Peruvian guano 151
Bones 151
Other nitrogenous manures 152
Oil-seeds and oilcakes 153
Other imported sources of nitrogen 153
Conclusion 153

APPENDIX TO CHAPTER III.
NOTE
I. Determination of the quantity of nitrogen supplied by
rain, as ammonia and nitric acid, to an acre of land
during one year 155
II. Nitrogen in soils at various depths 156
III. Nitrogen as nitrates in cropped soils receiving no
nitrogenous manures, in lb. per acre (Rothamsted soils) 157
IV. Nitrogen as nitrates in Rothamsted soils 157
V. Examples of increase of nitrogen in Rothamsted soils
laid down in pasture 158
VI. Loss by drainage of nitrates 158
VII. Examples of decrease of nitrogen in Rothamsted soils 159
VIII. Amount of drainage and nitrogen as nitrates in
drainage-water from unmanured bare soil, 20 and 60
inches deep 160

CHAPTER IV.--NITRIFICATION.

Process of nitrification 161
Occurrence of nitrates in the soil 162
Nitre soils of India 162
Saltpetre plantations 163
Cause of nitrification 165
Ferments effecting nitrification 167
Appearance of nitrous organisms 168
Nitric organism 169
Difficulty in isolating them 169
Nitrifying organisms do not require organic matter 169
Conditions favourable for nitrification--
Presence of food-constituents 170
Presence of a salifiable base 171
Only takes place i