Pierre, Bineau, and Ville. According to Ville's researches, which are
among the most recent, the amount does not exceed 30 _parts per thousand
million parts of air_.[29] Some conception of the value of this source
of nitrogen may be gained by estimating the quantity falling, dissolved
in rain, on an acre of soil throughout the year. Various estimations of
the total amount of combined nitrogen, which is in this way brought to
the soil, have been made. A certain amount of discrepancy, it is true,
is to be found in these various estimations, no doubt largely due to the
difference in the circumstances under which the investigations were
carried out. Mr Warington has made several investigations at Rothamsted,
and, according to his most recently published figures, the total
quantity only amounts to 3.37 lb. per acre per annum--of which only 2.53
lb. is as ammonia itself.[30]

As already mentioned, there can be little doubt that plants can absorb
nitrogen in the form of ammonia. The question of how far plant-leaves
are able to absorb ammonia is a much debated one. It is probable that if
they can do so, it is only to a very small extent.[31] The question as
to whether the plant's roots can absorb ammonia or not, is also a very
keenly debated one. The point is a very difficult one to decide, and is
much complicated by the consideration that ammonia, when applied to the
the soil, is so speedily converted into nitric acid. Despite, however,
these difficulties, and the vast amount of controversy on the point, the
experiments of Ville, Hosaeus and Lehmann, seem to indicate beyond doubt
that ammonia is a direct source of nitrogen. Lehmann's experiments would
seem, further, to indicate that there are certain periods of a plant's
growth when its preference for ammonia salts seems to be greater than at
other times. The point, however, it must be confessed, is still an
obscure one. The great difficulty in deciding it, as has just been said,
lies in the fact that ammonia salts, when applied to a soil, are, by the
process of nitrification, converted into nitrates. In experimenting,
therefore, with ammonia, and noting the results, it is wellnigh
impossible to say, except by subsequent analyses, whether the nitrogen
in the ammonia salts has not been converted into nitrates before
assimilation.

_Relation of Nitric Acid to the Plant._

Thirdly, as to nitrogen in the form of nitrates. While it is true that
plants can absorb nitrogen in c