BACTERIA IN THE SOIL 149 



pounds of ammonia, and nitrogen compounds of great com- 

 plexity stored up in the tissues and body of the animal. 

 The carbonic acid, water, and other simple substances like 

 them will return to nature and be of immediate use to vege- 

 table life. But otherwise the cycle cannot be completed, 

 for the more complex bodies are of no service as such to 

 plants or animals. 



I. In order that this complex material should be of service 

 in the economy of nature, and its constituents not lost, it is 

 necessary that it should be broken down again into simpler 

 conditions. This prodigious task is accomplished by the 

 agency of two groups of organisms, the decomposition and 

 denitrifying'- bacteria. The organisms associated with de- 

 composition processes are numerous; some denitrify as well 

 as break down organic compounds. This group will be 

 referred to under '* Saprophytic Bacteria.'' The reduction 

 by the denitrifying bacteria may be simply from nitrate to 

 nitrite, or from nitrate to nitric or nitrous oxide gas, or in- 

 deed to nitrogen itself. In all these processes of reduction 

 the rule is that a loss of nitrogen is involved. How that 

 free nitrogen is brought back again and made subservient to 

 plants and animals we shall understand at a later stage. 



Professor Warington has again recently set forth the chief 

 facts known of this decomposition process.^ That the 

 action in question only occurs in the presence of living 

 organisms was first established by Mensel in 1875 in natural 

 waters, and by Macquenne in 1882 in soils. If all living 

 organisms are destroyed by sterilisation of the soil, denitri- 

 fication cannot take place, nor can vegetable life exist. 

 ** Bacteria reduce nitrates,'' says Professor Warington, ** by 

 bringing about the combustion of organic matter by the 

 oxygen of the nitrate, the temperature distinctly rising 



^ ' ' Denitrifying " means reducing nitrates. 



^ R. Warington, M.A., F.R.S., Journ. Roy. Agricultural Soc, Eng.^ series 

 iii., vol. viii., pt. iv., pp. 577 et seq. 



