Most people believe N-P-K to stand for the nitrogen, phophorous, and potassium, but that is somewhat incorrect. These are commonly referred to as the major elements along with calcium, magnesium, and sulphur. They are the major contributing compounds in plant growth.
N: nitrogen, used to stimulate new vegetative growth and overall health.
P2O5: phosphorus oxide, used to stimulate flower development and rooting.
K2O: potassium oxide, used to stimulate stem growth and overall health
N-P-K ratings on the labels of fertilizers are misleading at best. They represent N: nitrogen, P2O5: phosphorus oxide, and K2O: potassium oxide. These molecular compound ratios are not the same as the elemental ppm of the associated primary element, except in the case of N. They are only the guaranteed minimum amount of the molecular compound.
For example:
A fertilizer labeled 30-10-10 can have up to a total of 80% N, along with 10% P2O5, and 10% K20 and still be “accurate”. It could have any combination of N, P2O5 and K2O adding up to 100%, as long as it has at least the MINIMUM listed of any of the three. You could have 50% nitrogen in 10-30-20. These types of labels are misrepresentative. Anyone using them to establish a controlled nutrient balance is not doing their plants any service.
As a general guidline, the N-P-K numbers can be roughly converted to elemental ppm. N, nitrogen is the only element to convert from the label at the ratio of 1 to 1. P2O5, phosphorus oxide, converts to elemental P, phosphorous at a ratio of 1 to .4. K2O, potassium oxide, converts to elemntal K, potassium at a ratio 1 to .8. This demonstrates the radical difference between an N-P-K of 1-1-1 and an actual elemental ratio of 1-.4-.8.
I have only found Botanicare and Green Air Products Genesis nutrients to give the actual ELEMENTAL ppm after dilution. I used to mix my own solutions from ammonium nitrate, calcium nitrate, magnesium nitrate, potassium nitrate, potassium phosphate and other reagents and know from nutrient analysis of the misleading nature of N-P-K labels.