Dont plan on using that concrete for anything else after you store fertilizer on it unless you put down some expensive plastic.

 I dont know the details of your situation, but if it is straight urea you use, I would look into changing over to a liquid application of that nutrient. Storing liquid N in most forms other than anhydrous, is easy and you can buy in the off season or anytime the price is at its projected low.

 Urea will draw moisture no matter what you try to do, it is extremely hygroscopic. There will be monthly shrink to the inventory, and a final cost of rehandling it. All equipment used for handling it will have additional maintainence costs. The concrete is porous and will take in the N imediately. Later usage will have to deal with a wet, slimy floor when moisture comes about.

  IMO.