and North American fertilizer companies developed their market share, forcing the English pioneer companies to merge, becoming Fisons, Packard, and Prentice Ltd. in 1929. Together they were producing 85,000 tonnes of superphosphate per annum by 1934 from their new factory and deep-water docks in Ipswich. By World War II they had acquired about 40 companies, including Hadfields in 1935, and two years later the large Anglo-Continental Guano Works, founded in 1917.

The post-war environment was characterized by much higher production levels as a result of the "Green Revolution" and new types of seed with increased nitrogen-absorbing potential, notably the high-response varieties of maize, wheat, and rice. This has accompanied the development of strong national competition, accusations of cartels and supply monopolies, and ultimately another wave of mergers and acquisitions. The original names no longer exist other than as holding companies or brand names: Fisons and ICI agrochemicals are part of today's Yara International[10] and AstraZeneca companies.

Major players in this market now include the Russian Uralkali fertilizer company Uralkali (listed on the London Stock Exchange), whose majority owner is Dmitry Rybolovlev, ranked by Forbes as 60th in the list of wealthiest people in 2008.

[edit] Inorganic fertilizers (mineral fertilizer)

Naturally occurring inorganic fertilizers include Chilean sodium nitrate, mined rock phosphate, and limestone (a calcium source).

[edit] Macronutrients and micronutrients

Fertilizers can be divided into macronutrients and micronutrients based on their concentrations in plant 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.

[edit] 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), which are known as N-P-K fertilizers or compound fertilizers when elements are mixed intentionally. They are named or labeled according to the content of these three elements, which are macronutrients. The mass fraction (percent) nitrogen is reported directly. However, phosphorus is reported as phosphorus pentoxide (P2O5), the anhydride of phosphoric acid, and potassium is reported as 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); this practice dates back to Justus von Liebig (see more below). Consequently, an 18-51-20 fertilizer would have 18% nitrogen as N, 51% phosphorus as P2O5, and 20% potassium as K2O, The other 11% is known as ballast and may or may not be valuable to the plants, depending on what is used as ballast. Although analyses are no longer carried out by ashing first, the naming convention remains. If nitrogen is the main element, they are often described as nitrogen fertilizers.

In general, the mass fraction (percentage) of elemental phosphorus, [P] = 0.436 x [P2O5]

and the mass fraction (percentage) of elemental potassium, [K] = 0.83 x [K2O]

(These conversion factors are mandatory under the UK fertilizer-labelling regulations if elemental values are declared in addition to the N-P-K declaration.[11])

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

B5A fertilizer [12] is a macronutritient fertilizer.

[edit] Agricultural versus horticultural

In general, agricultural fertilizers contain only 1 or 2 macronutrients. Agricultural fertilizers are intended to be applied infrequently and normally prior to or alongside seeding. Examples of agricultural fertilizers are granular triple superphosphate, potassium chloride, urea, and anhydrous ammonia. The commodity nature of fertilizer, combined with the high cost of shipping, leads to use of locally available materials or those from the closest/cheapest source, which may vary with factors affecting transportation by rail, ship, or truck. In other words, a particular nitrogen source may be very popular in one part of the country while another is very popular in another geographic region only due to factors unrelated to agronomic concerns.

Horticultural or specialty fertilizers, on the other hand, are formulated from many of the same compounds and some others to produce well-balanced fertilizers that also contain micronutrients. Some materials, such as ammonium nitrate, are used minimally in large scale production farming. The 18-51-20 example above is a horticultural fertilizer formulated with high phosphorus to promote bloom development in ornamental flowers. Horticultural fertilizers may be water-soluble (instant release) or relatively insoluble (controlled release). Controlled release fertilizers are also referred to as sustained release or timed release. Many controlled release fertilizers are intended to be applied approximately every 3–6 months, depending on watering, growth rates, and other conditions, whereas water-soluble fertilizers must be applied at least every 1–2 weeks and can be applied as often as every watering if sufficiently dilute. Unlike agricultural fertilizers, horticultural fertilizers are marketed directly to consumers and become part of retail product distribution lines.

[edit] Nitrogen fertilizer

Major users of nitrogen-based fertilizer[13][citation needed]
Country Total N consumption

(Mt pa)

of which used

for feed & pasture

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

Nitrogen fertilizer is often synthesized using the Haber-Bosch process, which produces ammonia. This ammonia is applied directly to the soil or used to produce other compounds, notably ammonium nitrate and urea, both dry, concentrated products that may be used as fertilizer materials or mixed with water to form a concentrated liquid nitrogen fertilizer, UAN. Ammonia can also be used in the Odda Process in combination with rock phosphate and potassium fertilizer to produce compound fertilizers such as 10-10-10 or 15-15-15.

