Our Base Formula for Biochar Composting
A. Utilize low temperature biochar production to optimize cation exchange capacity and the formation of aggregates. Research on Terra Preta has shown that char particles form aggregates. It also shows that aggregate formation is initiated through the surface charge attraction optimized by low temperature pyrolysis.
Research on soil organic matter indicates that environmental factors, particularly aggregate formation, play a much larger role in persistence than the molecular composition of the carbon. Aliphatic carbon, the portion of organic carbon molecules bonded to hydrogen, oxygen and nitrogen, predominates in aggregates, rather than aromatic carbon, and this aliphatic carbon often has radio carbon ages of thousands of years. Organic matter persistence and soil fertility go hand in hand.
Organic carbon with a high proportion of aromatic carbon (produced with high temperature pyrolysis at temperatures of 600° C and up) has been shown to be more stable than organic carbon with a higher proportion of aliphatics in laboratory experiments. But research has shown that it is aliphatic carbon that is much more stable in soil, simply because aliphatic carbon aggregates to minerals and clay, which protects it from decomposition.
B. Mill the biochar to sub-millimeter sized particles. Most of the char in Terra Preta soils is in the range of 10-20 microns in size. (For reference, human hair is 70 microns thick.) Only small particles form aggregates. Aggregation significantly enhances the fertility effects, such as nutrient and water retention, of a biochar compost.
Reducing biochar particle size also increases exposed particle surface area by a surprisingly large amount. It is predominately exposed surface area that drives biochar’s fertility benefits. For example, 10 micron particles will have 1000 times more exposed surface area than 1 centimeter particles of the same mass.
Biochar can be added to early stage compost at rates ranging from 5-25% by volume for example. Compost amended with biochar will generally mature more quickly, showing lower respiration rates, a higher concentration of humic substances, and a lower ratio of ammonia to nitrate compared to unamended compost.
See our Biochar Preparation and Rationale pages for more in-depth information on the advantages of low temperature pyrolysis and biochar particle size reduction.