humanity can't crush a bunch of cubic km of rock and spread over vast areas of land in a non-polluting way. It won't be carbon negative after drilling, blasting, digging, crushing, transporting, and spreading it around. If you can do it on your garden as a side-effect of hobby gardening, that is a nice benefit.

BTW, I worked as mechanic on heavy equip, crushers, drills, and haulage at many mines.