Terra Preta (biochar)
Terra preta ("dark earth" in Portuguese) refers to expanses of very dark, fertile anthropogenic soils found in the Amazon Basin. It owes its name to its very high charcoal content. It is also known as "Amazonian dark earth" or "Indian black earth". In Portuguese its full name is "Terra preta do índio" or "Terra preta de índio". Terra mulata is lighter or brownish in color.
Terra preta is characterized by the presence of low-temperature charcoal in high concentrations; of high quantities of pottery sherds; of organic matter such as plant residues, animal faeces, fish and animal bones and other material; and of nutrients such as nitrogen (N), phosphorus (P), calcium (Ca), zinc (Zn), manganese (Mn)[2]. It also shows high levels of microorganic activities and other specific characteristics within its particular ecosystem. It is less prone to nutrient leaching, which is a major problem in most rainforest soils. Terra preta zones are generally surrounded by terra comum, or "common soil;" these are infertile soils, mainly acrisols[2], but also ferralsols and arenosols.
Left, a nutrient-poor oxisol; right, an oxisol transformed into fertile terra preta
Terra preta soils are of pre-Columbian nature and were created by humans between 450 BC and AD 950. The soil's depth can reach 2 metres (6 feet). Thousands of years after its creation it has been reported to regenerate itself at the rate of 1 centimetre per year by the local farmers and caboclos in Brazil's Amazonian basin, and they seek it out for use and for sale as valuable compost.
Biochar is charcoal formed by low temperature pyrolysis. Higher temperature pyrolysis produces a more traditional charcoal. Ideally biochar is made in a way that achieves maximal bio-oil condensate retention. When used broadly, the term biochar simply refers to charcoal made from any biomass waste, and may or may not have a significant bio-oil condensate component. In this broader context biochar is simply charcoal used for agricultural purposes.
What are the benefits of using biochar in the garden?
- Enhanced plant growth
- Suppressed methane emission
- Reduced nitrous oxide emission (estimate 50%)
- Reduced fertilizer requirement (estimate 10%)
- Reduced leaching of nutrients
- Stored carbon in a long term stable sink
- Lowered soil acidity
- Lowered aluminum toxicity
- Increased soil aggregation due to increased fungal hyphae
- Improved soil water handling characteristics
- Increased soil levels of available Ca, Mg, P, and K
- Increased soil microbial respiration
- Increased soil microbial biomass
- Stimulated symbiotic nitrogen fixation in legumes
- Increased arbuscular mycorrhizal fungi
- Increased cation exchange capacity
How much biochar do I need to apply to achieve these benefits?
This is the subject of ongoing studies. The degree of benefit clearly increases with the application rate. If you are satisfied with a very rough estimate, we would venture that a target application rate of 5 kg/m2 (1 lb/ft2) would be sufficient to achieve these results in most gardens. However, there are substantial benefits related to soil biology at rates well below 1 kg/m2. This FAQ includes information on how to use small amounts of biochar in your garden to best advantage.
For more information:
Wikipedia: Biochar https://en.wikipedia.org/wiki/Biochar
Sage/Science Progress (2012) Biochar, and its potential contribution to improving soil quality and carbon capture. https://journals.sagepub.com/doi/pdf/10.3184/003685012X13445960110308
Natural Building Blog: More Details about Terra Preta (2013) https://naturalbuildingblog.com/more-details-about-terra-preta/
Photos from Cornell University https://www.css.cornell.edu/faculty/lehmann/research/terra%20preta/terrapretapictures.html
Also READ, on the Dirt Doctor site: Reinventing Agriculture for Environmental Enhancement
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