230 METABOLISM 



organic substances in the nitro-bacteria in an exhaustive manner, and their 

 results are summarized (in percentages) in the following table : — 



Nitrite formers. Nitrate formers. 



In the first column of each series are given the lowest percentages which 

 accelerate development, and in the second the doses which retard it. The 

 symbol > signifies ' more ', but not much more, than the dose following. 



We may deduce several important conclusions from this table. 



1. The different organic substances are by no means equally valuable, to the 

 nitro-bacteria, but may operate directly as antiseptics. This antiseptic effect is 

 not less than that of carbolic or salicylic acids in the case of ordinary Bacteria. The 

 nitro-bacteria are much more autotrophic than green plants ; owing to the fact that 

 the latter are at least facultatively heterotrophic, they may get on with organic 

 substances supplied to them from without. We might certainly suppose that the 

 same was true of the nitro-bacteria, if the first assimilation product were known. 



2. The very substances which form the best nutrients for ordinary Bacteria 

 and for heterotrophic plants inhibit nitrification most. 



3. The nitrite-bacterium is much more sensitive to organic substances 

 than the nitrate-bacterium. On the other hand, the nitrate-bacterium is 

 astonishingly sensitive to ammonia, and that substance is more antiseptic to 

 it than is corrosive sublimate to other organisms. 



All these facts are of significance, not only because they disclose to us 

 the remarkable differences and adaptations which occur in the organic world, 

 but also because they explain the role of nitrification in nature. The sensi- 

 tivity of the nitrite-bacterium to organic substances carries with it the con- 

 clusion that its development can commence only if all the organic materials 

 which are present in the soil (dead animals and plants, excrement, &c.) have 

 been completely decomposed by ordinary putrefactive organisms, so that 

 the carbon is present as carbon-dioxide, the nitrogen as ammonia, or even as 

 an element. The nitrate microbe is less sensitive to organic compounds, but 

 its development is inhibited by ammonia, and it can develop only after the 

 nitrite organism has operated first. The question now is whether the sharp 

 demarcation of nitrification from fermentation and putrefaction is of signi- 

 ficance in organic nature ; the answer to this is undoubtedly in the affirma- 

 tive. In many of the commonest fermentative processes, potassium nitrate 

 is reduced, whereby not only nitrite, but especially free nitrogen is formed 

 in large quantity. If nitrification sets in before the completion of fermentation, 

 the nitrates formed would be denitrified by these ferments instead of being avail- 

 able for the nutrition of the green plant. Such a denitrification does occur under 

 certain conditions, as we shall see in the next lecture. There we shall take 

 the opportunity of studying the combining of free nitrogen by organisms in 

 conjunction with its formation. 



At the present moment we may note that the nitro-bacteria do not confine 

 their activity to arable soil where ammonia is presented to them in the manure, 

 but that they establish themselves on bare rock containing lime, and make 

 use of the traces of ammonia brought down by rain. They then decompose 

 the lime, and thereby render this mineral available for the higher plant as well 

 as by forming nitric acid. 



