It is an accepted practice to express the nutrients in fertilizers as percentage of nitrogen (N), phosphate (P2O5), and potash (K2O). We know that plants do not take up these nutrients in any of these forms, and this practice is being continued because of some historical reasons. In olden days, plant ash analysis was the major procedure for studying plant nutrition, and scientists, while analysing plant ash for various elements, 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 (P2O5, K2O, CaO, MgO, etc.). Other elements that may not be present in plant ash are expressed in elemental forms only ( Examples, nitrogen ( N) and sulphur (S). This system has been accepted by the fertilizer industry to express grades of all fertilizers.
My question is: Why are we continuing this odd practice still now?. Why don’t we opt to express all the nutrients in elemental form?
Each nutrient has a role to play within the plant, in a particular process of biological processes.
There must be a percentage because it is calculated on the basis of plant absorption, say absorber for example 10% and so on
In conventional cultivation we are changing the amount of nutriens during cultivation. In the case of tomato production in greenhouse it is changed every 2 weeks to achive optimum generativ- vegetativ growth. Numerous publications show what changes are need - in greenhouse with precize cultivation we know it from analises of substrate (we prepare it every 2 - 3 weeks). It saw which element the plant has picked up and this needs to be replaced depends of growing fazis, climate etc.. .
Part of this practice is in reference to the traditional (historical) way of expressing nutrients as the oxide form. Early chemists ashed their samples so that nutrients that didn't volatilize were oxidized under the high temperatures remained as oxides. This is similar to what may be found, for example, in fly ash after burning coal or wood. Experimentally, you will find pH values of around 11 in the ash.
Probably, more importantly, if the nutrient contents of today's fertilizer materials (especially P and K) were changed to an elemental basis instead of an oxide basis, the fertilizer analyses would be lower. This then becomes a marketing issue so that if one vendor has a material with 30% P2O5 and another has a material with 13% P without either vendor clearly explaining that the two values are the same. As a buyer of the material without a clear understanding of the difference between the values but at the same price, which one would you be likely to buy? The one with the higher nutrient value because you think you are getting more for your money. From a fertilizer consumer standpoint, most fertilizer consumers are not as clearly attuned to to this as we might be. In addition, much of the literature available (fertilizer handbooks, extension materials) still use the oxide designation because this is what the fertilizer consumer in widely exposed to.
Thank you Larry Cihacek for your insightful comment. It seems manufacturers and vendors are against setting right an historical blunder!
I'm not sure that they are specifically against setting this right. These conventions have been used a long time and so they are entrenched in the discussion (language) of the science of fertilizers and soil fertility. I recall an effort by scientists in the US trying to move away from the oxide expression to the elemental expression of nutrients in the1970's, but this by that time had become an "immovable object". I'm not sure what the answer to this is other than scientists communicating the "elemental" message. But then again, farmers have been trained (conditioned?) to be familiar with the oxide expression so that it would be a massive undertaking to effect the change. However, it would be the more correct thing to do.
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