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The pervasive use of P2O5, K2O, CaO, MgO and other molecules that do not exist in soil or fertiliser bags
New Phytologist ( IF 9.4 ) Pub Date : 2021-09-05 , DOI: 10.1111/nph.17715
Hans Lambers 1 , N J Barrow 1
Affiliation  

Phosphorus (P) is very likely the most frequently misspelled nutrient in scientific literature (Lambers & Plaxton, 2015). Plant science, soil science and ecology journals often misspell the term as phosphorous. This error is not picked up by spell-checking software because phosphorous is a valid word. ‘Phosphorous’ is not an alternative spelling for phosphorus, and the difference is not due to British vs American English. The compound phosphorous acid (HPO(OH)2) is a reduced form of inorganic phosphate (Pi), in which one oxygen bonded to the P atom is replaced by hydrogen. One would hope that New Phytologist would make sure that P was invariably spelled correctly in all the papers it published, but, like all other journals we have checked, it offers numerous examples to support our claim (Ågren et al., 2012; Atkinson et al., 2012; Murik et al., 2019). However, our Letter is not about words, but about molecules. It deals with a far more serious matter than spelling.

Carl Sprengel and Justus von Liebig's seminal work on the chemical nature of nutrients necessary for plant growth (Lambers & Barrow, 2020a) was based on the doctrine of Berzelius (1814) (Lambers & Barrow, 2020b). This has created a problem, however, as outlined by Geoffrey Leeper in his well known textbook of 1948: ‘Unfortunately, archaic usages have lingered in soil science long past their time. Thus, the double-oxide theory of salts – the doctrine of Berzelius in 1820, that magnesium sulphate is MgO.SO3 – persists in two fields. First, many writers still record elements as their oxides; calcium appears not as the simple element, but as CaO (which does not exist in soil) and phosphate appears as P2O5, which is quaintly referred to as ‘phosphoric acid’. The phosphate radicle (PO4), which does exist, should surely be preferred, or alternatively the element (P), which many Americans have already adopted. These can be converted into one another on the basis 1.00 part of P is equivalent to 2.29 P2O5 and to 3.06 PO4.’ (Leeper, 1948). The conversion referred to here is on a mass basis.

Over 70 years later, P2O5, K2O and CaO still do not exist in soil, but they continue to be used in publications, and some fertiliser companies still label their products with P2O5; this may be to give consumers the impression that their product contains more than is in the bag. It would be so much more helpful if they informed the consumer in what form the P existed in the bag. Common forms are (single, ordinary or normal) superphosphate (Ca(H2PO4)2), which is produced by the action of concentrated sulfuric acid on powdered phosphate rock or (originally) ground animal bones; triple superphosphate, which is produced by exposing rock phosphate to phosphoric acid, and therefore has a higher P content (www.ipni.net/specifics). Other forms of P fertiliser include potassium phosphate, monoammonium phosphate and diammonium phosphate (Syers et al., 2008; Johnston & Poulton, 2019). Why FAO (http://www.fao.org/faostat/en/#data/RFN) and EU (https://eur-lex.europa.eu/eli/reg/2019/1009/oj) continue to use obsolete terms is puzzling, and it is a complete mystery to us why soil science analytical laboratories continue to present their data using obsolete terms from the 19th century. In a recent exchange of emails, when one of us shared Geoffrey Leeper's text we copied above, a colleague in Germany wrote: ‘Even in exams all this is still existing although I repeatedly argue against it in my lectures. I am going to forward the pdf to my students.’

Leading journals in agronomy also continue to use the nonsensical terms (Lopes & Guimarães Guilherme, 2016; Song et al., 2019), and authors still use P2O5 in horticultural journals (Ortas, 2019). We recently published a peer-reviewed Editorial in Plant and Soil (Lambers & Barrow, 2020a), and this journal now addresses this issue in its questionnaire for its reviewers. We were asked to paraphrase our Editorial to appear in another journal, which was equally keen to stop the use of nonsensical terms (Lambers & Barrow, 2020b). We received numerous responses, and one of our esteemed colleagues shared this with us: ‘The P2O5 K2O nonsense drives me crazy. I keep bringing it up in industry, who are the main culprits, and I will continue fighting to get rid of it, but it is like running against the wall. Their main argument is that by using elemental concentrations it would look like there were less nutrients in the bag. Sigh.’ Indeed, there are fewer nutrients in the fertiliser bag, and the oxygen that manufacturers write on the bag is not there in the claimed form.