The production of ammonia currently consumes about 5% of global natural gas consumption, which is somewhat under 2% of world energy production.[14] 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.[15] The price increases in natural gas in the past decade, among other factors such as increasing demand, have contributed to an increase in fertilizer price.

Nitrogen-based fertilizers are most commonly used to treat fields used for growing maize, followed by barley, sorghum, rapeseed, soyabean and sunflower. Study results have shown that using nitrogen fertilizer on off-season cover crops can not only increase the biomass of these crops, but can also have a beneficial effect on the nitrogen levels in the soil for the cash crop planted during the summer season.[16]

[edit] Health and sustainability issues

Inorganic fertilizers sometimes do not replace trace mineral elements in the soil which become gradually depleted by crops grown there. This 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.[17] One exception to this is in Western Australia where deficiencies of zinc, copper, manganese, iron and molybdenum were identified as limiting the growth of 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.

In many countries there is the public perception that inorganic fertilizers "poison the soil" and result in "low quality" produce. However, there is very little (if any) scientific evidence to support these views. When used appropriately, inorganic fertilizers enhance plant growth, the accumulation of organic matter and the biological activity of the soil, preventing overgrazing and soil erosion. The nutritional value of plants for human and animal consumption is typically improved when inorganic fertilizers are used appropriately.[citation needed]

There are concerns regarding arsenic, cadmium and uranium accumulating in fields treated with fertilizers. The phosphate minerals contain trace amounts of these elements and if no cleaning step is applied after mining the continuous use of phosphate fertilizers leads towards an accumulation of these elements in the soil; phosphate fertilizers also replace inorganic arsenic naturally found in the soil, displacing the heavy metal and causing accumulation in runoff. Eventually these heavy metals can build up to unacceptable levels and get into the produce. (See cadmium poisoning.)

Another problem with inorganic fertilizers is that they are presently produced in ways which cannot be continued indefinitely. Potassium and phosphorus come from mines (or from saline lakes such as the Dead Sea in the case of fertilizers) and resources are limited. Nitrogen is unlimited, but nitrogen fertilizers are presently made using fossil fuels such as natural gas. Theoretically fertilizers could be made from sea water or atmospheric nitrogen using renewable energy, but doing so would require huge investment and is not competitive with today's unsustainable methods. Innovative thermal depolymerization biofuel schemes are experimenting with the production of byproducts with 9% nitrogen fertilizer from organic waste[18][19]

[edit] Organic fertilizers

A compost bin

Naturally occurring organic fertilizers include manure, slurry, worm castings, peat, seaweed, sewage , and guano. Green manure crops are also grown to add nutrients to the soil. Naturally occurring minerals such as mine rock phosphate, sulfate of potash and limestone are also considered Organic Fertilizers.

Manufactured organic fertilizers include:

Other examples of 'natural' soil fertilization is the natural enzyme method[clarification needed] used by farmers to increase soil fertility and tilth[citation needed].

Some ambiguity in the usage of the term 'organic' exists because some of synthetic fertilizers, such as urea and urea formaldehyde, are fully organic in the sense of organic chemistry.

It is difficult to chemically distinguish between urea of biological origin and that produced synthetically[citation needed].

Adding to possible confusion between agricultural and chemical terminology, fertilizers approved for organic agriculture such as:

It is possible to over-apply organic fertilizers,[citation needed] however, by their nature, most organic fertilizers provide increased physical and biological storage mechanisms to soils, tending to mitigate risks of over-fertilization.[citation needed]

Further, organic fertilizer nutrient content, solubility, and nutrient release rates are typically much lower than mineral (inorganic) fertilizers.[citation needed]

[edit] Risks of fertilizer use

High application rates of inorganic nitrogen fertilizers in order to maximize crop yields combined with the high solubilities of these fertilizers leads to increased leaching of nitrates into groundwater. Eventually, nitrate-enriched groundwater makes its way into lakes, bays and oceans where it accelerates the growth of algae, disrupts the normal functioning of water ecosystems and kills fish, a process known as eutrophication. Water may become cloudy and/or discolored (green, yellow, brown, or red). About half of all the lakes in the United States are eutrophic, while the number of oceanic dead zones near inhabited coastlines are increasing.

