check your local rock quarry's if you have any. most of the time they will give you the real fine stuff for free or VERY cheap. i got one ton( yes a ton..) of rock dust for 5$..... more than i will ever need even for my large garden. i just had to make a few trips in my truck haha.
rock powders are good for minerals and stimulate soil microbial life.
For all things come from earth, and all things end by becoming earth.