126 BACTERIA IN THE SOIL 



Without it the reverse process, denitrification, occurs, and instead 

 of a building up we get a breaking down, with an evolution of 

 nitrogen gas. The amount of oxygen present has an intimate pro- 

 portion to the amount of nitrification, and with 16 to 21 per cent, of 

 oxygen present the nitrates are more than four times as much as 

 when the smallest quantity of oxygen is supplied. The use of tillage 

 in promoting nitrification is doubtless in part due to the aeration of 

 the soil thus obtained. 



3. A third condition is the presence of a base with which nitric 

 acid when formed may combine. Nitrification can only take place 

 in a feebly alkaline medium, but an excess of alkalinity will retard 

 the process. 



4. The last requirement is a favourable temperature. As low as 

 37 or 39 F. (3-4 C.) will suffice, but at a higher temperature it 

 becomes much more active. According to Schlosing and Miintz, at 

 54 F. (12 C.) nitrification becomes really active, and it increases 

 as the temperature rises to 99 F. (37 C.), after which it falls. A 

 high temperature or a strong light are prejudicial to the process. 



We are now in a position to consider shortly some of the char- 

 acters of these nitrification bacteria. They may readily be divided 

 into two chief groups, not in consideration of their form or biological 

 characteristics, but on account of the duties which they perform. 

 Just as we observed that there were few denitrifying organisms 

 which could break down ammonia compounds to nitrogen gas, so is it 

 also true that there are few nitrifying bacteria which can build up 

 from ammonia to the nitrates. Nature has provided that this shall 

 be accomplished in two stages, viz., a first stage from ammonia bodies 

 to nitrites (nitrosification), and a second stage from nitrites to 

 nitrates. The agent of the former is termed the nitrous organism, 

 the latter the nitric organism. Both are contributing to the final 

 production of nitrates which can be used by plant life.* 



The Nitrous Organism (Mtrosomonas). Prior to Koch's gelatine 

 method the isolation of this bacterium proved an exceedingly difficult 

 task. But even the adoption of this isolating method seemed to give 

 no better results, and for an excellent reason : the nitrifying 

 organisms will not grow on gelatine. To Winogradsky f and Percy 

 FranklandJ belongs the credit of separately isolating the nitrous 

 organism on the surface of gelatinous silica containing the necessary 



* The saltpetre beds of Chili and Peru are an excellent example of the applica- 

 tion of these facts. Nitrates are there produced from the faecal evacuations of sea- 

 fowl in such quantities as to form an article of commerce. A like form of utilisation 

 of the action of these bacteria was once practised on the continent of Europe. 

 Considerable nitrate deposits have recently been discovered in California, Economic 

 application is also seen in the treatment of sewage referred to elsewhere. 



f Ann. de VInst. Pasteur, 1890, p. 213. 



$.Phil. Trans. Roy. Soc., 1890, B. 107. 



