130 BACTERIA IN THE SOIL 



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 am)nonia being 

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

 solution of sufficient strength, we have selected conditions very un- 

 favourable to the growth of the nitric organism, and a few cultiva- 

 tions leave the nitrous organism alone in possession of the field " * 

 (Warington). Adeney has summarised conclusions respecting 

 nitrification as follows : (1) In organic solutions containing ammonia 

 nitrous organisms thrive, but nitric organisms gradually lose their 

 vitahty ; (2) nitrous organisms cannot oxidise nitrites to nitrates in 

 inorganic solutions ; (3) nitric organisms thrive in inorganic solutions 

 containing nitrites; (4) the presence of peaty or humous matter 

 appears to preserve the vitality of nitric organisms during the 

 fermentation of ammonia, and estabhshes conditions whereby it is 

 possible for the nitric organisms to thrive simultaneously in the 

 same solution as the nitrous organisms. Other conditions necessary 

 for nitrification are, of course, the presence of ammonia preceding 

 the appearance of nitrous or nitric acid, the presence of a fixed base, 

 not too high a degree of alkalinity, and darkness and free admission 

 of air. 



A word may be said upon the natural distribution of these nitrify- 

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

 water, and sewage. They are also said to be frequently present in 

 well-water. From experiments at Eothamsted it appears that the 

 organisms occur mostly in the first 12 inches, in subsoils of clay down 

 to 3 or 4 feet, and in sandy soils probably at even a greater depth. 

 These facts are of the first importance in relation to the biological 

 treatment of sewage. 



We have now given some consideration to the chief events in the 

 life-cycle of nature depicted in the table (p. 120). 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 a 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 

 into the air in the form of a free gas. This is the last stage of all 

 proteid decomposition, so that wherever putrefaction is going on 

 there is a continual "loss" of an element essential to life. Thus it 

 would appear at first sight that the sum-total of nitrogen food must 

 be diminishing. 



But there are other ways also in which nitrogen is being set free. 

 * Lawes Agricultural Trust Lectures, 1891, p. 63. 



