Soil and Climate - Soil: Reflections on the Basis of our Existence - page 130

Rainforest soil is poor in nutrients and has limited ability to retain nutrients like
potassium, magnesium and calcium ions as they flush easily away. Another down-
side is the tendency of phosphorus to combine with iron and aluminium to form
insoluble compounds. The rainforest flora is good at absorbing phosphorus at source
when it is released by decomposing leaf litter. But once the phosphate ions seep
down into the iron- and aluminium-rich soil, they quickly become unavailable.
Deep in the rainforests of eastern Ecuador I was told about Terra preta , the
“black soil”. Several million indigenous people inhabited the Amazon basin until
the first white settlers arrived in the sixteenth century. The Spanish conquistador
Francisco de Orellana reported large towns and well developed agriculture while
exploring the Amazon River. But western diseases wiped out so many of the natives
that only a fraction survived. As disease swept the jungle, communities disintegrated
and were abandoned. Behind them, the indigenous inhabitants left Terra preta—one
of the world's most fertile soils. The black soil is the result of the natives' mode of
agriculture, a modified form of slash-and-burn in which they covered fires with earth
and leaves to reduce oxygen flow. As a result, the earth was charred rather than burnt
and charcoal (known as biochar in this context) was left behind.
Biochar enhances soil quality. Many people know that clay and humus-rich soils
are high in nutrients and sandy soils are the opposite. Clay particles and humus have
a large surface area that nutrients can attach to. By contrast, nutrients tend to run
straight through sandy soil. The surface area of a teaspoon of sand grains would be
roughly the size of a table tennis table, while a teaspoon of clay would cover a tennis
court. Biochar is similar to clay in this respect. Magnify it and it looks sponge-like,
with a large surface area for nutrients to attach to. The pores also enhance the soil's
aeration. Adding biochar to a clay soil improves drainage and reduces the risk of
waterlogging, while adding it to a sandy soil helps to stop it from drying out.
Through the millennia, fish and animal bones and faeces and urine from indig-
enous settlements accumulated in the soil along the Amazon River. Nutrients such as
phosphorus, nitrogen and potassium from this human waste attached to biochar, cre-
ating a fertile, phosphorous-rich soil that was farmed for thousands of years. Normal
rainforest soil cannot retain nutrients; almost all nutrients are fixed in the forest flora.
When leaves and branches fall to the ground they quickly decompose and the nutri-
ents are absorbed directly by plants. This is why rainforest clearance is so devas-
tating: when the trees are removed, most of the nutrients disappear with them. It is
usually possible to grow corn for a few years, but the soil becomes rapidly depleted
and then requires 20-30 years to recover. Black soil is different: added nutrients
remain locked in the soil, allowing continuing cultivation with no drop-off in yields.
Plans are afoot to add charcoal to soil to combat climate change, the idea being
to sequester atmospheric carbon dioxide in biomass burned in oxygen-deprived
conditions to produce charcoal. This is a simple technique that would lend itself
to widespread application in developing countries as a means both to improve soil
fertility and sequester carbon dioxide. It would also offer a source of much-needed
income to participant nations by enabling them to join the global trade in emis-
sions rights, which involves rich countries effectively paying poorer nations to
store carbon in their soil. This enables developing countries to improve soil quality
and increase food production at the same time.