[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. [120] See Appendix, Note VI., p. 198. [121] For full analytical results see Appendix, Note VII., p. 198. [122] We find the least amount in the month of April. In the water, from a 20-and 60-inch gauge respectively, the amounts were 1.35 lb. and 1.61 lb. per acre (rainfall 2.25 inches). From then on to November the amount steadily increases. In the latter month it reaches its maximum--viz., 6.50 lb. (20-inch gauge) and 5.98 lb. (60-inch gauge) per acre (rainfall 2.30 inches). See Appendix to Chapter III., Note VIII, p. 160. APPENDIX TO CHAPTER IV. NOTE I. (p. 162). OLD THEORIES OF NITRIFICATION. According to the old theories, nitrification was regarded as a simple case of the oxidation of nitrogen by the oxygen of the air, or by ozone. The union of nitrogen and oxygen, however, probably takes place only at very high temperatures, such as are formed during electric discharges. It is needless to point out that the union of nitrogen and oxygen in this way is not likely to occur in soils. According to other theories, nitrification was effected by means of the oxidation of ammonia. Ammonia, however, can only be oxidised to nitric acid by means of certain powerful oxidising agents, such as ozone or hydrogen peroxide. As, however, these substances are not found in the soil, it is much to be doubted whether nitric acid is ever formed in the soil in this way. It is possible, however, as held by some, that ferric oxide is capable of inducing this conversion. On the whole, however, most evidence points to the conclusion that all nitric acid produced in the soil is formed through the agency of micro-organic life. NOTE II. (p. 170).