PLANT PHYSIOLOGY 253 



If the animal dies, the unchanged protein of the body de- 

 composes bacterially in the soil, yielding simpler compounds, 

 amino-acids, etc., and eventually ammonia. This is then 

 further changed as above described and the cycle is again 

 completed. Thus the nitrogen of animal protein, either as 

 dead animal protein or as excretion products of living animals, 

 is eventually converted into soil nitrate compounds and as- 

 similated by plants as their soil food for the synthesis of new 

 plant protein. 



During the decomposition of organic nitrogen compounds of 

 manure or dead animal bodies and the transformation of the 

 nitrogen into nitrate compounds, another class of bacteria known 

 as denitrifying bacteria cause the loss of some of the nitrogen 

 into the atmosphere as Jree nitrogen. Also some ammonia 

 fails to become oxidized and is volatilized as a gas. This 

 ammonia gas is absorbed by water and returned to the soil in 

 rain. The whole cycle of changes is controlled by conditions of 

 equilibrium, both biological and chemical, so that it cannot be 

 represented as a clean-cut series of reactions always going in 

 one direction. The final results are, however, those which have 

 been indicated. 



For the purpose of a review and to condense what has been 

 said into a short presentation, we may write the following 

 reactions as representing the general course of the cycle of 

 changes. 



