Properly washing plant fertilizer off stuff?

I've been using two plastic reptile water bowls (the ones that look like carved tree trunks and have ridges in them) as flower pots for the past few months, and the flowers have been fertilized many times... it's just because they looked good as flower pots LOL.
Now I'm wanting to use the bowls again for my snakes, if possible. How do you wash them to make them reptile-safe again?

I've just been filling the bowls with water for a few days and letting them soak. But if there's no way they're going to be 100% safe again, I won't use them.