I know is far from you and don't know if it still happens in San Mateo and maybe is for those close to SM
Back in the day of doing work for Comcast, every Saturday off of 3rd St towards the bay, a big load of soil was dumped for people to come and grab as much as they liked; I think it is/was from composting.
There are people offering free soil/dirt, even free delivery but you don't know what's in it since big loads of dirt have to have a soil analysis or something like that per local/state/federal law?