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Superphosphate is a chemical fertiliser first synthesised in the 1840s by reacting bones with
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
. The process was subsequently improved by reacting
phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
coprolites with sulfuric acid. Subsequently, other phosphate-rich deposits such as
phosphorite Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentoxi ...
were discovered and used. Soluble phosphate is an essential nutrient for all plants, and the availability of superphosphate revolutionised agricultural productivity.


History

The earliest phosphate-rich fertilisers were made from
guano Guano (Spanish from ) is the accumulated excrement of seabirds or bats. Guano is a highly effective fertiliser due to the high content of nitrogen, phosphate, and potassium, all key nutrients essential for plant growth. Guano was also, to a le ...
, animal manure, or crushed bones. So valuable were these resources during the
Industrial Revolution The Industrial Revolution, sometimes divided into the First Industrial Revolution and Second Industrial Revolution, was a transitional period of the global economy toward more widespread, efficient and stable manufacturing processes, succee ...
that graveyards and
catacombs Catacombs are man-made underground passages primarily used for religious purposes, particularly for burial. Any chamber used as a burial place is considered a catacomb, although the word is most commonly associated with the Roman Empire. Etym ...
across Europe were pillaged for human bones to satisfy demand. In 1842, the Reverend John Stevens Henslow found
coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name ...
s – fossilised dinosaur dung – in the cliffs of south
Suffolk Suffolk ( ) is a ceremonial county in the East of England and East Anglia. It is bordered by Norfolk to the north, the North Sea to the east, Essex to the south, and Cambridgeshire to the west. Ipswich is the largest settlement and the county ...
in England. He was aware of previous research in Dorset by
William Buckland William Buckland Doctor of Divinity, DD, Royal Society, FRS (12 March 1784 – 14 August 1856) was an English theologian, geologist and paleontology, palaeontologist. His work in the early 1820s proved that Kirkdale Cave in North Yorkshire h ...
which showed that coprolites were rich in phosphate that could be made available for plants by dissolution in sulfuric acid. John Bennet Lawes, who farmed in
Hertfordshire Hertfordshire ( or ; often abbreviated Herts) is a ceremonial county in the East of England and one of the home counties. It borders Bedfordshire to the north-west, Cambridgeshire to the north-east, Essex to the east, Greater London to the ...
, learnt of these discoveries and conducted his own research at his farm at Rothamsted (later an agricultural research station), naming the resultant product "super phosphate of lime". He patented the discovery, and in 1842, started producing superphosphate from fossilised dinosaur dung on an industrial scale; this was the first chemical manure produced in the world. Edward Packard, recognising the significance of Lawes' work, converted a mill in
Ipswich Ipswich () is a port town and Borough status in the United Kingdom, borough in Suffolk, England. It is the county town, and largest in Suffolk, followed by Lowestoft and Bury St Edmunds, and the third-largest population centre in East Anglia, ...
to produce this new fertiliser from coprolites excavated in the village of Kirton. He moved his operation in the 1850s to Bramford next to a similar new factory operated by Joseph Fisons. These operations were destined to form part of the
Fisons Fisons plc was a British Multinational corporation, multinational pharmaceutical, scientific instruments and horticultural chemicals company headquartered in Ipswich, United Kingdom. It was listed on the London Stock Exchange and was once a cons ...
fertiliser company. The street where the original mill stood is still called Coprolite Street.


Agricultural significance

All plants and animals need phosphorus compounds to carry out their normal metabolism even though in the case of plants it may constitute as little as 2% of their dry matter. The phosphorus can be in the form of soluble inorganic phosphates or organic compounds containing phosphorus. In the living cell, energy is accumulated or expended using a complex range of biochemical processes which involve the transformation of
adenosine triphosphate Adenosine triphosphate (ATP) is a nucleoside triphosphate that provides energy to drive and support many processes in living cell (biology), cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis. Found in all known ...
to
adenosine diphosphate Adenosine diphosphate (ADP), also known as adenosine pyrophosphate (APP), is an important organic compound in metabolism and is essential to the flow of energy in living cells. ADP consists of three important structural components: a sugar backbon ...
when energy is being expended and the reverse when energy is being accumulated as in
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
. Superphosphate is relatively cheap compared to other available sources of phosphate. The lower price contributes to its widespread adoption, particularly in developing regions where the costs of agricultural inputs are a significant consideration. The fate of phosphates in soil is complicated as they readily form complexes with other minerals such as clays, and aluminium and iron salts, and may be generally unavailable to plants except by weathering and through the action of bacterial and the soil
microbiome A microbiome () is the community of microorganisms that can usually be found living together in any given habitat. It was defined more precisely in 1988 by Whipps ''et al.'' as "a characteristic microbial community occupying a reasonably wel ...
. The advantage of superphosphate fertilisers is that a significant proportion of the phosphate content is soluble and is immediately available to plants. It thus provides a very quick boost to plant growth. However, the complex soil dynamics tend to immobilize phosphate in mineral complexes or organic
ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
s reducing the availability to plants. Phosphates are also lost to the soil and plant environment when crops are harvested or consumed by animals or otherwise lost to the local system. Phosphates tend to be tightly bound to fine sediments in the soil. Leaching of sediments from soil can lead to elevated phosphate concentrations in the receiving watercourse. The addition of phosphorus as super-phosphate enables much greater crop yields. Although there is some replenishment of soil phosphorus from mineral sources and release from soil complexes by physical and biological mechanisms, the rate of re-solubilisation is too low to support modern agricultural productivity. Organic phosphorus contained within plant or animal matter is much more readily re-solubilised as the material decomposes through
microbial A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
action. However, the key quality that made superphosphate so attractive—the solubility of the phosphate—also created an ongoing demand for the product as the soluble phosphorus salts and phosphate bound to fine sediments are eluted from fields into rivers and streams where they became lost to agriculture but help to encourage unwelcome
eutrophication Eutrophication is a general term describing a process in which nutrients accumulate in a body of water, resulting in an increased growth of organisms that may deplete the oxygen in the water; ie. the process of too many plants growing on the s ...
.


