56 ELEMENTARY AGRICULTURAL CHEMISTRY 



5. Absence of Strong Light, — Sunlight stops the action of, 

 and, if continued, destroys the organisms. 



6. Abundance of Oxygen, 



Denitrification. — This is a process resulting in the libera- 

 tion of free nitrogen from nitrates, brought about by micro- 

 organisms which are probably nearly always present in soil, 

 but which can only act when free oxygen is absent — that is, the 

 organisms are anaerobic. If a soil be consolidated, water- 

 logged, or highly charged with oxidisable carbonaceous matter 

 the conditions become favourable for denitrification. The 

 application of very large dressings of dung along with nitrate 

 of soda sometimes causes a considerale loss of nitrogen from 

 this cause. 



Fixation of Atmospheric Nitrogen in Soils.— Certain 



micro-organisms, able to absorb free nitrogen from the 

 air and to convert it into compounds capable of being 

 utilised by plants, have been discovered. Indeed, some years 

 ago cultures of such organisms were prepared for sale under 

 the name of Alinit, but the success attending their employ- 

 ment was doubtful, and their manufacture has ceased. 

 Recently further attention has been directed to the subject, 

 and other organisms possessed of similar powers have been dis- 

 covered. Among others, a large bacterium, which has been 

 named Azotobacter^ is said to possess this power, in soils rich 

 in /)rganic matter, the necessary energy being believed to 

 be derived from the oxidation of the carbonaceous material. It 

 is thought that the fertility and richness in nitrogen of forest 

 or prairie soil is largely due to the activity of this and similar 

 organisms, which would find suitable conditions for growth in 

 the large quantity of organic carbonaceous matter contained in 

 ^uch soils. 



Gases in a SoiL — The interstices of a soil are usually 

 occupied by air, but, in consequence of the chemical changes 

 going on in the f-oil, this air becomes robbed of some of its 