The use of ammonium nitrate in fertilizers is particularly damaging - plants absorb the ammonium ion preferentially to the nitrate ion - this means that the nitrate ions are not absorbed and therefore are free to be dissolved by rain leading to eutrophication. [20]

As of 2006, the application of nitrogen fertilizer is being increasingly controlled in Britain and the United States. If eutrophication can be reversed, however, it may take decades before the accumulated nitrates in groundwater can be broken down by natural processes.

Storage and application of some nitrogen fertilizers in some weather or soil conditions can cause emissions of the greenhouse gas nitrous oxide (N2O). Ammonia gas (NH3) may be emitted following application of inorganic fertilizers, or manure or slurry. Besides supplying nitrogen, ammonia can also increase soil acidity (lower pH, or "souring"). Excessive nitrogen fertilizer applications can also lead to pest problems by increasing the birth rate, longevity and overall fitness of certain pests.[21] [22] [23] [24] [25] [26]

The concentration of up to 100 mg/kg of cadmium in phosphate minerals (for example, minerals from Nauru[27] and the Christmas islands[28]) increases the contamination of soil with cadmium, for example in New Zealand.[29] Uranium is another example of a contaminant often found in phosphate fertilizers, also radioactive Polonium-210 contained in phosphate fertilizers is absorbed by the roots of plants and stored in it tissues. Tobacco derived from plants fertilzed by rock phosphates contains Polonium-210 which emits alpha radiation estimated to cause about 11,700 lung cancer deaths each year worldwide. [30][31] [32] [33] [34] [35]

For these reasons, it is recommended that knowledge of the nutrient content of the soil and nutrient requirements of the crop are carefully balanced with application of nutrients in inorganic fertilizer especially. This process is called nutrient budgeting. By careful monitoring of soil conditions, farmers can avoid wasting expensive fertilizers, and also avoid the potential costs of cleaning up any pollution created as a byproduct of their farming.

[edit] Toxic Fertilizer

Toxic fertilizers are recycles of industrial waste that introduce several different types of toxic materials into farm land, garden soils, and water streams. The consumption levels of toxic fertilizer are increasing lately in the U.S. from citizens who are purchasing the wrong chemicals for their gardens as well as choosing the wrong company to purchase it from. This fact is leading to a major problem with the environment due to the fact of toxic waste being processed and planted into our land and water. The most common toxic elements in this type of fertilizer are mercury, lead, and arsenic.[36] [37]

[edit] Global issues

We throw away nutrients for our plants in underground sewage systems. We do this in such a way that pollutes underground water tables. Then we buy manufactured "nutrients" for our plants which aren't as good as what we threw away. This is modern day wastewater "technology".
Michael Reynolds - Earthship Vol.2: Systems and Components

The growth of the world's population to its current figure has only been possible through intensification of agriculture associated with the use of fertilizers.[38] There is an impact on the sustainable consumption of other global resources as a consequence.

The use of fertilizers on a global scale emits significant quantities of greenhouse gas into the atmosphere. Emissions come about through the use of: [39]

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

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.[40]

