I have a large window box (12" wide/ 80" long/ 18" deep) which is part of the builing and is made of concrete. the bottom is covered with tar and there are drainage holes at each end. what i'd like to do is put gravel (a few inches) and then soil near to the top. is this safe? (we're on the 6 th floor and there is a side walk below) does anyone have any experience with this sort of thing? is there anyone or book we could consult about this? (we're located in jackson heights ny, which is part of new york city)