G60 Agricultural Gazette of N.S.TF, [Aug. 3, 1908. 



acid into nitric acid is pcrfoniiod by another type, which completes the 

 complex process of nitrification. It will be seen that it requires quite a number 

 of different bacteria to convert the organic nitrogen compounds into an 

 assimilable form for plants. The different putrifying organisms break up 

 the multiform compounds into elementary substances, and whilst ammonia 

 is evolved this is fastened on to by a distinct oxidising organism ; and then 

 by another distinct type, that cannot oxidise ammonia, it is further oxidised 

 to the final stage. These breaking-up and oxidising bacteria, it must be 

 borne in mind, woik simultaneously from the start of the decay, and in this 

 manner a continuous development of nitric acid is proceeding, always presum- 

 ing that all the types necessary for this evolution are present. As a rule 

 they are present in every soil that is sufficiently aerated. I found the nitri- 

 fying bacteria absent in the samples from very sour or waterlogged localities 

 only, in the many samples of Australian soils I have examined for the purpose. 



The importance of the presence of these different bacteria in the soil, and 

 their co-operation, will readily be perceived. All organic matter would 

 remain intact were it not for the putrifying organisms, and unless the nitrify- 

 ing bacteria were present the nitrogenous gases evolved during the decompos- 

 ing fermentation w^ould escape into the air. It would be useless to add fertilisers 

 to the land, whether in the shape of green plants, stable manure, or artificial 

 nitrogenous manures (except such containing nitric acid), were it not for the 

 activity of these silent workers. The whole tribe of this soil flora is necessary 

 to prevent a loss of plant food ; but if a preference of importance may be 

 assigned to any of them it is the nitrous-acid former which fixes the very 

 volatile ammonia. 



These infinitesimally minute plants, which persistently though invisibly 

 retain by their activity in the soil the most precious of plant foods, cannot 

 be too highly esteemed, for upon them depends entirely the productiveness 

 of the land, and the sustenance of life generally. 



A few words concerning the effect produced by tilth, and consequent 

 aeration upon the soil flora, is not out of place here. The greatest value of 

 aeration is generally attributed to the oxidising effect it has upon the 

 mineral plant foods contained in the soil, by which these become soluble, and 

 consequently assimilable by the crops. But the effect it has upon the bacteria 

 in the soil, probably, is no less important. The nitrifying bacteria require a 

 liberal supply of oxygen dictated by their functions, whilst on the other 

 hand the greater number of the putrifying bacteria are anaerobic. Aeration 

 therefore encourages the multiplication of the nitrifying bacteria, which is 

 of the utmost importance, as their number is always smaller than that of any 

 other kind, owing to their much slower grow'th and reproduction, and at the 

 same time the development of the anaerobic putrifying bacteria is retarded 

 by the admission of air into the soil and a too violent fermentation may be 

 prevented. Aeration balances the functions of the different kinds of soil 

 bacteria. 



