Maggie -

That is a nice gift, even if it requires Herculean strength to move.
What you intend to plant in it would determine the answer, though.

A tree or a rosebush -- I'd recommend using the whole pot. But if you plan to do a container "garden" type with more shallow-rooted plants mixed up with varied depth and height -- then absolutely, I'd think you could fill up the bottom half with whatever you want, provided that it is nontoxic to the plants and allows for easy drainage through the bottom.

I've attached an example to the post.

Thank you Sugi, beautiful picture ...I found same on pinterest ! Right now I'm leaning toward a canna upright, coleus fillers and possibly petunia trailers ...all moisture loving pigs, lol ;-D tho, open to suggestions and appreciate the pictures

That's only 6 cu ft....2 bags of pine bark fines plus amendments

Thanks, I appreciate all the personal emails and pictures; and yes abnorm can figure lol .... : D