I am new to this forum so please bear with me.

I am running a school garden that is in the middle of a quad area. It is 5 beds (3 x 6 foot and 11inch deep) wood with no bottoms. They sit on concrete. They were previously lined with landscape fabric but this has posed a problem with far too much water loss. So this year we lined with plastic with drainage holes cut into it and now I have the major problem of putting soil back.

Unfortunatly the soil I found at the bottoms and realy for most of the container was construction leftovers. It has rocks, and is realy dense.

Can I amend it with vermiculite or moss? Or do I have to start over with potting mix?

Any advice would be helpful.

I will post pictures shortly.

Thanks!