One of us (HL) did a quick search in his EndNote library, to be astounded by the number of papers and journals that popped up when searching for P2O5 in his PDF files, even when restricting the search to publications after 2000. Is there any journal in which the nonsensical chemical formulas do not appear in this century? New Phytologist has published papers showing Australian native plants were flushed daily with a balanced nutrient solution [24 (% N)–8 (% P2O5)–16 (% K2O)] (Lamont & Groom, 2002) and that trees used P2O5 (Edwards et al., 2015; Weber et al., 2018). Some researchers interested in mycorrhizas feed their plants Terragreen®, which contains c. 0.84% P2O5, they claim (Leigh et al., 2009). Others feed this stuff to Arabidopsis thaliana (Remy et al., 2012) or add it to tropical lowland forest in Amazonia (Lugli et al., 2021). A preprint of a handbook that is in press at New Phytologist also contains a reference to P2O5 in fertiliser bags (Freschet et al., 2020); when alerted to this, the authors made changes, so the same mistake will not appear in the final version (Freschet et al., 2021).

Our editorials have not yet had the desired effect on New Phytologist's policy, as evidenced by a very recent paper (Gross et al., 2021). The dust that is supposed to be a source of P for plants is reported to contain an even longer list of molecules that do not exist in soil than we cared to mention in our Editorial. That is when we contacted the authors and New Phytologist, and received an invitation to write a Letter, which again reiterates our original Editorial but focuses a bit more on New Phytologist. The final question to be addressed is: does this really matter beyond a need to be linguistically and chemically correct, or is this just the last gasp of a couple of curmudgeons? The following examples clearly illustrate that it does matter. As we pointed out in our original Editorial (Lambers & Barrow, 2020a), some ecologists prefer to use P2O5 in their experiments, because they believe they can then vary the amount of P without worrying about cations at the same time. Unfortunately, that is not an option, because they do not add P2O5. Similarly, when authors claim to use a balanced nutrient solution of 24 (% N)–8 (% P2O5)–16 (% K2O) (Lamont & Groom, 2002), it becomes very complicated to work out what the balance actually is for someone used to N : P : K in mass ratios of these elements. When 45.13 ± 0.1% of dust is erroneously claimed to be CaO (Gross et al., 2021), and the total of all identified compounds, including CaO and several others that do not occur in dust, adds up to 100%, it becomes hard to work out what is the real composition of that dust. Similarly, when Weber et al. (2018) report that they provided plant-available nutrients nitrogen (N, 260 mg l−1), phosphate (P2O5, 180 mg l−1) and potassium (K2O, 480 mg l−1), the reader has really no idea in what forms N, P or K were supplied. At a meeting in a Chinese field, a scientist communicated to HL how much N and P were used to fertilise the crop. He was shocked by the vast amount of P, until his Chinese friend and translator pointed to the fertiliser bag, and he realised the amount had to be divided by 2.29 to know how much P that really was. So, continuation of the use of obsolete terms will obscure what is done in laboratory experiments and in field studies.

If some fertiliser manufacturers wish to retain the outdated practice of labelling their product to include phosphorus, calcium, magnesium and potassium attached to oxygen that are not really in the fertiliser bag (http://ifadata.fertilizer.org/ucSearch.aspx), then that is, literally, their business, but we would like to see them and especially FAO and EU update their nomenclature. In academic writing, the use of P2O5, CaO, K2O and MgO must really stop. This might be achieved by including a specific question to be addressed by reviewers, as Plant and Soil has done. We have moved on since Berzelius (1814) and von Liebig (1855). It is high time we acted upon Geoffrey Leeper's advice (Leeper, 1948), and used chemical formulas that belong in the 21st century, rather than the 1800s.

更新日期:2021-11-03
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