What has all along characterised these valuable experiments has been their practical nature. While their aim has been entirely scientific, the scale of the experiments and the conditions under which they have been carried out, have been such as to render them essentially _technical_ experiments. For this reason their results possess, and will always possess, a peculiar interest for every practical farmer. The greatest services the Rothamsted experiments have rendered agricultural chemistry have been the valuable contributions they have made to our knowledge of the function of nitrogen in agriculture; its relation in its different chemical forms to plant-life; and the sources of the nitrogen found in plants. Researches of a most elaborate nature have been carried out on what is still one of the most keenly debated questions of the present hour--viz., the relation of the "free" nitrogen in the atmosphere to the plant. Of the very highest value also have been the elaborate researches of Mr R. Warington, F.R.S., on the important question of _Nitrification_, which have been in course in the Rothamsted Laboratory for the last fifteen years, and to which full reference will be made in the chapter on Nitrification. To the Rothamsted experiments also we owe the refutation of Liebig's mineral theory. In fact it may safely be said that no experimenters in the field of agricultural chemistry have made more numerous or valuable contributions to the science than these illustrious investigators. _Review of our present Knowledge of Agricultural Chemistry._ Some attempt may now be made to indicate briefly our present knowledge of the more important facts regarding plant physiology, agronomy, and manuring. _Proximate Composition of the Plant._