And what's worse... The mushroom compost varies considerably from batch to batch...
One batch will be "hot" ( lots of nitrogen), while the next batch may be mostly carbon, and useless except as mulch.
Seems a waste of a valuable water holding container... I'd put it where it could catch the water off the roof, and stock it with water lilies and goldfish...
Or... Having filled it with soil... Grow a bog garden... But... I do live in an area where there is zero water retention in the bottomless white sand... And the ability to hold moisture is something very desireable here...
Hmm... Florida... Rainy season down there, isn't it?
I'd be hesitant about using a galvanized tub for growing food...
By the way...
Welcome to ATP!