Biochar

The addition of biochar to soils, if done correctly, greatly increases the capacity of soils to hold water, nutrients and carbon. Use of biochar can be a “carbon negative” process. The central biochar
in this image is the size of a pinhead, and the crystalline, porous surface hints at its sponge-like qualities.

http://biochar-us.org/soil-water-benefits-biochar
https://phys.org/news/2017-08-biochar-benefits-manure-lagoon.html
https://e360.yale.edu/features/as_uses_of_biochar_expand_climate_benefits_still_uncertain