Fertilizer storage and longevity

Provided they are kept quite dry I wouldn't expect there to be any change at all. If wetted all manner of things could happen.

David

Why are you buying three or four years' worth of fertiliser all at once?

If I were worried about needing a supply that far into the future, then I would be inclined to start doing "humanure" composting immediately. Although my main disincentive at the moment is just space and privacy issues.

They are fairly stable salts that will maintain their ability to kill the ecology of you soil. You may as well go hydroponic.

Kept dry they should present no problems but may go lumpy and the sack may also degrade and burst when you try to lift. Store them in a plastic trash bag. A soil test will insure you get the best use of it as well.