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This week’s Book of the Week feature is The Modern Grower’s Guide to Terra Preta, by Caroline Pfützner.
From Chapter 2: The Ingredients
This chapter discusses the ingredients you can use to make terra preta.
The most important element is biochar, which, together with rock dust and effective microorganisms, produces the ideal mixture for rich soil life and strong plant growth. How these substances work—and how they interact with each other—will be the subject of the following pages.
Biochar is the central component of terra preta. It contributes to stable humus formation like no other soil additive can. Thanks to its diverse characteristics, however, its effects go well beyond soil creation. From environmental protection and wastewater treatment to agriculture and animal health, it is employed in countless ways. Biochar’s effectiveness is greatly dependent on how it is produced.
What Is Biochar?
Biochar is the result of the incomplete burning of biomass at high temperatures with limited or no access to oxygen. Complete burning simply produces ashes. This charring process is referred to as pyrolysis, or carbonization. At the beginning of the pyrolysis process, residual moisture evaporates as temperature increases. Volatile components (gases) are then released. As these gases burn, the initial biomaterial carbonizes, resulting in a product that is mostly just carbon. If the pyrolysis is not stopped at this point, the charcoal will continue to burn into ash.
During this process, a portion of the emitted gases form PAHs (polycyclic aromatic hydrocarbons) and dioxins—two groups of substances that are extremely damaging to our health.
When producing charcoal for use in the soil or for other agricultural purposes, pyrolysis needs to be steered in a way that causes these pollutants to fully burn off into steam and CO2. Charcoal prepared in this way is called biochar, both in order to differentiate it from the charcoal used for grilling, which often contains damaging substances, and to emphasize the fact that it can be made not only from wood but from practically any organic material.
How Is Biochar Made?
Up until well into the nineteenth century, Europeans produced the charcoal they needed for iron and steel manufacturing in large charcoal piles. The piles were created by stacking layers of logs in the general shape of a cone near a water source, which was used to extinguish the finished charcoal. The cones were covered with light material like foliage, hay, grass, or moss. They were then sealed on all sides with soil or clay such that they were airtight up to a layer in the middle. Depending on the size of the pile, the carbonization process would take days or weeks, and it had to be constantly monitored and its airflow precisely regulated so that it would char but not burn. There was no way in those days to make use of the resulting heat, and the quality of the charcoal that was produced would not meet modern standards.
Today, large quantities of high-grade biochar are produced industrially. Factories use either a continuous process, in which biomass is constantly fed in and carbonized, or a batch process, in which production takes place in sealed containers whose contents are replaced each time the pyrolysis process finishes. Wood gasifiers, whose primary function is energy production, also produce a small amount of low-contaminant biochar as a by-product.
The major advantages of industrial biochar production are the largely autonomous nature of the pyrolysis process and the fact that we have ways to utilize the excess heat it produces. Industrial producers also generally adhere closely to the maximum recommended limits for PAHs, heavy metals, and other pollutants. Many of them have the European Biochar Certificate (EBC) to prove it.
Regular people can make equally high-quality biochar, though, as long as certain conditions are observed (page 108). A completely different method of biochar production is hydrothermal carbonization (HTC). However, there is not yet any suitable way to use the material it produces in the soil (see page 169 of the appendix).
Due to its enormous surface area, biochar has a very high storage capacity for water and nutrients, and it can also bind to (adsorb) contaminants of all sorts. The temperatures that occur during pyrolysis play a big role in this.
At temperatures under 450°C, biochar rarely has a surface area
of more than 150 m2/g; above 600°C, though, its surface area usually increases to more than 300 m2/g, which is the desirable level for both biochar and feed biochar (page 151).
The higher the temperature, the more water the biochar will later be able to store in its pores, and thus the larger the number of micro- organisms that will be able to populate them. An increase in temperature also increases the proportion of long-lasting stable carbon in the biochar as well as its pH value: at temperatures above 500°C, the pH will be 10 to 12—in the strongly basic part of the spectrum.
Similarly, at high temperatures, biochar’s cation exchange capacity also increases (CEC, page 24), as does its adsorption capacity: the ability to bind to other materials—primarily nutrients, but also drug residues, dioxins, PCBs (polychlorinated biphenyls), heavy metals, and pesticides.
The toxic PAHs resulting from pyrolysis reliably burn away at temperatures above 600°C; even 400°C is sufficient if the material is exposed to such temperatures for at least an hour.
Maximum biochar yields are achieved at temperatures around 450°C. Production gradually decreases as temperature continues to increase, although the proportion of long-lasting, stable carbon continues to increase.
About an Author:
Caroline Pfützner is a passionate hobby gardener with many years of terra preta experience. As a young entrepreneur at TerraTirol KG, which has been producing high-quality soil using the Terra Preta method since 2014, she was awarded the Tyrolean Regional Environment Prize in 2016. She passes on her practical knowledge in numerous workshops and lectures.
Titles of Similar Interest:
Humusphere, by Herwig Pommeresche
Burn: Using Fire to Cool the Earth, Kathleen Draper