Dry chemical fertilizer does not expire if it is whole and kept dry.

Organic fertilizers can loose some potency if they older and have had a lot of moisture contact (they are not sealed like chemical fertilizer is, in little balls). Good thing is, with organics you can just apply a little stronger and it is fine.

Osmocote and other time release fertilizers loose potency when exposed to humidity and over time.

I have been studying up on fertilizers. Finally can use some of the stuff in my head!

GGG

Thanks! I was also wondering about the water solubles, Miracle Grow and those types..... I appreciate the response.
Susan.

Miracle Grow can loose some potency because the chemicals are bound to salt. When salt gets really humid it gets wonky.
That said, Many people recommend using lower rates of these types of products than listed on the label because of the salts that bind. So you should be OK at regular strength. It doesn't take too much humidity to make MG lumpy and all humid like.

GGG

Maybe that's why MiracleGro is blamed on so many tree and plant deaths.