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QUESTION:I think it is a redox reaction, as many thousand years old clay that never
has been exposed to oxygen or any microorganisms is blue. After e few days in
contact with air it becomes grey. Are we talking topsoil or subsoil in this instance ? If it is topsoil
('black ground' for out Scottish subscribers) that is under discussion
then I must let others such as Oz, who has a pracitcal understanding
of the paper-thin covering of live soil that he farms, answer the
demanding question as to why the soil on the tractor-tyre changes
colour overnight. that a tractor should not be subjected to, instead of his usual
activity of x-posting to uk.people.rural, Anyway, Andrew, that's the answer to your question. It oxidises,
unless it's topsoil you were talking about, in which case I don't
have a clue.
ANSWER: I have researched this further (mostly in a conversation with a
biologist working at Butser as he drove me to the arable farm which
hosts UKagriculkture.com with *no* outside support, an invitation
which I accepted after my post on the subject to the group).
Jon suggests it is a bacteria which digests iron. Its energy source is
in oxidising the ferric to ferrous, in anaerobic conditions the
resulting blueing occurs. Further interesting point is that the
bacteria and dogs mercury are mutually exclusive as a result of the
presence of ferric ions. This is a fast reaction as today the 3wheel
drive Case failed to get into a new piece of the site. We gave up
immediately and used the Kubota mini excavator to carry the stakes in
(and it was surprisingly good at pressing them into the ground). By
4:30 when I retrieved the tractor the area the area directly under the
tyre was blued.
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