9. Biochar – an ancient solution to a modern problem?

Ultimately we all need food. It is an undeniable, unavoidable human necessity. We either eat plants or we eat animals, which in turn have eaten plants. Plants grow in the soil… therefore soil is fundamentally the basis of all life. Without it we wouldn’t and couldn’t exist.

For this post I wanted to draw on material I have learnt in my Soil Science course and tie it in with the work I am doing for my dissertation. For my dissertation I am running crop trials on a vegetable farm in Kenya investigating whether adding charcoal to agricultural soils could maintain or improve crop yields under reduced artificial fertilizer and irrigation scenarios.

To explain the whole wacky idea of adding charcoal to soils…. I’m going to go back to the beginning of the story…

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When you think of the Amazon, you tend to think of lush green rainforest thriving with plant and animal life. It seems reasonable, therefore, to assume that agriculture would also flourish in such a region. However, it does not. Why?

Typical lush vegetation in the Amazon rainforest

Tropical regions are warm and wet. Consequently, microbial activity is high. This means that any organic matter that falls on the ground is rapidly decomposed and only a very thin layer of soil is able to develop. This layer is maintained by the substantive amounts of organic matter reaching the ground and by the protection the canopy provides against heavy rain.

This photo shows a cross-section through a typical tropical soil - very thin fertile topsoil overlying an expanse of clay

However, as soon as the forest vegetation is removed to make way for agriculture these two phenomena disappear and the thin layer of soil rapidly gets washed away and degraded.

Example of deforestation in the Amazon

The inability of tropical soils to support large-scale agriculture is often sited as one of the fundamental reasons why temperate nations tend to be more advanced than tropical ones (Robinson & Acemoglu, 2012).

Somewhat overgeneralised depiction of an advanced temperate society on the left (New York), and a less advanced tropical society of the right (small village in Borneo)

However, archaeological evidence shows that in the past, over 2,000 years ago, the Amazon basin was home to some unexpectedly complex agricultural societies (Heckenberger, 2009; Mann, 2008). How were these societies able to overcome the issue of soil infertility?

Images showing the complexity of these ancient Amazonian societies - including ceramic pots,  mounded settlements and early forms of writing and rock art

In the 1870s, archaeologists reported anomalously dark and fertile soils (referred to as ‘terra preta’) in certain regions of the Amazon (Hartt, 1885; Smith, 1879). These characteristics result from a higher than usual carbon content.

Colour comparison of a normal Amazonian oxisol (on the left) and a dark terra preta soil (on the right)

A number of different theories were put forward to try to explain the existence of such soils. Some suggested that they were the result of volcanic fallout from the Andes (Camargo, 1941). Whereas, others suggested they were the result of indigenous soil management (Gourou, 1949). The second of these two theories has since become widely accepted. The Pre-Columbian Amazonians are believed to have added charcoal to the soil to improve its agricultural performance. Charcoal is the carbon-rich product obtained when biomass is burnt at high temperatures (~600 °C) under low oxygen conditions, a process called pyrolysis. When charcoal is used for soil amendment rather than for fuel it is referred to as biochar (Lehmann & Joseph, 2009).

Charcoal

Both the physical and chemical properties of biochar can help improve crop yields. Its porous physical structure improves soil water retention. This increases soil moisture content, reduces nutrient leaching and provides a moist environment for the bacteria and funghi required for nutrient exchange.

Scanning electron microscope image of biochar compared to structure of a common bath sponge

Chemically, the surface of biochar is negatively charged. This means that it electrostatically attracts positive cations in the soil. These cations are ‘adsorbed’ onto the biochar, which increases the ‘cation exchange capacity’ of the soil (a measure of the soil’s ability to hold positively charged ions). Cations such as potassium (K⁺), magnesium (Mg²⁺) and calcium (Ca²⁺) are all essential plant macronutrients.

In summary, the use of biochar to improve soil fertility is an ancient technique that was successfully used by the pre-Columbian Amazonians over 2,000 years ago.

Food security is arguably one of the greatest challenges facing the world today. Perhaps, adding biochar to soils could help us overcome this problem by increasing agricultural productivity through both agricultural expansion (into previously infertile areas) and agricultural intensification (increasing current yields). In the last decade, interest in biochar has significantly increased, extensive research has been conducted and the ‘International Biochar Initiative’ has been set up.

References:

Camargo, F. (1941). Estudo de Alguns Perfilsdo Solos Coletados em Diversas Regiões da Hiléia. Belém: Instituto Agronômico Do Norte.

Gourou, P. (1949). Civilisations et malchance géographique? Annales. Histoire, Sciences Sociales, 4(4), 445–450.

Hartt, C. . (1885). Contribuição para ethnologia do valle do Amazonas (pp. 10–14). Archivos do Museu Nacional de Rio de Janeiro.

Heckenberger, M. (2009). Lost Cities of the Amazon. Scientific Amercan, 301(4), 64–71.

Mann, C. (2008). Ancient Earthmovers Of the Amazon. Science, 321(5893), 1148–1152.

Robinson, J., & Acemoglu, D. (2012). Why Nations Fail: The Origins of Power, Prosperity, and Poverty.

Smith, H. H. (1879). Brazil: The Amazons and the Coast. New York: Charles Scribner’s Sons.