Scientific Agriculture, 



[November, 1908. 



but the ammonia, unless the fermenta- 

 tion is very violent, and the bacteria 

 cannot consume it, is fastened on to by 

 a certain type of organism an<t con- 

 verted by it into nitrons acid. This is 

 the first stage of nitrification, the 

 organisms bringing the ammonia to a 

 lower state of oxidation than that, re- 

 quired by the higher plants. The 

 further oxidation of nitrous acid into 

 nitric acid is performed by another type, 

 which completes the complex process of 

 nitrification. Ib will be seen that it 

 requires quite a number of differeut 

 bacteria to convert the organic nitrogen 

 compounds into an assimilable form for 

 plants. The differeut putrifying organ- 

 isms 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 break- 

 ing-up and oxidising bacteria, it must 

 be borne in mind, wor k 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 suffi- 

 ciently aerated. I found the nitrifying 

 bacteria absent in the samples from very 

 sour or waterlogged localities only, in 

 the many samples of Australian soils I 

 have examiutd lor 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 putri- 

 fying organisms, and unless the nitri- 

 fying bacteria were present the nitro- 

 genous gases evolved during the decom- 

 posing fermentation would escape into 

 the air. It would be useless to add 

 fertilizers 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 effects 

 produced by tilth, and consequent aera- 

 tion upon the soil flora, is not out of 

 place here. The greatest value of aera- 

 tion is generally attributed to the 

 oxidising effect it has upon the mineral 

 plant foods contained in the soil, by 

 which these become soluble, and conse- 

 quently assimilable by the crops. But 

 the effect it has upon the bacteria 

 in the soil, probably, is no less im- 

 portant. The nitrifying bacteria re- 

 quire a liberal supply of oxygen 

 dictated by their functions, whilst on 

 the other haud the greater number of 

 the putrifying bacteria are anaero- 

 bic. Aeration therefore encourages 

 the multiplication of nitrifying bacteria, 

 which is of the utmost importance, as 

 their number is always smaller than 

 that of any other kiud, owing to their 

 much slower growth 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. 



Besides the nitrifying bacteria, a 

 series of other organisms occur in soil, 

 the activity of which is even more 

 mysterious and which benefit higher 

 life to a considerable extent. Tnese 

 are the 



Nitrogen-fixing Bacteria, 



The inertness of nitrogen has already 

 been mentioned, and, in reality, only 

 two phenomena in nature are known 

 to oxidise and fix it, The one is the 

 discharge of electricity during thunder- 

 storms, and the other the activity of 

 certain bacteria. At the high temper- 

 ature produced by the lightning flash 

 the nitrogen combines with the oxygen 

 of the air, How the bacteria fix the 

 elementary nitrogeuis an entire mystery. 

 Nevertheless quite a number have been 

 credited with the characteristic of being 

 able to fix nitrogen in a greater or 

 lesser degree. European investigators 

 have proved one or the other, or 

 several, to occur in almost every kind 

 of soil. All Australian soils probably 

 contain similar organisms. In two 

 examined by me I found considerable 

 assimilating power. The quantitative 

 fixation of nitrogen from the air is not 

 very great in these free living organisms, 

 but study may reveal substances that 

 will encourage their multiplication, and 

 consequently produce greater fixation 

 results. They are naturally all aerobic, 

 and therefore thrive best in open, well 

 aerated soils, A greater assimilating 



