NITROSOMONAS AND NITROBACTER ,39 



case is already fixed and must at some previous period have 

 been derived from atmospheric nitrogen from one of the 

 sources ah'cady mentioned (i-iv). Such material, of wliicji 

 there is a plentiful supply in fertile soils, is known as Immus. 

 Decaying matter contains nitrogen locked up, and normally 

 it serves as food for bacteria and fungi and other saprophytic 

 organisms which contain no chloroi^hyll. Some of these are 

 able to convert the dead nitrogenous matter into forms which 

 are available for the plant to take up. If certain putrefactive 

 organisms are absent or external conditions prevent their 

 activity, decay does not take place and one sometimes finds 

 dried-up mummies of animals who have escaped what is 

 believed to be the universal fate. As animals decay ammonia 

 is set free ; this may escape into the air and be washed down to 

 the soil by the rain. Here the ammonia forms ammonium 

 carbonate. Now there is all over the world a bacterium known 

 as Nitrosomonas, which is capable of converting ammonia into 

 nitrous acid. Nitrous acid can combine with various salts, 

 such as soda, or potash, or lime, to form nitrites, and there is 

 still another bacterium equally widely distributed, known as 

 Nitrobacter, which converts the nitrites into nitrates, the com- 

 monest source of nitrogen for green plants. 



Finally there is one group of bacteria which act in the 

 reverse direction, taking in nitrogenous compounds and 

 liberating the free element. These "de-nitrifying" forms are 

 prevalent in badly aerated or ill-drained soils and probably 

 account in part for their relative sterility. 



For purposes of illustration we may comj^are the organic 

 world with a business in which nitrogen is substituted for 

 money. When the balance sheet is made out, we have on the 

 credit side of the account firstly the nitrogen derived from 

 decaying organic matter by bacterial action, and secondly 

 atmospheric nitrogen rendered available by bacterial and 

 electrical action. On the expenditure side there are two items : 

 one is a very large one, being the constant using up of nitrogen 

 compounds by both animals and plants ; the other is the loss 

 due to the denitrifying organisms. 



In Nature, as in business, the accounts must be made to 

 balance, since if the expenditure is greater than the credit 

 then life cannot continue owino- to lack of food. If the balance 



