Plant food

[Squirrel]

In what proportions does a plant contain oxygen, carbon and hydrogen, and other significant elements / compounds? I'm wondering what my potted ones are growing from; - on the bottle of 'Plant Food' it says to add 5-10 drops of food to every litre of water. And on the side of the bottle it says:

' N10.6%
' P₂O₅4.4% (P 1.9%)
' K₂O1.7% (K 1.4%)

But it still doesn't seem very much. After all, they get enough C, H and O from air and water, but all the green bits of the plant, and other parts, seem to grow and die faster than I add this tiny amount of food, and some of them have been in the same soil for years.

[Squirrel]

Any ideas what they're growing from? Am I being incredibly short-sighted?

[j2buttonsw]

Over 90% of the plant is derived from C, O, H and N, all of which can be derived from the atmosphere (if the plant can fix nitrogen). If it is not a legume, nitrogen has to be added (hence the high proportion of N in your feed). The micro-nutrients include K, P, Na, Cl. Zn, S, Fe, Mo, Mn and Mg which are required in much less proportion than the big 4 macro-nutrients, which is why you need so little of the feed. The green bits are still predominantly C, N, H and O which puzzled many scientist who realised that the increase in plant weight gained was disproportionate to the amount the soil in which they grew decreased.

[Squirrel]

Thanks, j2buttonsw, that is just the kind of answer I was hoping for.