BACTERIA IN THE SOIL 159 



abundantly distributed in the superficial soils. They act 

 together and in conjunction, and for one common purpose. 

 They are separable by employing favourable media. 



" If we employ a suitable inorganic solution containing potas- 

 sium nitrite, but no ammonia, we shall presently obtain the nitric 

 organism alone, the nitrous organism feeding on ammonia being 

 excluded. If, on the other hand, we employ an ammonium 

 carbonate solution of sufficient strength, we have selected con- 

 ditions very unfavourable to the growth of the nitric organism, 

 and a few cultivations leave the nitrous organism alone in pos- 

 session of the field " (Warington). 



A word upon the natural distribution of these nitrifying 

 bacteria before we leave them. They belong to the soil, 

 river water, and sewage. They are also said to be fre- 

 quently present in well water. From some experiments at 

 Rothamsted it appears that the organisms occur mostly in 

 the first twelve inches, and in subsoils of clay down to three 

 or four feet. In sandy soils nitrification may probably 

 occur at a greater depth. These facts should be borne in 

 mind when arranging for the purification of sewage by in- 

 termittent filtration. 



We have now given some consideration to the chief 

 events in the life-cycle of nature depicted in the table. 

 There is but one further process in which bacteria play a 

 part, and which requires some mention. It will have been 

 noticed that at certain stages in the cycle there is more or 

 less appreciable " loss " of free nitrogen. In the process of 

 decomposition brought about by the denitrifying bacteria, 

 a very considerable portion of the nitrogen is dissipated 



nitrites in an ammoniacal solution can be determined by the character of the 

 bacterial culture with which the solution is seeded, and that in a solution of 

 potassium nitrite conversion into nitrate can be determined by the introduction 

 of the nitric organism. Professor Warington still adheres to the opinion, in 

 favour of which he has produced so much evidence, that the formation of 

 nitrates in the soil is due to the nitric organism which soil always contains. 



