There is a lot of confusion over the way in which the nutrient content of fertilizers is expressed. The scientific symbol for nitrogen is N and the amount in a fertilizer is expressed as % N. However, although the symbol for phosphorus is P, its nutrient content is expressed as % P2O5.

First, it should be noted that no fertilizer contains the pure elements such as N, P or K. In their pure forms, these elements are useless to plants and some, such as P and K, are actually dangerous substances. Plants can use these elements only in certain combinations with other elements. For example, nitrogen must be either combined with three oxygen atoms as NO3- or with four hydrogen atoms as NH4+. In Muriate of Potash, the potassium does not exist by itself, but in combination with chlorine as KCl, in potassium nitrate, the potassium is in combination with nitrogen and oxygen as KNO3.

 For each nutrient, different combinations are possible and each has a different effectiveness as a fertilizer. The amount of each nutrient in its combined state does not tell us how useful a fertilizer is. For example, potassium nitrate contains 61 per cent NO3 and ammonium sulfate contains 27 per cent NH4. However, potassium nitrate is a far less effective source of nitrogen than ammonium sulfate. Expressing the nutrient content as the element is a far better indication of a fertilizer's usefulness. Therefore, stating that potassium nitrate has 13.5 per cent N and that ammonium sulfate has 21 percent N truly tells us which one provides more nitrogen to the plant, even though neither one contains uncombined N.

 In the case of phosphorus (P), potassium (K) calcium (Ca) and magnesium (Mg), the situation gets a little more complicated. Instead of expressing their nutrient contents as the uncombined elements, they are traditionally expressed as the oxides even though in most cases, the fertilizers do not contain oxides. Therefore P is measured as P2O5, K as K2O , Mg as MgO and Ca as CaO.

 Why was this done? It probably had something to do with the laboratory methods used to analyze these nutrients in the early days of chemistry. However, it is really an inconsistent and confusing way to express nutrient contents.

Why don't we simply change to using only the elements and not oxides? The present approach is internationally accepted and is even law in most places. It will take a world-wide effort to make the change. Also, a fertilizer that now has the formula 20.20.20 will become 20.8.16 which may cause some people to suspect that they are not receiving the nutrients they paid for.

 

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 Elements and Oxides