184 



animals, after these have changed from the active state called life, to 

 that of inactivity or death, and by this process are redissolved periodi- 

 callv, these substances wonld be entirely lost to succeeding gensritions 

 of plants and animals, and this constant drain from the resources now- 

 found on the surface of the globe would ultimately exhaust their supply 

 and make life impossible. 



That putrefaction and other processes of decomposition are produced 

 T)y bacteria was suspected for a considerable time, but this was not 

 definitely proved till 1875 by Menzel and subsequently verified by 

 others The action of these organisms is so variable that in the de- 

 composition of any given substance probably a dozen species partici- 

 pate. 



From this indisputable achievement of scientific research il will be 

 Seen that it is in the fiist instance entirely due to the activitv of the 

 minutest organisms thnt what is of earth goes back to it again by the 

 dissolution into simpler substinces of the complex and intricate 

 combinations. Chemical action, no doubt, has also much to do with 

 the redis>olution ; but it is now accepted that this activity in the 

 generality of instances is secondary to the oacterial, and takes place 

 after these have broken up the compounds. 



"W^ithout the one process the other would not take place, and from 

 this it is evident that micro-org .nisms are more closely connected with 

 the productiveness of the soil trian w^s dreamt of less than thirty 

 years ago, and that not only in medicine and industrial pursuits but 

 also from an agronomic point of view bacteriology is becoming daily 

 of greater importance. 



The organisms intimat ly connected with the fertility of the soil 

 may conveniently be divided into two groups, namely, assimilating and 

 destructive bacteria. 



Under the first group we would classify the nitrifying and the ni- 

 trogen- fixing bacteria ; and in the seco id all species which cause pu- 

 trefaction and decomposition may be included. 



The first group, as previously stated, includes nothing bat beneficial 

 species which are occupied either in buildingupnitrogenouscompaunds 

 in the soil, the nitrifying species, or in fixing nitrogen, from the atmos- 

 phere, the nitrogen-fixing bacteria the iatter of which, besides assisting 

 plants to use this element, frequently also enrich the soil itself. Im- 

 portant as is the numerous second group oa account of the reducing 

 power of its species, it includes several that act disadvantageously in 

 regard to agriculture. 



These objectionable species are the denitrifying organisms. By den- 

 itrification is understood the deoxidation of nitrates and nitrites, which 

 in each case involves a loss by either nitrogen or ammonia being given 

 off. Their activity is perceptible by the pungent smell of ammonia 

 rising from fresh stable manure, particularly Irora that of horses. 

 Nitrogen being odourless, its loss cannot be perceived by the senses ; 

 it nevertheles takes place to some extent during every process of de- 

 composition where nitrogenous compounds are present. 



