The Role of Biochar in the Soil:
The role of biochar in the soil is electron transfer and adhering to a coating of materials that help it capture and exchange nutrients. See this article explaining what co-composting does to biochar. Nearly all of the benefits of biochar in the soil come from the coating of decomposition products that adheres to its surface:
Carbon coating gives biochar its garden-greening power
Organic coating on biochar explains its nutrient retention and stimulation of soil fertility
In order to achieve this coating in the least time and with the greatest effect, mix biochar with compostable materials at a rate of about 10% by bulk volume. (It is very forgiving; anywhere from 10-50% yields beneficial results, but 10-15% seems to make the biochar go the furthest.) Then, let them go through the composting process together.
We do not recommend directly adding biochar to soil. Charcoal that has gone through high temperature processes tend to be aggressive adsorptive media, comparable to a charcoal filter. If you mix this with soil by itself, it will adsorb and cling onto nutrients and the substances that plants need, and the plants will not be able to get these substances off. When this happens, it can take up to two years for the soil to recover from the stunted growth and nutrient depletion. This is not a trivial effect, and many of the early papers that reported negative effects from the use of biochar were due to attempts at directly adding it to the soil.
However, when you mix biochar with compostable materials and let them decompose together, many benefits are obtained. Firstly, the odour is dramatically reduced. Ammonia odours (and therefore, ammonia emissions, which are harmful), in particular, are significantly reduced when biochar is mixed together with compostables. Secondly, the compost tends to compost quite hot when conductive (high temperature processed) biochar is part of the mix. At Gill Tract Farm, our main composting partner, they observed that adding biochar to the compost caused it to compost at nearly 68-70˚C for most of a month, whereas prior to the addition of biochar, the compost would rarely get hotter than 54˚C. The resulting compost was much more thoroughly decomposed, and seemed to retain far more fertilizer value. We now know why: co-composted biochar retains a tremendous amount of nitrates which are normally lost as leachate or as harmful emissions:
Plant growth improvement mediated by nitrate capture in co-composted biochar
I’d like to share a couple of galleries of growing comparisons and growth results that our composting partners have observed with the use of biochar:
LCN Biochar paired results
Biochar not only is fantastic for soil fertility, but it also has two major climate benefits:
- the charcoal itself does not revert to CO2 without combustion. The gradual accumulation of biochar in soil amounts to reverse coal mining— the production of solid black carbon whose carbon was drawn from the air, and the interment of this carbon in the ground.
- When co-composted biochar is added to the soil, it triggers many additional effects which continue to draw down additional carbon in the form of increased soil biology. This effect is quite significant; there can be a 5-10x multiplication of the amount of carbon added to the soil over the subsequent decade due to biological carbon storage effects, especially increased soil fungi.