By Dave of Darlington

In the days when people in England had open fires, it was common practice for gardeners to put soot from their chimneys onto the soil in their gardens or allotments. In doing so, they believed (quite rightly) that they were fertilising the soil with necessary elements like sulphur, phosphorus and nitrogen. But they were also (unwittingly, perhaps) adding something potentially much more important to the soil, namely black carbon.

Black carbon occurs in most soils, but in some parts of the world, especially in Russia, Germany, Brazil, Australia and North America, there are soils with a particularly high black carbon content. Such soils are known as black earth or more often by the Russian word chernozem or the Portuguese terra preta. This black earth is notable for its great fertility and its ability to maintain that fertility over a relatively long period of time. In the Amazon rainforest, for example, where the soil, once deforested, quickly becomes infertile, the isolated areas of terra preta are known to stay fertile and hence to be much more suitable for conversion to arable farming.

So where does this black carbon come from? It has two main origins. Much of the older black carbon comes from natural forest fires, some of which may have taken place thousands of years ago. But a considerable part of the black carbon in soil is believed to originate from human activity. The terra preta in Brazil probably came from the camp fires of ancient forest dwellers. Much of the black carbon in European soils may come from the burning of coal in domestic fires and industrial processes.

The importance of black carbon in soil is that it is an extremely recalcitrant form of organic matter. It is hardly degradable by any micro-organisms, so there is a great likelihood of it remaining in the soil, even when, as a result of repeated cultivation, other organic matter has been oxidised away. This explains the ability of black earth to stay fertile for a longer time.

Editor’s note: Dave of Darlington would have been interested in projects which have taken this forward. See www.biochar.org.uk

This article appeared in Growing Green International magazine Num 27 (Summer 2011), p11.