INVESTIGATIONS ON THE PRODUCTION OF PLANT FOOD. 179 



Scheme showing Similarity between Process of Decomposition of 

 Protein tn Soil and that of Hydrolysis in the Laboratory. 



T T , , Amino- acids 

 In Laboratory (various) 



Diamino- acids 

 (Histidine, 

 Arginine) 



Purin bases 

 (various) 



Humus 



Amino- acids 



Diamino- acids 







Found in Soil not 



(Histidine 



Purin bases 



Humus 



identified 



Arginine, &c.) 







In the soil, however, the decomposition proceeds further, and 

 the various intermediate products break down to yield ammonia. 

 So far as is known, this change is mainly brought about by bacteria 

 and possibly other organisms ; recent investigations at New Jersey 

 suggest that moulds also may be effective. It is certain that con- 

 siderable numbers of different organisms are able to bring about 

 the change. There are also indications that some of the remarkable 

 chemical ferments known as enzymes are active. 



Ammonia is the last stage in the decomposition of the proteins. 

 If it were left to itself it would change into the carbonate, a substance 

 which in small quantities can be taken up and utilized by the plant, 

 but which is harmful in larger quantities, besides having rather a bad 

 effect on heavy soils, making them very sticky. 



But the ammonia is not left to itself. It is at once seized upon by 

 another set of bacteria, quite distinct from all others, which oxidize 

 it to nitrite ; and this in turn is seized upon by an organism, also quite 

 distinct from any other, and oxidized to nitrate. 



Stages in the Decomposition of Protein. 

 Protein 



i 



Amino- acids 



I 



Ammonium compounds 



Nitrates 



The decomposition of the protein thus involves two completely 

 distinct processes : the production of ammonia, which takes in 

 at least two stages, and the production of nitrate, which also 

 involves at least two stages. It so happens that in natural conditions 

 the last stages proceed more rapidly than the earlier ones, so that each 

 set of organisms may be regarded as having to wait for those engaged 

 in the earlier stages. This arrangement seems at first sight wasteful, 

 but it is really advantageous, because it prevents any accumulation of 



