Feb. 1897.] BOTANICAL SOCIETY OF EDINBURGH 29 



introduced bacteria would rapidly multiply and spread. 

 This is a favourable place to notice one of the values of 

 stable manure. " Dung," as Hilltner has expressed it, " is 

 the leaven of the agricultural soil," meaning that in adding 

 it one gets, over and above other well-known advantages, 

 a supply of vigorous nitrifying organisms. 



2. Access of Oxygen. — This is why the bacteria of nitri- 

 fication occur in greatest number towards the surface, and 

 decrease with the depth. A new meaning is hereby given 

 to tillage and good cultivation, as these ensure a good 

 circulation of the necessary air. 



3. Light acts injurious! g. 



4. A certain moisture is necessary, but marshy ground 

 will lack the germs. 



5. A suitable temperature. — In this case, say, between 

 90° and 100° F. 



6. A base with vjhich the nitric acid, on its appearance, 

 can combine. — It is an interesting fact that as a result 

 of the activities of soil organisms, their own further 

 development may be inhibited. Thus, yeast is killed by 

 excess of alcohol produced by itself, and so, too, with 

 bacteria. Their own products may forbid their further 

 growth. In the world of medicine, by inoculation of the 

 matter prejudicial to the life of certain bacteria, we can 

 put an end to them. The presence of a base then prevents 

 the accumulation of nitric acid. A favourite base is lime, 

 and among the numerous advantages of lime in a soil, I 

 desire at present to single out its favouring so largely the 

 development of the nitrifying bacteria. 



7. The other necessary 7:>/rt«^ foods. 



And now there is a dark side to this story. Eesearch 

 has proved the presence in soils of denitrifying organisms, 

 which may bring the work of the useful organisms to 

 nought. Their work is a reducing one, the nitrates being 

 decomposed with an ultimate giving off of free nitrogen. 

 Instead of being a gain, these denitrifying bacteria occasion 

 a loss. Generally it may be stated that the conditions 

 under which these harmful organisms flourish are the 

 opposite to these we have mentioned as favourable to the 

 nitrifying germs. 



