by the various natural processes
constantly going on, as well as by the crops, and how far their natural
sources are capable of making good this loss--in short, to clearly
understand the economic reasons for the application of artificial
manures.

FOOTNOTES:

[62] See Chapter on Basic Slag.

CHAPTER III.

THE POSITION OF NITROGEN IN AGRICULTURE.

Of manurial ingredients, nitrogen is by far the most important, and on
the presence and character of the nitrogen it contains, the fertility of
a soil may be said to be most largely dependent. Most soils, as a rule,
are better supplied with available ash ingredients than with available
nitrogen compounds. The expensive nature of most artificial nitrogenous
manures also gives to nitrogen the first position from an economic point
of view. A thorough study, therefore, of the different forms in which it
exists in nature, of the numerous and complicated changes it undergoes
in the soil, by which it is prepared for the plant's needs, of the
relation of its different forms to plant-life, and of the natural
sources of its loss and gain, is of the highest importance if we are to
hope to understand the difficult question of soil-fertility.

_The Rothamsted Experiments and the Nitrogen question._

The position of nitrogen in agriculture is a question of great
difficulty and complexity. It has engaged much attention, and has had
devoted to its elucidation much elaborate and painstaking research. To
the Rothamsted experiments we owe most of the information we possess on
the subject, and the facts contained in this chapter are almost entirely
derived from the results of these famous experiments, as embodied in the
memoirs and writings of Messrs Lawes, Gilbert, and Warington.

_Different forms in which Nitrogen exists in Nature._

We have already referred to the nitrogen question in the historical
introduction. In order, however, to have a comprehensive view of the
subject, it may be well to recapitulate some of the facts there
mentioned.

Nitrogen, as we have already seen, exists in the "free" or elementary
condition, as nitrates and nitrites, as ammonia, and in a large number
of different organic forms.

_Nitrogen in the Air._

It occurs in greatest abundance (amounting to about 80 per cent) in the
first of these forms in the air. That this free nitrogen, which is
practically unlimited in quantity,[63] has originally been the source of
all its other forms, i