Do fertilizers expire?

[tuvok]

I have a 1/4 full bottle of "Leaf Zone" aquarium plant food by Aquarium Pharmecuticals, however, it has been around for quite some time. I'm not sure exactly how old it is though as a got it from someone else. My question is do these products expire or lose their effectiveness over time? For reference the ingredients are:
Soluble Potash 3.00%
Iron 0.10%

I also did not notice an expiry date on the bottle.

[SnakeIce]

the chemicals used to feed plants for the most part are pretty stable, Iron may oxidize in some conditions but the rest don't change over time.

It should be good