[110] See Appendix, Note II., p. 196, and Note III., p. 197.
[111] See Appendix, Note V., p. 198.
[112] This is shown by the fact that nitrification will only continue in a solution of carbonate of ammonia till one-half the ammonia is nitrified. It then stops. The base, with which the nitrous acid combines as it is formed, being at that stage entirely used up, nitrification is no longer possible. With regard to urine solutions the same is the case. Nitrification thus will only take place where there is a sufficiency of base.
[113] See Appendix, Note IV., p. 197.
[114] It would seem that an alkalinity much exceeding four parts of nitrogen per million is prejudicial to the process.
[115] According to Warington, solutions containing 50 per cent of urine become nitrifiable when sufficient gypsum is added. The gypsum neutralises the alkalinity of nitrifying solutions by converting the alkaline ammonium carbonate into neutral ammonium sulphate, the calcium carbonate being precipitated.
[116] See Chapter on Farmyard Manure.
[117] As practically illustrating this fact, a solution kept at 10° C. required ten days, while a solution kept at 30° C. required only eight days for nitrification.
[118] In sixty-nine trials no failure to produce nitrification by seeding with soil from a depth, of 2 feet was experienced. Similarly in eleven trials only one failure took place with soil from a depth of 3 feet. With clay soil from a depth of 6 feet success took place to the extent of 50 per cent. No nitrification was obtained with clay from a depth of 8 feet. Entire failure was experienced with chalk subsoil. The process thus diminishes in activity the lower down we go.
[119] Koch has found that in soils he has examined few organisms were found at a depth below 3 feet.