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 Müntz, 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. Müller 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.
[74] The comparatively insignificant effect the addition of various nitrogenous manures have in increasing the total soil-nitrogen is strikingly illustrated in the tables given in the Appendix, Note IV., p. 157.
[75] See Storer's Agric. Chem., vol. i. p. 357.
[76] See Chapter IV., Appendix, Note VII., p. 198.
[77] See Appendix, Note III., p. 157.
[78] See Appendix, Note IV., p. 157.
[79] See Appendix, Note I., p. 155.
[80] The original source of the nitrogen in the soil must have been the nitrogen in the air. When plants first begin to grow on a purely mineral soil, they must obtain nitrogen from some source. The small traces washed down in the rain will supply sufficient nitrogen to enable a scanty growth of the lower forms of vegetable life; whereas these by their decay furnish their successors with a more abundant source, which rapidly increases, until we have a fair percentage of humus accumulated.
[81] See Appendix, Note V., p. 158.
[82] See Historical Introduction, pp. 40-45.
[83] The evidence demonstrating this is to be found in the fact that the amount of carbon found in different soils rises or falls in proportion to the nitrogen. See p. 126.
[84] See Chapter IV. on Nitrification.
[85] Diffusion as well as capillary attraction is a means of bringing nitrates again to the surface-soil after rain.
[86] See Appendix, Note VI., p. 158, and Note VIII., p. 160; also p. 154.
[87] See Appendix, Note VII., p. 159.
[88] See following Chapter on Nitrification, p. 178.
[89] According to the Agricultural Returns for 1888, the number of cows in milk in Great Britain amounted to 2,450,444. If we multiply this number by 22 the result is 54,000,000 lb., or in tons 24,107. This quantity represents 154,067 tons of ordinary commercial nitrate of soda.
[90] For 1878 (p. 146 et seq.) The reader interested in the subject is referred to the paper itself.
[91] In tons 4464, and represents 28,530 tons of nitrate of soda.
[92] This in tons 162,946, which represents 1,041,384 tons of nitrate of soda.
[93] This in tons 40,625, which represents 259,633 tons of nitrate of soda. See paper in 'Journal of Science' already referred to.
[94] Europe's total production may be stated at 200,000 tons.
[95] 10,500 tons of which were as guano.
[96] Mr Warington estimates this at about 8 lb. See p. 141.