74 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



very much less than nitrates, and ammonia none at all ; therefore to change 

 the ammonia into nitrites, and nitrites into nitrates, a process of oxidation 

 is required, and this is what the nitrifying bacteria perform, the process 

 being termed nitrification. Although they are definite and different pro- 

 cesses, yet denitrification and nitrification dovetail into each other, and in a 

 manure heap, for example, both processes may be going on concomitantly. 



The conditions necessary for nitrification are (1) the presence of 

 oxygen in excess of the supply of organic matter, for without it the 

 reverse process of denitrification occurs, and instead of a building up we 

 get a breaking down. The use and value of tillage in promoting nitri- 

 fication is, no doubt, due to the aeration the soil gets. The second (2) con- 

 dition is the presence of a base with which the nitric acid when formed 

 may combine. The presence of bases in the soil also neutralises the 

 resulting nitrous or nitric acid, and they thus protect the bacteria from 

 injury from this source. In the soil this work is usually performed by the 

 calcium cabonate or lime that generally is present. The third (3) condition 

 is a favourable temperature. The nitrifying organism can act at a tem- 

 perature as low as 37° F., but at 54° F. it becomes really active, and its 

 activity increases till the temperature rises to 99° F., after which it falls. 

 A very high temperature or a strong light is prejudicial to its action. The 

 fourth (4) condition is the presence of suitable food materials, phosphates 

 being essential ; but the main point of interest in this connection is that 

 the nitrifying organisms, or, at any rate, some of the species of nitrifying 

 organisms, can apparently feed with equal ease on organic or inorganic 

 matter. Again, like the denitrifying bacteria, there is division of labour 

 among the nitrifying bacteria, and they are classifiable into two sharply 

 divided sub-groups, the nitroso-bacteria and the nitro-bacteria. The 

 nitroso-bacteria oxidise ammonia to nitrous acid and nitrites, but go no 

 further. For example, they are unable to alter nitrites. On the other 

 hand, the nitro-bacteria cannot attack ammonia, but alter nitrous acid 

 into nitric acid, nitrites into nitrates. 



We have progressed so far in our knowledge of the nitrifying bacteria 

 that two types of the nitrous organism — the nitroso-bacteria — can be 

 differentiated. One is found in all the soils of Europe, Asia, and Africa, 

 and is termed nitroso-monas, while the other type is found in Australia 

 and America alone, and is termed nitroso-coccus. 



The nitric organism — the nitro-bacteria (or nitro-monas, as it was 

 once termed) — is allied to the nitrous organism, but so far no subdivision 

 has been attempted. This may be due to the fact that they are the 

 smallest of all known organisms. Both the nitrous and nitric organisms 

 are widely and abundantly distributed in soils. The nitroso-bacteria are, 

 however, the more active of the two, the nitro-bacteria, on the other 

 hand, immediately oxidising the product made by the first. The one 

 converts ammonia into nitrite, the other converts this nitrite into nitrate. 



We have seen that in the decomposition brought about by denitrifying 

 bacteria a portion of the nitrogen is dissipated into the air in the form of 

 a free gas. To complete the nitrogen cycle this " loss " must be recovered, 

 and to perform this vital function is the duty of the nitrogen-fixing 

 bacterial the third group of economic bacteria in our list. These micro- 

 organisms exist in groups and colonies in the nodules, which appear on 



