ant source
of plant-food. Reference has already been made to the early theory of
Sir Kenelm Digby regarding the value of nitrates.[28] De Saussure, as we
have also already seen, was fully impressed with the importance of
applying nitrogen to the soil as a manure. Liebig's early attitude on
this question was to the effect, that to apply nitrogen in manures was
quite unnecessary, as the plant had a sufficient source in the ammonia
present in the air, which he erroneously supposed was sufficient in
quantity to supply all the needs of the crops. Despite this early
recognition of the value of combined nitrogen to the plant, it is only
of recent years that we have obtained any definite knowledge as to the
respective value of its different compounds as manures, or as to the
form in which it is assimilated by the plant. It exists in three
forms--(1) as organic nitrogen; (2) as ammonia salts; (3) as nitrates
and nitrites. Much experimental work has during late years been devoted
to studying the comparative action and merits of these three forms.

_Relation of Organic Nitrogen to the Plant._

First, as to the relation of organic nitrogen to the plant. There is a
large number of different organic compounds which contain nitrogen. That
the plant is able to assimilate certain of these organic compounds,
seems, from several experiments, to be extremely probable. From certain
researches, carried out as far back as the year 1857, Sir Charles
Cameron concluded that the plant could assimilate one of them--viz.,
_urea_. From what, however, we have subsequently learned regarding the
process of "nitrification," it is quite probable that the nitrogen in
these experiments was first converted into nitrates before being
assimilated. At any rate, as the plants were not tested for urea, the
experiments must be regarded as leaving the problem unsolved.

Other experiments were carried out of a similar nature by Professor S.
W. Johnson, the different kinds of nitrogen experimented with being
_uric acid_, _hippuric acid_, and _guanine_. But here, again, no
definite conclusion can be drawn, as no analyses were made of the
plants. More recently, however, Dr Hampe has carried out experiments
with _urea_, _uric acid_, _hippuric acid_, and _glycocoll_. These
experiments may be held as demonstrating the fact that at least one
organic compound of nitrogen is capable of being assimilated, as urea
was actually identified as being present in the plants exp