Superphosphate

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Superphosphate is a chemical fertilizer first synthesised in the 1840s by reacting bones with sulphuric acid. The process was subsequently improved by reacting phosphate containing rocks with sulphuric acid. Soluble phosphate is an essential nutrient for all plants, and the availability of superphosphate revolutionised agricultural productivity.

Formulations[edit]

All formulations of superphosphate contain a significant proportion of soluble and available phosphate ions which is the key quality that have made them so essential for modern agriculture.[1]

Triple superphosphate[edit]

Triple superphosphate is a component of many proprietary fertilizer that primarily consists of monocalcium phosphate, Ca(H2PO4)2. It is obtained by treating phosphate rock with phosphoric acid. Many proprietary fertilizers are derived from triple superphosphate, e.g. by blending with ammonium sulfate and potassium chloride. Typical fertilizer-grade triple superphosphate contains 45% P2O5eq, single superphosphate 20% P2O5eq.[2]

Single superphosphate[edit]

Single superphosphate is produced using the traditional method of extraction of phosphate rock with sulfuric acid, an approximate 1:1 mixture of Ca(H2PO4)2 and CaSO4. [2]

Double superphosphate[edit]

Refers to a mixture of triple and single superphosphate, resulting from the extraction of phosphate rock with a mixture of phosphoric and sulfuric acids.[2]

History[edit]

The earliest phosphate rich fertilizers were made from guano, animal manure or crushed bones.[3]. So valuable were these resources during the Industrial revolution that graveyards and catacombs were pillaged across Europe to satisfy demand. [3] In 1842 the Rev Hounslow found coprolites in the cliffs of south Suffolk in England. He was aware from previous research in Dorset by William Buckland that coprolites were rich in phosphate which could be made available for plants by dissolution in sulphuric acid. John Bennet Lawes, who farmed in Hertfordshire learnt of these discoveries and conducted his own research at his farm at Rothamstead (later an agricultural research station) and coined the product that he made as "super phosphate of lime".[4] He patented the discovery and in 1842 started producing superphosphate from fossilised dinosaur dung on an industrial scale, which was the first chemical manure produced in the world. [3]

Edward Packard, realising the significance of this, converted a mill in Ipswich to produce this new fertilizer. He moved his operation in the 1850s to Bramford next to a similar factory that had sprung up operated by Mt Fisons. These operations were subsequently destined to form part of the Fisons fertilizer company. The street where the original mill stood is still called Coprolite Street.[5]

Agricultural significance[edit]

All plants, and all animals, need phosphorous 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[6]. The phosphorous can be in the form of soluble inorganic phosphates or organic compounds containing phosphorous. In the living cell, energy is accumulated or expended using a complex range of biochemical processes which involve the transformation of adenosine triphosphate to adenosine diphosphate when energy is being expended and the reverse when energy is being accumulated as in photosynthesis. [7]

The fate of phosphates in soil is complex as they readily form complexes with other minerals such as clays[6] and may be generally unavailable to plants except by weathering and through the action of bacterial and the soil microbiome.[6] Phosphates are also lost to the soil and plant environment when cops are harvested and consumed by animals or otherwise lost to the local system.

Although there is some replenishment of soil phosphorous from mineral sources and release from soil complexes, the rate of re-solubilisation is too low to support modern agricultural productivity. Organic phosphorous contained within plant or animal matter is much more readily re-solubilised as the material decomposes through microbial action [6]The addition of phosphorus as super-phosphate enabled much greater crop yields.

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 are eluted from fields into rivers and streams where they became lost to agriculture but help to encourage unwelcome eutrophication.[7]

Manufacture[edit]

Superphosphates are manufactured in all the main industrial centres of the world. Including Europe, China and the US.[8] 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[9]

Benefits[edit]

  • Superphosphate is a concentrated source of phosphorus, a nutrient essential for plant growth and development. Its rapid solubility ensures quick uptake by plants, promoting robust root development and overall vitality.
  • Superphosphate it is applicable for a wide variety of crops and horticultural plants, but not all.[10]
  • It is relatively cheap compared to other available sources of phosphate which contributes to its widespread adoption, particularly in developing regions where the costs of agricultural inputs are a significant consideration.
  • One of the key advantages of superphosphate is its immediate effect on plant growth. Unlike slow-release fertilizers, which may take time to break down and release nutrients, superphosphate provides an instant boost to plants, addressing acute phosphorus deficiencies.

