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 in slightly alkaline solutions 172 Action of gypsum on nitrification 173 Presence of oxygen 173 Temperature 175 Presence of a sufficient quantity of moisture 176 Absence of strong sunlight 176 Nitrifying organisms destroyed by poisons 176 Denitrification 177 Denitrification also effected by bacteria 178 Conditions favourable for denitrification 178 Takes place in water-logged soils 179 Distribution of the nitrifying organisms in the soil 179 Depth down at which they occur 180 Action of plant-roots in promoting nitrification 181 Nature of substances capable of nitrification 181 Rate at which nitrification takes place 183 Nitrification takes place chiefly during summer 183 Process goes on most quickly in fallow fields 184 Laboratory experiments on rate of nitrification 185 Certain portions of soil-nitrogen more easily nitrifiable than the rest 187 Rate of nitrification deduced from field experiments 187 Quantity of nitrates formed in the soils of fallow fields 188 Position of nitrates depends on season 188 Nitrates in drainage-waters 188 Amount produced at different times of year 189 Nitrification of manures 190 Ammonia salts most easily nitrifiable 191