CHAPTER XXXVI. 



THE NITRIFYING BACTERIA. 



203. The Recognition of Nitrification as a Physiological 



Process. 



THE nitrogen excreted from the animal body as urea has not, when converted 

 into ammonium carbonate (see chap, xxxii.), yet attained the form in which it is 

 usually taken up by plants. Although it is indubitable that plants in general 

 can obtain their requirements of nitrogen from the ammonia salts, it is never- 

 theless certain both as a result of manuring experiments on the small scale and 

 also from the experience of agricultural practice that the majority of cultivated 

 plants absorb the element in question more rapidly and abundantly when it is 

 offered them in the form of nitrates. In fact, for some of them, e.g. maize, buck- 

 wheat, and tobacco, JUL. LEHMANN (I.) puts forward the well-grounded assumption 

 that they derive their nitrogen exclusively from nitrates. Here again Nature 

 has made provision for the necessities of the case by converting into nitrates the 

 ammonia salts which partly as a result of decomposition and partly as artificial 

 manures find their way into the soil. 



This process, long known and briefly termed nitrification, was defined in 

 1846 on the basis of an experiment by J. DUMAS (II.) as a purely chemical 

 process of oxidation. This observer regarded chalk as the intermediary facili- 

 tating the intimate combination of ammonia and atmospheric oxygen. Fifteen 

 years later this role of " go-between " was ascribed by MILLON (I.) to the porous 

 humic bodies in the soil a view that still remained destitute of any convincing 

 proofs when revived in 1863 by BLONDEAU (II.). 



Ten years later other opinions began to arise. The first adverse hypothesis 

 was expounded in 1873 by ALEX. MULLER (I.), but was not based on any 

 solid foundation, nor was it followed up any farther. Four years afterwards 

 SCHLOESING and Mtrxxz (I.), relying on the results of their researches in this 

 direction, hazarded the opinion that the formation of nitre in the soil is due to 

 the vital activity of organised ferments (soil bacteria). In a subsequent com- 

 munication these two workers detailed some of the conditions requisite for the 

 inception and course of nitrification. The operation is almost stagnant below 

 5 C., but becomes apparent at 12 C., and attains its maximum at 37 C. As 

 the temperature rises still higher the reaction becomes weaker, and ceases 

 altogether at 55 C. It proceeds the more rapidly as the degree of moisture in 

 the soil increases, provided aeration is not thereby impeded. A faintly alkaline 

 reaction facilitates the progress of nitrification, which, moreover, may not ahva\ > 

 result in the production of nitrates, but at times does not extend beyond the 

 formation of nitrites, especially at low temperatures (below 20 C.) and with a 

 restricted ;lmi.-sion of air. 



Both workers also endeavoured to obtain pure cultures of the organisms 

 under investigation. The result of their endeavours will not be judged too 

 harshly when the existing lack of any reliable method of pure cultivation at that 

 time is remembered. When the introduction of the Koch gelatin plate a Horded 

 a new appliance for this purpose, it was pressed into the service now under 



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