Disadvantages[edit]

  • Continuous use of superphosphate can lead to soil acidification, particularly on poorly buffered soils, altering pH levels and potentially limiting nutrient availability.[11] This necessitates careful monitoring and management of soil pH to prevent long-term soil degradation.
  • 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.[12] Other sources note that assuming all the sulfur for the sulfuric acid is recovered from oil and gas sweetening [13]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[12]
  • 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 [10]. Sustainable fertilization practices, including soil testing and targeted applications, are essential to mitigate this risk.
  • The availability of suitable phosphate rich rocks is limited and it is estimates of "peak phosphorous" vary between 30 years from now[14] or somehwere between 2051 and 2092[15]. As the human population increases and demand for food increases the availability of superphosphate fertilizers in the future may be less secure suggesting that alternative sources of phophate may need to be developed.
  • A significant number of plants, especially those that evolved in Gondwanaland have a sensitivity to excess phosphorous[10], getting all that they need from associations with Arbuscular mycorrhiza. Examples of plants that are intolerant of application of superphosphate include Hakea prostrata , and Grevillea crithmifolia. Many terrestial orchids which rely on mycorrhizal associations may have similar sensitvities to elevated phosphate levels [16] and populations may be suppressed by applications of superphosphate containing fertilizer.

References[edit]

  1. ^ "Phosphorus: a finite resource essential for life, critical for agriculture and food security". CSIRO _ Australia's Science Agency. 26 June 2019. Retrieved 28 March 2024.
  2. ^ a b c Kongshaug, Gunnar; Brentnall, Bernard A.; Chaney, Keith; Gregersen, Jan-Helge; Stokka, Per; Persson, Bjørn; Kolmeijer, Nick W.; Conradsen, Arne; Legard (2014). "Phosphate Fertilizers". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. 1–49. doi:10.1002/14356007.a19_421.pub2. ISBN 978-3527306732.
  3. ^ a b c O'Connor, Bernard (2005). "The Origins and Developments of the British Coprolite industry" (PDF). Mining History:The Bulletin of the Peak District Historical Society. 14 (5). Archived from the original (PDF) on 2017-02-02. Retrieved 27 March 2024.
  4. ^ Ivell, David M. (2012). "Phosphate Fertilizer Production – From the 1830's to 2011 and Beyond". Procedia Engineering. 46: 166–171. doi:10.1016/j.proeng.2012.09.461. Retrieved 28 March 2024.
  5. ^ "The Story of Corpolite Street". Ipswich Maritime Trust. 26 October 2019. Retrieved 28 March 2024.
  6. ^ a b c d "Phosphorus Basics: Understanding Phosphorus Forms and Their Cycling in the Soil". Alabama A&M and Auburn Universities. 19 April 2019. Retrieved 28 March 2024.
  7. ^ a b "Why Phosphorous is important". New South Wales Department of Primary Industries. Retrieved 28 March 2024.
  8. ^ "Normal superphosphates" (PDF). EPA. Retrieved 28 March 2024.
  9. ^ "Superphosphate above 35% - production". Knoema. Retrieved 28 March 2024.
  10. ^ a b c "Super-sensitive plants" (PDF). University of Western Australia. April 2024. Retrieved 28 March 2024.
  11. ^ Horsnell, LJ (1985). "The growth of improved pastures on acid soils. 1. The effect of superphosphate and lime on soil pH and on the establishment and growth of phalaris and lucerne". Australian Journal of Experimental Agriculture. 25. CSIRO: 149. doi:10.1071/ea9850149. Retrieved 28 March 2024.
  12. ^ a b "Table 7: Greenhouse Gas Emission Factors for Phosphate Fertilisers" (PDF). Stanford University. June 2004. Retrieved 28 March 2024.
  13. ^ "Mineral Resource of the Month - Sulfur". The American Geological Institute. July 2023. Retrieved 29 March 2024.
  14. ^ "Approaching peak phosphorus". Nature Plants. 8. 15 September 2022. Retrieved 28 March 2024.
  15. ^ "Risks and Opportunities in the Global Phosphate Rock Market" (PDF). The Hague Centre for Strategic Studies. ISBN 978-94-91040-69-6. Retrieved 29 March 2024.
  16. ^ Davis, B.; Lim, W. H.; Lambers, H.; Dixon, K. W.; Read, D. J. (12 May 2022). "Inorganic phosphorus nutrition in green-leaved terrestrial orchid seedlings". Annals of Botany. 129 (6): 669–678. doi:10.1093/aob/mcac030. PMC 9113155. PMID 35247265.