Biochar typically has a cation exchange capacity 6 to 7 times that of soil humic substances and is significantly more stable. However, it can lack mineral nutrients, particularly if made from woody biomass. If fresh biochar is added directly to soil, it can rapidly adsorb much of the minerals available in soil solution, compromising early growth and the epigenetic response of young plants.
The growth pattern of agricultural plants tends to be set by their earliest experience of nutrient and water availability.
Compost, particularly nitrogen-rich compost produced with manure, tends to mineralize (decompose) rapidly, releasing the greenhouse gases nitrous oxide, methane and carbon dioxide, as well as ammonia, to the atmosphere. Composts also leach nitrates and phosphates into surface and ground water, both during the composting process and after they are added to soil. These significant nutrient and carbon losses not only affect the environment, but also decrease the value of the finished compost.
Biochar improves the composting process by significantly reducing nutrient losses and greenhouse gas and ammonia emissions. Research has shown that nitrogen losses can be reduced by 30-50%, and it is very likely possible to improve these values with higher concentrations of biochar or an optimization of the biochar production process. A finished biochar compost will retain much more of the nutrient content of the biomass feedstock in an ideal, plant available, slow release form. Any initial undesirable side effects that may arise from freshly produced biochar being added to soil, particularly at the beginning of a growing season, are avoided, because the biochar is loaded with nutrients and populated with soil bacteria and fungi after composting.
The Amerindian tribes of the central Amazon figured out how to dramatically improve poor soils thousands of years ago. They created highly fertile patches of soil by composting charcoal and biological waste together. The local inhabitants call them “Terra Preta”, meaning black earth. Surrounding soils that have not been amended with this compost are pale yellow or white, and very infertile.
It is too hot and humid in the Amazon basin to allow any organic carbon to naturally accumulate.
Amazingly, Terra Preta persists thousands of years after it was created, and it is still highly fertile today. How is that possible? In the hot, humid jungle, it should have all decomposed long ago, its mineral content leached away in heavy tropical rains, or consumed by the plants and trees growing in it.
Through our modern scientific and technological capabilities, we are able to understand Terra Preta’s remarkable persistence and fertility, place them in the broader context of known soil fertility traits, and replicate them efficiently in our biochar production and recommended biochar composting processes.
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.