Thirdly, as to nitrogen in the form of nitrates. While it is true that plants can absorb nitrogen in certain organic forms and as ammonia salts, it is now a well-known fact that the chief, and by far the most important, source of nitrogen is nitric acid. Probably more than 90 per cent of the nitrogen absorbed by green-leaved plants from the soil is absorbed as nitrates. The tendency of all nitrogen compounds in the soil is towards conversion into nitric acid. It is the final form of nitrogen in the soil. The precise method in which this conversion takes place is a discovery of only a few years' standing. The great economic importance of this discovery, made by the French chemists Schloesing and Muentz, and associated in this country with the names of Warington, Munro, and P. F. Frankland, is only gradually being appreciated. It is without doubt one of the most interesting made in the domain of agricultural chemistry of late years. _Nitrification._ It was in the year 1877 that the two French chemists above referred to published the results of some experiments they had carried out, which proved that nitrification--the name given to the process by which ammonia or other nitrogen salts are converted in the soil into nitric acid--was due to the action of micro-organic life. The basis of the theory rests upon the fact that dilute solutions of ammonia salts or urine, containing all the necessary constituents of plant-food, if previously sterilised, may be kept for an indefinitely long period of time, provided the air supplied be filtered through cotton wool,--so as to prevent the entrance of micro-organisms--without any formation of nitrates. Introduce, however, into such a solution a little fresh soil, and nitrification will soon follow. The conditions under which the nitrification ferment acts, as well as the nature of the ferment, or rather ferments, have subsequently been carefully studied by Schloesing and Muentz, Winogradsy, Deherain, Kellner, and other Continental observers, and especially by Warington, Munro, and P. F. Frankland in this country. These conditions cannot be gone into here. They will be fully discussed in the chapter on Nitrification. Briefly stated, they are a certain range of temperature (between slightly above freezing-point and 50 deg. C., the maximum activity taking place, according to Schloesing and Muentz, at about 30 deg. C.); a plentiful supply of atmosphere oxygen (hence the fact observed by Warington, that nitrification is chiefly limited to the surface-soil); a certain amount of moisture; and the presence of certain of the necessary mineral plant constituents, and the presence of carbonate of lime.