Life in the Soil - Soil: Reflections on the Basis of our Existence - page 57

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Nature's great larder. Eating soil is a tradition in a wide variety of cultures. Do pregnant women
in Africa eat loamy soil due to calcium or zinc deficiency—or because the clay neutralises toxins
in the stomach and alleviates diarrhoea? Evidence suggests the nutritional value of certain types
of food rises if they are eaten in combination with clay. Acorns are widely ignored as human
food due to their high tannin content. But Native Americans in California and the indigenous
population of Sardinia would mix acorn meal with clay because it neutralised the acorn tannins,
rendering the bread they baked edible. In the West we have used clay as medicine. The highest-
selling diarrhoea remedy in the USA—kaopectate—originally contained kaolin, an active ingre-
dient that is a kind of clay

avoid. Tinned vegetable manufacturers take great care to wash all soil traces away
to ensure no Clostridium bacteria enter the tin. If the contents are not heated for
long enough, the bacteria may start reproducing in the anaerobic conditions inside
the tin, giving off a nerve gas that causes botulism, a life-threatening disease. But
Clostridium bacteria occur in soil in such small concentrations that they pose no
risk to humans who inadvertently swallow a little soil.
Swallowing soil is not confined to children. In some cultures, soil is eaten for
medicinal reasons, which may not be as misguided as it sounds. Modern science
has shown that some plant toxins are neutralised if mixed with loamy soil. It is well
known, for example, that the potato plant is poisonous. Yet the tubers we eat carry
no poison if they grow in the dark so they do not go green. The indigenous wild
potato in South America also had toxic tubers, which indigenous people neutralised
by soaking them in loamy soil slurry (the toxins attached themselves to the loam
particles). Pregnant women in some African tribes eat soil for its calcium, which
they need for breast milk - a custom particularly prevalent among people who do
not keep dairy animals. Many animal species eat soil, both as a source of salt and
other minerals and to purge themselves of toxins absorbed from plant food.
Long known as an important reservoir of biodiversity, soil is difficult to study
because many of its resident organisms are small and hard to detect. The standard way
of measuring biological activity in soil is to analyse the volume of carbon dioxide pro-
duced by soil-dwelling organisms. Microscopic fungi and bacteria contribute the lion's
share, emitting ten times more carbon dioxide than all other animals put together.
I once attended a lecture by a soil microbiologist who told the audience,
“Imagine yourself as a birdwatcher, and the only birds you've seen so far are a
hen, a goose, a budgerigar and a duck. Then someone takes you to a tropical rain-
forest and shows you the array of birdlife there, overwhelming you with all the
brightly coloured parrots and the hummingbirds zipping around from flower to
flower. That's what it's like for soil microbiologists. In the past, we were only able
to study species you could cultivate in a Petri dish, which equates to a few percent
of all the fungi and bacteria that are actually present in the soil. Using modern
techniques, we can extract DNA from soil samples and divide the sequences into
fragments that enable us to identify the organisms. We often discover unique DNA
sequences from species new to science.
Take archaea, for example. They resemble bacteria and used only to be
recorded in extreme environments like salt lagoons, hot springs or cows' stomachs,