In conclusion, it may be asked how far are the artificial sources of nitrogen able to make good the loss? In the opinion of such a reliable authority as Sir John Lawes, they do not. There are some soils which depend almost entirely upon imported fertility, and could not be cultivated without it. Upon some of them it is possible that the imports of nitrogen are in excess of the exports. Taking the agricultural acreage as a whole, however, he is of opinion that there is a decided loss of nitrogen, which he estimates at _from 15 lb. to 20 lb. per acre per annum_.[96] FOOTNOTES: [63] The total amount of nitrogen in the air has been estimated approximately at four million billion tons. [64] See Introductory Chapter, pp. 40 to 45. [65] Although ammonia is more abundant than nitrates and nitrites, it only amounts to a few parts per million of air. According to Muentz, the air at great heights contains more ammonia than in its lower strata. The opposite, however, is the case with regard to nitrates, which are only found in air near the surface of the earth. See p. 49. [66] Nitric acid may also be formed by the oxidation of ammonia by ozone, or peroxide of hydrogen. [67] According to Schloesing, the chief source of the ammonia present in the air is the tropical ocean, which yields gradually to the atmosphere, under the action of the powerful evaporation constantly going on, a large amount of nitrogen in this form. The sources of the nitrogen of the ocean are the nitrates which it receives from the drainage of land, animal and vegetable matter, sewage, &c. [68] See Appendix, Note I., p. 155. [69] To illustrate this point, it may be mentioned that on the least windy of days, when the wind is only moving at the rate of two miles an hour--and this, it may he added, is so slow as to be scarcely noticeable--the air in a space of 20 feet is changed over five hundred times in an hour. The combined nitrogen thus absorbed is probably entirely in the form of ammonia. It would seem so at any rate, from some experiments by Schloesing. See p. 132. [70] No vegetable or animal cell exists which does not contain nitrogen. [71] This is less on the whole than what has been found in subsoils by Continental investigators. Thus, for example, A. Mueller found the average of a number of analyses of subsoils to be .15 per cent., and the late Dr Anderson found the nitrogen in the subsoil of different Scottish wheat-soils to run from .15 per cent to .97 per cent. [72] See Appendix, Note II., p. 156. [73] "Under prolonged kitchen-garden culture the subsoil becomes enriched with nitrogenous matter to a far more considerable depth; this has been shown by the analyses of the soil of the old kitchen-garden at Rothamsted. This is doubtless due to the practice of deep trenching employed by gardeners."--R. Warington, 'Lectures on Rothamsted Experiments.' U.S.A. Bulletin, p. 24.