[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.