ty years ago, when Chemistry and Vegetable Physiology began to be
applied to Agriculture, the opinion was firmly held among scientific
men, that the organic parts of humus--by which we understand decayed
vegetable matter, such as is found to a greater or less extent in all
good soils, and _abounds_ in many fertile ones, such as constitutes the
leaf-mold of forests, such as is produced in the fermenting of stable
manure, and that forms the principal part of swamp-muck and peat,--are
the true nourishment of vegetation, at any rate of the higher orders of
plants, those which supply food to man and to domestic animals.

In 1840, Liebig, in his celebrated treatise on the "Applications of
Chemistry to Agriculture and Physiology," gave as his opinion that these
organic bodies do not nourish vegetation except by the products of their
decay. He asserted that they cannot enter the plant directly, but that
the water, carbonic acid and ammonia resulting from their decay, are the
substances actually imbibed by plants, and from these alone is built up
the organic or combustible part of vegetation.

To this day there is a division of opinion among scientific men on this
subject, some adopting the views of Liebig, others maintaining that
certain soluble organic matters, viz., crenic and apocrenic acids are
proper food of plants.

On the one hand it has been abundantly demonstrated that these organic
matters are not at all essential to the growth of agricultural plants,
and can constitute but a small part of the actual food of vegetation
taken in the aggregate.

On the other hand, we are acquainted with no satisfactory evidence that
the soluble organic matters of the soil and of peat, especially the
crenates and apocrenates, are not actually appropriated by, and, so far
as they go, are not directly serviceable as food to plants.

Be this as it may, practice has abundantly demonstrated the value of
humus as an ingredient of the soil, and if not directly, yet indirectly,
it furnishes the material out of which plants build up their parts.

2. _The organic matters of peat as indirect food to plants._ Very nearly
one-half, by weight, of our common crops, when perfectly dry, consists
of _carbon_. The substance which supplies this element to plants is the
gas, carbonic acid. Plants derive this gas mostly from the atmosphere,
absorbing it by means of their leaves. But the free atmosphere, at only
a little space above the soil, contains o