NEWS
volves time, labour and machinery that
they can’t afford.
But it only takes hours or a few days to
collect residues and make biochar. And
it doesn’t have to involve expensive
technology.
When biomass is exposed to tempera-
tures between 300°C and 800°C with lit-
tle, or no, oxygen present it is converted
into combustible gases, oils and bio-
char. There is a broad range of systems
that can carry out this process. Some
are basic, such as domestic cook stoves
that have a chamber with a natural or
forced air draft. Other systems may
be larger and more complex, generat-
ing heat and electricity through highly
controlled processes. These can serve
whole communities.
Using these systems, biochar can be
generated from crop residues and
mixed directly into agricultural soils, if
done cautiously.
Potential of biomass waste
In our study, we measured the resi-
dues produced by five crops over
two growing seasons. These five crops
form the major food staples across our
study area and other parts of Uganda.
For maize and sorghum we sampled
two quadrants of 4m² in 14 fields. For
rice, millet and groundnut we studied
four quadrants of 1m² in 25 fields.
We interviewed 60 farmers and record-
ed how many of them use specific crop
residues for animal fodder, as cooking
fuel, to put on soil and for construction.
This revealed between 39% and 60% of
cereal straw is potentially available for
biochar, as is 88% to 100% of non-straw
residues like shanks, chaff and hulls.
Nearly all respondents said they burned
crop residue to get rid of it.
We then worked out how much carbon
could be stored in the soil of smallhold-
er farms using the available biochar.
This analysis showed there is a lot of po-
tential to absorb carbon dioxide from
the air under a wide range of farming
scenarios.
We found that biochar derived from res-
idues of the studied crops could com-
pensate for 19% to 77% of all green-
house gas emissions from Uganda each
year, depending on how much residue
is left unused by farmers.
From potential to action
Sub-Saharan African countries have the
ability to remove large amounts of car-
bon from the atmosphere with biochar
from crop residues. But there’s a long
way to go before farmers start using this
technique.
First, more awareness must be created
about its benefits for climate, agricul-
ture and energy.
It will also take a great effort to adapt bi-
omass transport models, gasifier energy
appliances and biochar farming practic-
es to suit different agricultural, ecologi-
cal and socioeconomic contexts.
The economic viability of sequester-
ing carbon in soils through biochar has
proved to be limited in parts of the
world where benefits for agriculture and
energy are less pronounced.
Insights like those from our study point
to a bigger role for sub-Saharan Africa
in adopting the practice. The debate
and research on biochar needs to take
this into account.
Photo Competition Entry: Research in Action
Early morning start to Phymaspermum acerosum herbicide control trial Photo: Michelle Keith
21
Grassroots
Vol 19
No 4
November 2019