The main product I’ve used for adding carbon and humic acid to my soil has been The Andersons Humichar DG (available on the product links page). It has done a solid job of adding carbon to my red clay soils. The soil profile when pulling soil samples has become richer and darker brown within the first 2” of depth from the surface.
As the weather warms up, I am switching my program over to Dirt Booster Plus, also made by The Andersons. This product claims to be an all organic soil amendment product, not a fertilizer specifically. I used a small portion of it last year to recover an entirely DEAD strip of sod that didn’t get hit by irrigation for 5 days.
Dirt Booster Plus contains corn distillates, molasses particles, biochar, humic acid, microbes, and mycorrhizal fungi. The research on mycorrhizal fungi seems to be fairly conclusive that it will essentially extend your root systems reach and efficiency in attaining nutrients. The video below explains better than I can. 👇🏻
Video of my Dirt Booster Plus application here👇🏻