In the past couple of decades, biochar has emerged as an exciting new tool for land management and soil improvement in agriculture, and as a climate solution for carbon sequestration. One of the most exciting recent developments has been the emergence of a best-practice—sending biochar through the composting process. This has emerged after various experiments found that the direct addition of plain biochar to soil often proved not to be immediately beneficial. In these cases, it appeared that biochar behaved like an aggressive adsorbent (think of it as a carbon filter) that ended up adsorbing and binding to nutrients too tightly, suppressing crop yields until the biochar had matured or “aged” to the point where it could carry out nutrient exchange. The practice of inoculating biochar with cultured microbes, mixing it with compost, or charging it with fertilizer before applying it to soil was adopted in an attempt to overcome this period of suppressed yields. The post-processing of biochar to prepare it for use in soil has become an area of intense investigation, because it has become abundantly clear that there is more going on in fertile biochar-amended soils than merely the presence of charcoal.
Co-composting biochar has emerged from this search for the best post-processing method, and appears to be a universal best practice for all types of biochar, at least as for agronomic applications of biochar. Let us take a closer look at what co-composting entails, and why it is a best practice.
“Co-composting” vs Composting
Before we take a look at the practice of co-composting biochar, and its many benefits, let’s first discuss the odd terminology—why does the literature on biochar refer to “co-composting” as opposed to merely “composting”? Because composting is deliberately managed decomposition, and for the most part, biochar does not decompose; it influences the decomposition process, and interacts with the microbes involved, and even interacts with the decomposition products, but is not decomposed the way compostable materials are. The biochar is with (co-) the compost, while not being decomposed by the composting process. Because of this, the process of putting biochar through the compost is referred to as “co-composting”.
Why not merely refer to it as “biochar compost”? Because we need to emphasize that this is not merely the mixing of biochar and finished compost, which is a fairly common practice. The biochar is to be mixed with compostable materials before composting, mingling with the decomposing materials throughout the composting process. The finished product is a mixture of compost and biochar; the compostable materials break down and shrink during composting, while the volume biochar remains or even swells somewhat, resulting in a mixture that consists of considerably larger fraction of biochar than the initial mixture. This is the product we refer to as “co-composted biochar”.
How does the presence of biochar benefit compost?
In order to understand how the presence of biochar benefits the composting process and how it improves the finished product, we should first consider what goes on in the composting process when it happens properly, and what happens when it goes wrong.
Aerobic vs. anaerobic compost
The terms aerobic and anaerobic respectively refer to either the presence of air or the absence of air. Whereas there are anaerobic methods of composting, such as bokashi, which entail pickling the compostable materials before interring them in the ground, the most common method of composting is aerobic composting, where the compost is decomposed in the presence of air, with the air being introduced by regularly turning the pile or the compost tumbler. Aerobic composting heats up to the point where pathogens and weed seeds are sterilized, and should not produce foul odors— if the compost is ideally formulated with a correct ratio of carbon-rich materials to nitrogen-rich materials, and if there is enough bulk to produce high temperatures, and if it is sufficiently aerated. If the compost fails to meet these requirements, it can stink with stomach-turning odors, or fail to decompose thoroughly, while weed seeds and pathogens survive and infect the plants that receive the compost.
Biochar can greatly assist in aerobic composting by making it much more forgiving on all three of these criteria. The addition of biochar to compost at a rate of 10% by bulk volume significantly abates the emission of foul odors, while the biochar stimulates microbial activity in the compost, causing the compost pile to heat up and decompose more thoroughly. Even modest compost piles can achieve and maintain elevated temperatures when biochar is part of the mix.
Consider, for example, our own observations from composting with biochar at Gill Tract Community Farm in Albany, California. We observed that biochar significantly alters the behavior of compost piles, resulting in compost with noticeably different characteristics. In 2018, Gill Tract Community Farm partnered with the Local Carbon Network as an early adopter of our biochar. Prior to adding biochar to their compost piles, the piles would heat up to about 130˚F, but the heat would dissipate when the piles were turned, and would not recover. After adding biochar to the compost pile, the compost would heat up to 155˚F, and would remain that hot for nearly a month. By the end of six weeks of composting, the compost piles with biochar would often be at temperatures in excess of 130˚F (as a point of reference, 132˚F is the internal temperature of medium-rare steak. That compost got hot enough to cook meat), at the same time, the unpleasant odors of compost, including the odor of ammonia, went away.