[edit] See also

[edit] References

  1. ^ http://www.extension.umn.edu/distribution/cropsystems/DC6288.html
  2. ^ http://www.etymonline.com/index.php?search=manual&searchmode=none
  3. ^ Crop Fertilization Improves Soil Quality
  4. ^ Nitrogen Fertilization: General Information
  5. ^ Avoiding Fertilizer Burn
  6. ^ http://www.ipm.ucdavis.edu/TOOLS/TURF/SITEPREP/amenfert.html
  7. ^ Haber & Bosch Most influential persons of the 20th century
  8. ^ History of Fisons at Yara.com
  9. ^ Competition Commission report
  10. ^ History of Yara at Yara.com
  11. ^ UK Fertilizers Regulations 1990, Schedule 2 Part 1, Para. 7.
  12. ^ B5A fertilizer
  13. ^ Food and Agricultural Organization of the U.N. Table 3.3 retrieved 9 Aug 2007
  14. ^ IFA - Statistics - Fertilizer Indicators - Details - Raw material reserves (2002-10; accessed 2007-04-21)
  15. ^ Sawyer JE (2001). "Natural gas prices affect nitrogen fertilizer costs". IC-486 1: 8. 
  16. ^ Nitrogen Applied Newswise, Retrieved on October 1, 2008.
  17. ^ Lawrence, Felicity (2004). "214". in Kate Barker. Not on the Label. Penguin. pp. 213. ISBN 0-14-101566-7. 
  18. ^ Discover Magazine May 2003
  19. ^ Discover Magazine Apr 2006
  20. ^ Roots, Nitrogen Transformations, and Ecosystem Services here http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.arplant.59.032607.092932
  21. ^ Jahn GC (2004). "Effect of soil nutrients on the growth, survival and fecundity of insect pests of rice: an overview and a theory of pest outbreaks with consideration of research approaches. Multitrophic interactions in Soil and Integrated Control". International Organization for Biological Control (IOBC) wprs Bulletin 27 (1): 115–122. .
  22. ^ Jahn GC, Sanchez ER, Cox PG (2001). "The quest for connections: developing a research agenda for integrated pest and nutrient management". International Rice Research Institute - Discussion Paper 42: 18. 
  23. ^ Jahn GC, Cox PG, Rubia-Sanchez E, Cohen M (2001). "The quest for connections: developing a research agenda for integrated pest and nutrient management. pp. 413-430,". S. Peng and B. Hardy [eds.] "Rice Research for Food Security and Poverty Alleviation". Proceeding the International Rice Research Conference, 31 March – 3 April 2000, Los Baños, Philippines. Los Baños (Philippines): International Rice Research Institute.: 692. 
  24. ^ Jahn GC, Almazan LP, Pacia J (2005). "Effect of nitrogen fertilizer on the intrinsic rate of increase of the rusty plum aphid, Hysteroneura setariae (Thomas) (Homoptera: Aphididae) on rice (Oryza sativa L.)". Environmental Entomology 34 (4): 938–943. .
  25. ^ Preap V, Zalucki MP, Nesbitt HJ, Jahn GC (2001). "Effect of fertilizer, pesticide treatment, and plant variety on realized fecundity and survival rates of Nilaparvata lugens (Stål); Generating Outbreaks in Cambodia". Journal of Asia Pacific Entomology 4 (1): 75–84. .
  26. ^ Preap V, Zalucki MP, Jahn GC (2002). "Effect of nitrogen fertilizer and host plant variety on fecundity and early instar survival of Nilaparvata lugens (Stål): immediate response". Proceedings of the 4th International Workshop on Inter-Country Forecasting System and Management for Planthopper in East Asia. 13-15 November 2002. Guilin China. Published by Rural Development Administration (RDA) and the Food and Agriculture Organization (FAO): 163–180,226. 
  27. ^ Syers JK, Mackay AD, Brown MW, Currie CD (1986). "Chemical and physical characteristics of phosphate rock materials of varying reactivity". J Sci Food Agric 37: 1057–1064. doi:10.1002/jsfa.2740371102. .
  28. ^ Trueman NA (1965). "The phosphate, volcanic and carbonate rocks of Christmas Island (Indian Ocean)". J Geol Soc Aust 12: 261–286. 
  29. ^ Taylor MD (1997). "Accumulation of Cadmium derived from fertilisers in New Zealand soils". Science of Total Environment 208: 123–126. doi:10.1016/S0048-9697(97)00273-8. 
  30. ^ Hussein EM (1994). "Radioactivity of phosphate ore, superphosphate, and phosphogypsum in Abu-zaabal phosphate". Health Physics 67: 280–282. doi:10.1097/00004032-199409000-00010. 
  31. ^ Barisic D, Lulic S, Miletic P (1992). "Radium and uranium in phosphate fertilizers and their impact on the radioactivity of waters". Water Research 26: 607–611. doi:10.1016/0043-1354(92)90234-U. .
  32. ^ Scholten LC, Timmermans CWM (1992). "Natural radioactivity in phosphate fertilizers". Nutrient cycling in agroecosystems 43: 103–107. doi:10.1007/BF00747688. 
  33. ^ American Public Health Association, Framing Health Matters, Waking a Sleeping Giant: The Tobacco Industry’s Response to the Polonium-210 Issue: Monique E. Muggli, MPH, Jon O. Ebbert, MD, Channing Robertson, PhD and Richard D. Hurt, MD [1]
  34. ^ Journal of the Royal Society of Medicine, The big idea: polonium, radon and cigarettes, Tidd J R Soc Med.2008; 101: 156-157 [2]
  35. ^ The Age Melbourne Australia, Big Tobacco covered up radiation danger, William Birnbauer [3]
  36. ^ http://www.pirg.org/toxics/reports/wastelands/
  37. ^ http://www.mindfully.org/Farm/Toxic-Waste-Fertilizers.htm
  38. ^ Vaclav Smil, e.g.: Nature 29 July 1999: Detonator of the population explosion
  39. ^ Food and Agricultural Organization of the U.N. retrieved 9 Aug 2007
  40. ^ "Rapid Growth Found in Oxygen-Starved Ocean ‘Dead Zones’", NY Times, Aug. 14, 2008

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