Most fertilizers contain high levels of phosphorus, nitrogen, and potassium which can all be lethal to the the aquatic environment if it ever got in any water source. Some fertilizers can even contain pesticides and herbicides which are also hazardous chemicals.
If it's still good - give it away! See if a neighbor or friend wants it.
Another option is check the bag! Some fertilizers have instructions for disposal on the packaging.
Fertilizer can't be thrown away and must wait until the Household Hazardous Waste event. Check our Upcoming Events page to see if we have scheduled our next one.