Since P exists in oxide form under natural condition..you can express in P also...Phosphorous forms phosphorous penta oxide in presence of oxygen, whereas phosphoric acid is formed once these oxides react with water depending upon number of hydrogen atoms replaced with phosphates...
Since P exists in oxide form under natural condition..you can express in P also...Phosphorous forms phosphorous penta oxide in presence of oxygen, whereas phosphoric acid is formed once these oxides react with water depending upon number of hydrogen atoms replaced with phosphates...
thank you for your reply,
when we said Triple super phosphate has 46 % P2O5, how much P exist in 100 kilograms of TSP?
You need to multiply by 0.44 to convert P2O5 into P. That way , P will be 20.24 kg in 100kg TSP...very simple calculation..
thank you Dr.Rajan for your kind reply, I general we said P2O5 because: 1- P naturally exists in oxide form and 2- when we said 46% sounds better than we said 20.24 kilogram P in each bag of triple super phosphate .....!
Good justifications! The fertilizer manufacturers also have a stake to popularize their products.
Back the day Liebig discovered the plant nutrients, he stated that P2O5 was a plant nutrient because he found high level in plant ashes. And then it persists nowdays. Phosphate anion (H2PO4-) is the chemical form the plants uptake.
Mostly it is a convention based on Fertilizer industry who use high numerical figures to attract farmers.
You are right Dr Ghafoor...whichever value is higher , fertilizer companies highlight those figures only...
I think because P naturally found in oxide form ,furthermore in this formula we can easily calculate net P amount in the fertilizers.
Some fertilizers mentioned for Phosphorus as P2O5 and some as P and also phosphorus recommendation for crops as P2O5 or as P so that simple conversion as per Dr Annop K. Srivastava. But P converted to P2O5 by multiply by 2.29. For example Di ammonium phosphate (DAP) content about 46% P2O5.
Since first determination of phosphorus & potash was determined in terms of Phosphate (as oxidised form) and Potash (as K2O) it has been expressed in terms of % P2O5 & % K2O
I quote "In earlier times, before current lab instrumentation was developed, chemists used a gravimetric (weighing) method after ignition to determined the phosphorus and potassium content of fertilizers in the form of phosphorus oxide (P2O5) and potassium oxid e (K2O). By convention, the amounts (or analysis grade) of phosphorus and potassium in fertilizers are still expressed in this oxide form. The Association of American Plant Food Control officials have developed a uniform state fertilizer bill which says that available P2O5 and soluble K2O must be guaranteed by the manufacturer and so the guaranteed analysis must still be expressed in the oxide form. Nitrogen content has always been expressed as simply N."
The above answer (Indranil Das) is the most correct answer to the actual question; being WHY is it expressed as the pentoxide not P. It is simply a convention that, while no longer used in science, is still used widely in the international fertilizer industry, though interestingly not in countries such as New Zealand. The convention came about in the 1800s when by far the most important analysis being conducted was the P and Ca content of phosphate rock (which contains little or no N). If you express the mineral nutrients in these roasted rocks as the most stable oxidation-state oxide, the total adds up to 100%, which gave a 'tidy'-looking result. This doesn't apply to manufactured fertilisers, especially those containing N, so N has always been expressed as N.
The answers explaining the mathematical conversion from one to the other are correct in themselves, but miss the point of the question.
It is simplistic to blame it on the industry just 'wanting the highest number'. The entire industry in a given country would have to agree to make the change (or be legislated to change, as happened in New Zealand originally) to avoid those companies who did not change gaining an advantage in a confused market. And then there is labelling confusion in the case of internationally traded fertilizer. So a successful change really requires international agreement; in today's world can anybody really see that happening? We can't even agree to all go metric!
What all of us should work towards is encouraging the labelling of phosphorus content both in terms of elemental P and as the pentoxide.
In whatever form P exists in fertilizer, plants take it in the form of H2PO4 - , H2PO4 -2
Scientists and Industrialists should resolve the issue together and determine the price on the basis of plant's actual intake of P .
