I was thinking about getting some 5 gallon buckets, with lids, or one of the plastic deck boxes they have on sale.

Most clay-substrate spreadable fertilizers will experience caking if stored in a humid environment. This makes them difficult to apply. Soluble fertilizers, e.g. those intended to be mixed with water and applied with a hose end sprayer should be OK.

Might be worth reading the label on the fertilizer package. Typically they recommend storage in a cool, dry place, in the original packaging.

Keep it in a cool, dry place.

I think storing garden fertilizer outside is fine as long as you keep it in something waterproof like those 5-gallon buckets with lids or a plastic deck box. I’ve used similar containers for my garden stuff, and they work well to keep moisture out. Just be careful with extreme temperatures since heat can mess with the fertilizer's potency. If you can, try to place it in a shaded area to help it last longer.

Speaking from experience: I bought 4 bags of Scotts weed and feed on sale - post season. Stored them in a Tupperware-type deck box (live in mid-NC). Box isn't airtight, just weather proof. Went to use said fertilizer and I had 4 bags of concrete-like fertilizer. Broke the clumps up - but a pain.

Ditto to behindyou's experience. If fertilizer isn't kept in an airtight container, any moisture in the air will make it get hard. When I was growing up on the farm, we usually had a hammer with the corn planter to break up any clods that might have been present in bagged fertilizer.

I'd long forgotten about that a few years ago when I left half a bag of fertilizer in the garage over the winter. I had to get a hammer out.

Mine caked solid.

Not recommended. As others have said. Even if it's almost airtight, which 5 gal buckets are not unless you get a sealing snap on sealing screw top adapter, the heat cat still get you in the long run. Depending on the chemistry of the product, heat can activate chemical degredation even if it's dry. Like older IMR reloading powders over time even when stored in air conditioned space, sometimes time is your enemy with fertilizers. If you have to store a certain quantity that takes a long time to use, prob best to buy smaller quantities if possible. I've thrown out lots of degraded fertilizer. These days I buy it and use it, I don't store it for any length of time.