232 Plant Physiology 



(3) By the action of certain soil bacteria and fungi 

 which are also able to utilize atmospheric nitrogen. 



(4) By the ammonia returned to the soil as a result of 

 rainfall; but since this, in general, is that which escapes 

 from the soil into the air, it is negligible. 



(5) As a result of electrical discharges nitrous and nitric 

 acids may be produced in the air and through rains brought 

 to the soil, but this amount is relatively inconsiderable, 

 consisting, under the most favorable conditions (in the 

 moist tropics), of about five pounds per acre annually. 



126. Ammonification. The remains of plants and 

 animals are " returned " to the soil through processes of 

 decay and putrefaction brought about largely by means 

 of fungi and bacteria. Decay is a relative term usually 

 implying decomposition without the production of mal- 

 odorous compounds, and commonly taking place with 

 access of oxygen. In putrefaction ill-smelling compounds 

 result, usually from the decomposition of nitrogenous 

 substances, taking place, as a rule, with poor oxygenation. 



A result of both of the above processes is that nitrog- 

 enous compounds are broken down into ammonia, carbon 

 dioxid, and other products. This reduction to ammonia 

 constitutes what is known as ammonification. Under 

 favorable conditions a large part of the ammonia is held 

 in the soil by entering into combinations with the soil 

 bases. During this decomposition many substances more 

 or less injurious may be at least temporarily set free in 

 the soil. 



Numerous species of bacteria and fungi affect decom- 

 position. Bacteria are particularly important in arable 

 soils, especially such species as Bacillus mycoides and B. 



