Charcoal, used as a filter in sewage systems, could be used to remediate soils for little cost, and would benefit the soil's greatly.
Top quality soils are black, which is actually the free carbon in the soil, and the biome needs this carbon for carbohydrate.
The soil C:N ratio is the key to long term soil fertility.
Biochar, what is it..
https://www.youtube.com/watch?v=jGzMDUaYROA