to
controvert the humus theory, he certainly dealt it its death-blow. He
reasserted de Saussure's conclusions, and by some simple calculations
showed very clearly that it was wholly untenable. One of the most
striking of the arguments he brought forward was the fact that the humus
of the soil itself consisted of the decayed vegetable matter of
preceding plants. This being so, how, he asked, could it be the original
source of the carbon of plants? To reason thus was simply to reason in a
circle. He pointed out, further, that the comparative insolubility of
humus in water, or even in alkaline solutions, told against its
acceptance as correct.

_His Mineral Theory._

Having thus controverted the humus theory, he then goes on to deal with
the question of the source of the various plant constituents. In
treating of the relation of the soil to the plant, he puts forward his
"mineral" theory. It cannot be doubted that, while the advance of
science since Liebig's time has induced us to considerably modify his
mineral theory, it contained the statement of one of the most important
facts in the chemistry of plant physiology. He was the first to fully
estimate the enormous importance of the mineral portion of the plant's
food, and point the way to one of the chief sources of a soil's
fertility. Up to this period the ash constituents had been generally
considered to be of minor importance. By emphasising the contrary
opinion, and insisting upon their essentialness to plant-life, he gave
to agricultural research a fresh impetus upon the right lines. His
statement of his mineral theory was in the main true, but was not the
whole truth.

De Saussure, as has already been pointed out, to a certain extent,
anticipated Liebig's mineral theory. He was of the opinion that whatever
might be the case with some of the mineral constituents of plants,
others were necessary, inasmuch as they were always found in the ash. Of
these he instanced the alkaline phosphates. "Their small quantity does
not indicate their inutility," he sagaciously remarks. Sir Humphry Davy,
as has already been pointed out, missed recognising the true importance
of the ash constituents. It was left to Liebig, then, to restate the
important doctrine of the essentialness of the mineral matter, already
implied to some extent by de Saussure.

Liebig says: "Carbonic acid, water, and ammonia are necessary for the
existence of plants, because they contain the elements fro