Mujibur Rahman Khan gave good suggestion for price fixation o f P fertilizers.
I have my own reservation , price fixation of P-fertilizers would be possible based on actual intake/uptake of P...If , this is the case for P-fertilizers , then why should it be restricted to p-fertilizers only..
"actual intake/uptake of P" is a term subjected to be associated with other avenues like soil type and its physico-chemical and microbial characteristics, plant type and its characteristics for uptake of specific nutrients, climatic condition and others. How can these be related to fertiliser price fixation. Even if so there will be lot of fluctuation in price for different condition and plant. It should be based on the plant available form of nutrients derived from that specific fertiliser when applied.
The percentage of P205 in single and triple superphosphate are 18 and 48 respectively and must be multiplied with 0.44 to get percent P for each fertiliser type,This must be taken note in fertiliser application for phophate
Phosphatic fertiliser application should be based on soil available phosphate status for particular crop and accordingly it should be applied based on Recommendations by specified authority (usually recommendations are based on soil available P2O5). hence conversion of P2O5 is not so important. Further it would be difficult for the farmers to use this conversion all time they apply.
I would not recommend to use P2O5, it refers to the correct form P4O10 which is used to dry. Why use it in agriculture, soil science and for fertilizers? It is easiere to use P and that is what is required in most scientific journals anyway. Use P for the content of P and in case of fertilizer add the technical name, for example triplephosphate. P in the soil is in many forms, plant available P depends on the extractant, give P for the concentration. I find the usage of P2O5 completely useless.
Prof. Manfred argument seems to be ok. We should use P instead of P2O5.
H3PO4 is phosphoric acid which is stable. As you have write HPO4 having -2 charge when Ca combine with HPO4 it form Single and Triple super phosphat which is a complet phosphatic fertilizer while P2O5 is the content of Phosphorus in it.
It is a common question which received a lot of interest from readers and followers but has interesting historical and scientific reason.I appreciate the comments/responses of collegues.In 1833 Graham suggested the term metaphosphoric acid for HPO3.Within the context of the dualistic theory used at the time,acids were considered to be hydrates of corresponding acidic oxides,and thus Graham viewed metaphosphoric acid as a monohydrate of P2O5(i.e.,P2O5.H2O=2HPO3),derived via dehydration from parent phosphoric acid,which he viewd as trihydrate of P2O5(i.e.,P2O5.3H2O=2H3PO4).Further details are available in the paper cited/attached below.So it is the anhydride form of phosphoric acid the fertilizer industry took to express the P content of fertilizer.As mentioned by one colleague the structural formula for phosphorus pentoxide is P4O10 and the emperical formula is P2O5,the anhydride of phosphoric acid.Though term is not used now in expressing P concentration in soil or plant material,the fertilizer industry is still using the term/expression P2O5 to mention the P content of fertilizer materials.
The origins of the Ortho-Meta- and Para Nomenclature by Jensen,W.B. J.Chem.Educ.,2006,83,356
The references given in the paper can be consulted for more information. www.che.uc.edu/jensen/W.B.Jensen/Reprints/126. Ortho,Meta,Para.pdf
I think We are making fertilizer recommendation for various crop based on P2O5 content only. we can express in P form also but it is universally used form. If it should be in P form I agree with idea of Mr. Fred Vocasek
Because of this naming issue, it is very important that we state our units precisely in our discussions. This becomes very confusing when we work with the animal industry which uses elemental P in feed rations. Since we want to use the P in manure properly, the labs sometimes report both, so in our extension efforts we need to explain why we would convert from elemental P to P2O5.
I was told long ago that the fertilizer industry did not want to use P, because it made their products seem more expensive: 0-46-0 would become 0-20-0 and the price would not change.
P fertilizers like DAP , 46 percent P2 O5 but 42 percent water soluble that is easily available by plant.
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