The Soil's the Limit on Adding Biochar

How much biochar should you add to your soil? The answer hinges on an understanding of biochar.

Sand, clay and silt consist of minerals. They provide a substrate for the formation of organic complexes, which bind the soil particles. Very slow weathering and dissolution by microbes can bring minerals into circulation in the soil food web. Biochar is not a mineral, but it behaves the same way. Like minerals, it is inert in the soil. However, it has a vastly greater surface-to-volume ratio. This is what supercharges soil life and productivity.

Biochar does not directly enhance soil productivity. Its tremendous contribution to soil ecology is indirect, through increasing the surface area required by the organic complexes that constitute soil life.

Our recent trials indicate that biochars that are engineered to build soil carbon and placed in the rhizosphere and … applied every year … can build an extra 1 tonne of carbon in 1-2 years. Suggest you read Han Weng's work on this which we summarised in Joseph et al. (2021), How biochar works, and when it doesn't: A review of mechanisms controlling soil and plant responses to biochar.

– Stephen Joseph PhD, Renewable Energy and Biochar Researcher and Consultant, UNSW

Any char is good. Some are better. Cooking at above 450°C provides permanence. Below that more SOC (food) but less biochar (structure).

– Albert Bates, coauthor of Burn and Terra Preta

Low temperature biochars can have a long lifetime if they form an organomineral coating on their surface and are taken into the soil microaggregate structure.  This often occurs quickly in many tropical and sub tropical soils that have a high clay content.

– Stephen Joseph

How much is enough? The limit depends on whether labile organic matter (food) and water are in good supply. You will read that diminishing returns and possibly even lower yields start above 10% biochar by volume in the root zone. This may be because mineral availability begins to become limiting above that percentage. However, the exact level will probably depend on your mineral soil structure, so your mileage might vary accordingly. Best to conduct experiments in your own garden.