The phenomena of vegetation, no less than those of the animal world,
had, however, during the last fifty years been placed by the chemist
on an entirely new basis.
Liebig, in 1860, asserted that the whole of the carbon of vegetation
was obtained from the atmospheric carbonic acid, which, though only
present in the small relative proportion of four parts in 10,000 of
air, was contained in such absolutely large quantity that if all the
vegetation on the earth's surface were burned, the proportion of
carbonic acid which would thus be thrown into the air would not be
sufficient to double the present amount. That this conclusion was
correct needed experimental proof, but such proof could only be given
by long-continued and laborious experiment.
It was to our English agricultural chemists, Lawes and Gilbert, that
we owed the complete experimental proof required, and this experiment
was long and tedious, for it had taken forty-four years to give a
definite reply.
At Rothamsted a plot was set apart for the growth of wheat. For
forty-four successive years that field had grown wheat without the
addition of any carbonized manure, so that the only possible source
from which the plant could obtain the carbon for its growth was the
atmospheric carbonic acid. The quantity of carbon which on an average
was removed in the form of wheat and straw from a plot manured only
with mineral matter was 1,000 lb., while on another plot, for which a
nitrogenous manure was employed, 1,500 lb. more carbon was annually
removed, or 2,500 lb. of carbon were removed by this crop annually
without the addition of any carbonaceous manure. So that Liebig's
prevision had received a complete experimental verification.