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.
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.
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
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.
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.
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.