In case of nutrient management, traditionally Phosphorus and Potassium content in fertilizers are mentioned as P2O5 and K2O. What is the logic of using these types of expression at present time which sometimes causes confusion? Wouldn't it be simpler if we just use P and K content?
This practice of expressing certain nutrients in elemental form and certain others in oxide form has a history. Earlier, plant ash analysis was the major procedure for studying plant nutrition. While analyzing plant ash for various elements, scientists observed that when these elements were expressed as oxides, they summed up to 100 percent indicating total analysis. Consequently, it became a practice to express various plant nutrients found in plant ash in oxide form. For example, P is expressed as P2O5, K as K2O, Ca as CaO, Mg as MgO and so on. The other elements that may not be present in plant ash are expressed in elemental forms only, for instance, nitrogen as N and sulphur as S. This procedure has been accepted by the fertilizer industry, and is used in the expression of grades of all fertilizers, organic or chemical, for which the manufacturer guarantees availability of nutrients. Now, all of us are conditioned to this practice, and if we change the pattern suddenly, it may confuse the industry as a whole!
For convenience, it is Ok. However, technically those are different.
The origin of the practice of expressing the mineral content as oxide (e.g. P2O5) is purely historical and does not reflect the chemical reality in soils and fertilisers. Phosphorus in the soil is mainly present and absorbed as monovalent H2PO4- and divalent HPO4 2- orthophosphate. Many initiatives have been undertaken in the past to change this practice and to finally express globally P, K and cationic micronutrients on an elemental basis. The fertiliser industry is certainly not interested to change this practice as their products would suddenly and apparently declare a lower analysis on the label. 22,9 kgof P2O5 sounds clearly better than 10 kg of P.
As mentioned, mineral content of fertilizers is described as oxide form of elements that form on heating of substance, although these are not the actual forms in commercial fertilizers.To convert between P2O5 and K2O and elemental form, the conversion factors are as under
% P2O5 X0.44 = % P
%P x 2.29 = % P2O5
% K2O x0.83 = % K
%K x1.2 =% K2O
Legislative change may be slow but the reality is that it's better to express the composition of most substances on an element basis. For example, that gets rid of issues such Cd contamination in phosphates because the ratio remains fixed independent of mixing the P source with other materials. Legislation needs to follow, as others have said, because representation as the oxides is typically false.
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