Manufacture

Superphosphates are manufactured in all the main industrial centres of the world, including Europe, China and the US. In 2021, about 689,916 tonnes of superphosphate were produced with more than half from Poland and substantial amounts from Indonesia, Bangladesh, China and Japan.


Formulations

All formulations of superphosphate contain a significant proportion of soluble and available phosphate ions which is the key quality that has made them essential for modern agriculture.


Single superphosphate

Single superphosphate is produced using the traditional method of extraction of phosphate rock with
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
, an approximate 1:1 mixture of and .


Double superphosphate

The term, "double superphosphate", refers to a mixture of triple and single superphosphate, resulting from the extraction of phosphate rock with a mixture of phosphoric and sulfuric acids.


Triple superphosphate

Triple superphosphate is a component of many proprietary
fertiliser A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrition, plant nutrients. Fertilizers may be distinct from Liming (soil), liming materials or other non- ...
s. It primarily consists of
monocalcium phosphate Monocalcium phosphate is an inorganic compound with the chemical formula Ca(H2PO4)2 ("AMCP" or "CMP-A" for anhydrous monocalcium phosphate). It is commonly found as the monohydrate ("MCP" or "MCP-M"), Ca(H2PO4)2·H2O. Both salts are colourless so ...
, . It is obtained by treating
phosphate rock Phosphorite, phosphate rock or rock phosphate is a non- detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite (or grade of phosphate rock) varies greatly, from 4% to 20% phosphorus pentox ...
with phosphoric acid. Many proprietary fertilisers are derived from triple superphosphate, for example by blending with ammonium sulfate and
potassium chloride Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a sa ...
. Typical fertiliser-grade triple superphosphate contains 45% eq, single superphosphate 20% eq.


Adverse impacts of superphosphate

Continuous use of superphosphate can lead to soil acidification, particularly on poorly buffered soils, altering pH levels and potentially limiting nutrient availability. This necessitates careful monitoring and management of soil pH to prevent long-term
soil degradation Soil retrogression and degradation are two regressive evolution processes associated with the loss of equilibrium of a soil health, stable soil. Retrogression is primarily due to soil erosion and corresponds to a phenomenon where succession revert ...
. Production and transport produce significant quantities of CO2 amounting in some estimates to 1.2kg/kg for the manufacture of superphosphate and 238 g/kg for transport. Other sources note that assuming all the sulfur for the sulfuric acid is recovered from oil and gas sweetening, and the reaction to produce superphosphate is exothermic: provided that the heat generated is fully re-used, the whole cycle may have a negative carbon footprint as low as -518 g/kg for production alone. While superphosphate enriches soil with phosphorus, excessive or imbalanced application can disrupt nutrient ratios, leading to deficiencies or toxicities in plants. Evidence is emerging that elevated levels may be associated with deadly infections by '' Phytophthora cinnamomi''. Sustainable fertilisation practices, including soil testing and targeted applications, are essential to mitigate this risk. The availability of suitable phosphate-rich rocks is limited and estimates of " peak phosphorus" vary between 30 years from 2022, or somewhere between 2051 and 2092. As the human population increases and the demand for food increases, the availability of superphosphate fertilisers in the future may be less secure, suggesting that alternative sources of phosphate may need to be developed. A significant number of plants, especially those that evolved in Gondwanaland, have a sensitivity to excess phosphorus, getting all that they need from associations with Arbuscular mycorrhiza. Examples of plants that are intolerant of the application of superphosphate include '' Hakea prostrata '' and '' Grevillea crithmifolia''. Many terrestrial orchids which rely on mycorrhizal associations may have similar sensitivities to elevated phosphate levels and populations may be suppressed by applications of superphosphate containing fertiliser. Eutrophication of rivers, lakes and the sea has a very well-documented link to increasing phosphate concentrations. However, teasing out the contribution made to this problem by the use of superphosphate is difficult because of the wide range of other sources of phosphorus compounds in both human and animal waste streams. Recent issues on the
River Wye The River Wye (; ) is the Longest rivers of the United Kingdom, fourth-longest river in the UK, stretching some from its source on Plynlimon in mid Wales to the Severn Estuary. The lower reaches of the river forms part of Wales-England bor ...
have been traced back to intensive poultry rearing with the excess phosphate coming from poorly-managed chicken manure.


References

{{Reflist Phosphates Calcium compounds Inorganic fertilizers