A lot of canon fodder is made from zink WW that boolit shooters don't want , it is cheap material & marginally lighter than lead. I haven't read of anybody using modern hollow (two piece, lead soldered together) molded iron balls but, while expensive, it could be done. DGW in Tennessee used to sell cannon ball molds up to 5" but I don't think they do anymore.
That is just sod being displaced by the mortar balls. some of the NSSA matches are held on soft soggy ground and I've read of the mortar balls embedding themselves 3 ft in the ground...a lot of digging to recover...