Ah yes, miracle grow lacks certain micro nutrients or lacks them in sufficient quantity. I must say though, for straight dirt and miracle grow you're doing remarkably well. Keep doing what your doing with water, that isn't the problem or you would have killed the plant before it got so big. Mulch will definitely help it retain moisture though which sounds like it needs if you're needing to flood it like that every day. The problem is most likely the miracle grow lacking micro nutrients and your soil not having any either after beng flooded and leached like that every day. Look for Jacks' Classic 20-20-20 if you absolutely insist on buying your nutes from a hardware store.