[katog]

We have a private water supply filtered from hill stream water through sand then carbon which got changed recently after ?years.
I kept some bags of it - it looks like black sand/grit.
Do you think it could be of use either in the soil or potting mixes?
Perhaps it would be harmful?
Any suggestions?

[deepinthewoods]

i would definitely try to use it, maybe try a few broad beans in pots with different mixes in, the y should sprout easily now and any problems will be shown in the bb leaves.

[Fleecewife]

I know that when you make a bottle garden you have to put charcoal below the soil so it doesn't go smelly.  Maybe your carbon would work like that in the bottom of pots.  Do you know what kind of carbon it is?  Coal to diamonds 
Check first that there are no chemicals in there.