Ciao rachel, la dicitura sembrerebbe questa.
Buon lavoro :)

The mass fraction (percent) nitrogen is reported directly. However, phosphorus is reported as phosphorus pentoxide (P2O5), the anhydride of phosphoric acid, and potassium is reported as potassium oxide (K2O), which is the anhydride of potassium hydroxide. Fertilizer composition is expressed in this fashion for historical reasons in the way it was analyzed (conversion to ash for P and K); this practice dates back to Justus von Liebig (see more below). Consequently, an 18-51-20 fertilizer would have 18% nitrogen as N, 51% phosphorus as P2O5, and 20% potassium as K2O, The other 11% is known as ballast and may or may not be valuable to the plants, depending on what is used as ballast. Although analyses are no longer carried out by ashing first, the naming convention remains. If nitrogen is the main element, they are often described as nitrogen fertilizers.
In general, the mass fraction (percentage) of elemental phosphorus, [P] = 0.436 x [P2O5]
and the mass fraction (percentage) of elemental potassium, [K] = 0.83 x [K2O]
(These conversion factors are mandatory under the UK fertilizer-labelling regulations if elemental values are declared in addition to the N-P-K declaration.)
An 18−51−20 fertilizer therefore contains, by weight, 18% elemental nitrogen (N), 22% elemental phosphorus (P) and 16% elemental potassium (K).
http://www.isragri.com/inorganic_fertilizers.html

Mass fraction conversion
"Mass fraction" conversion factors are mandatory under the UK fertilizer-labelling regulations if elemental values are declared in addition to the N-P-K declaration.[9])

In phosphorous pentoxide, the element phosphorous constitutes about 43.6% of the total mass of the compound. Thus, the official UK mass fraction (percentage) of elemental phosphorus, [P] = 0.436 x [P2O5]

Likewise, the mass fraction (percentage) of elemental potassium, [K] = 0.83 x [K2O]

Thus an 18−51−20 fertilizer therefore contains, by weight, 18% elemental[citation needed] nitrogen (N) , 22% elemental phosphorus (P) and 16% elemental potassium (K).

http://en.wikipedia.org/wiki/Fertilizer#Chemical_Content

http://www.google.com/search?hl=en&ie=ISO-8859-1&q="mass fra...

Il titolo dichiarato di un fertilizzante indica il suo contenuto percentuale rispetto al prodotto tal quale in N, P2O5 e K2O riportati sempre in quest'ordine in etichetta prodoto.
E' possibile riportare il titolo anche di altri elementi nutritivi, che saranno allora espressi con riferimento a ossido di Ca, CaO, ossido di Mg, MgO, ossido di Na, Na2O, anidride solforosa, SO3 o zolfo S o direttamente al singolo elemento negli altri casi.
http://www.fertirrigazione.it/ita_790/

Sulle confezioni dei concimi vengono indicate delle cifre, separate da trattini, o a volte interspaziate da segni di addizione. Se ci troviamo davanti a un composto minerale che riporta il titolo 15-3-15, vuol dire che su 100 Kg. di prodotto 15 saranno di Azoto, 3 di Fosforo e ancora 15 di Potassio.
http://www.guidaconsumatore.com/agricoltura_giardinaggio/con...