In earlier times, before current lab instrumentation was developed, chemists used a gravimetric (weighing) method after ignition to determined the phosphorus and potassium content of fertilizers in the form of phosphorus oxide (P2O5) and potassium oxid e (K2O). By convention, the amounts (or analysis grade) of phosphorus and potassium in fertilizers are still expressed in this oxide form. The Association of American Plant Food Control officials have developed a uniform state fertilizer bill which says that available P2O5 and soluble K2O must be guaranteed by the manufacturer and so the guaranteed analysis must still be expressed in the oxide form. Nitrogen content has always been expressed as simply N.
According to the conventional fertilizer standards, a 100 pound bag of 10-10-10 contains 10% or 10 pounds of nitrogen, 10% or 10 pounds of P2O5 and 10% or 10 pounds of K2O. Since P2O5 is really only 44% actual elemental phosphorus and K2O is only 83% actual elemental potassium, a 100 pound bag of 10-10-10 contains 10% or 10 pounds of nitrogen, 4.4% or 4.4 pounds of elemental phosphorus and 8.3% or 8.3 pounds of elemental potassium.
Perhaps the reluctance of the fertilizer industry to convert to expressing the nutrients in the elemental forms is due to the perception that less fertilizer is being purchased for the same amount of money. A 100 pound bag of 10-10-10 containing N, P2 O5 and K2O would be equivalent to a 100 pound bag of 10-4.4-8.3 containing N, P and K.
Once the fertilizer is added to the soil, the oxide forms, P2O5 and K2O, are no longer used when discussing these two nutrients. The amount of these nutrients analyzed in the soil is expressed as the pounds per acre of P and K. The plant content is expressed as the percentage of P and K.