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, is of course obvious. But this conversion of free nitrogen into the various compound forms in which it occurs throughout the mineral, vegetable, and animal kingdoms, has been a process effected by a variety of indirect methods, and only at the expense of a vast amount of time. For practical purposes, the free nitrogen of the air may be regarded chiefly as a non-available source for most bodies containing it. It may be described as of all forms of nitrogen the least active, as far as plant-life is concerned. _Relation of "free" Nitrogen to the Plant._ The relation of the "free" nitrogen to the plant has formed the subject of much research, more especially during the last few years, and a brief epitome of the main results arrived at has already been given in the Introductory Chapter.[64] That this source of nitrogen is not so inaccessible to the plant as was formerly believed, has now been abundantly proved. As the considerations which have led to this conclusion, and have suggested the very recent elaborate experiments on the fixation of free nitrogen by the plant--the results of which bid fair, it would seem, to largely revolutionise our agricultural practice--have been due to the study of the relation of the soil-nitrogen to the plant, it will be best to defer further discussion of this question till we have dealt with the other sources of nitrogen. _Combined Nitrogen in the Air._