LITERATURE 153 



time may elapse before any of the nitrogen becomes available for 

 higher plants. Under these conditions, the excess of carbo- 

 hydrates, especially the cellulose, keeps the available nitrogen at a 

 minimum until they have nearly all decomposed. 



The ammonia liberated as a result of decomposition of the 

 nitrogenous constituents of plants, animals, and microbes is either 

 utilized directly by plants or is changed by certain specific groups 

 of bacteria to nitrites, then to nitrates. This process, known as 

 nitrification, is of considerable importance in soils because it is 

 believed that the majority of higher plants obtain their nitrogen 

 from the soil in the form of nitrate. 



The nitrate thus produced, if not assimilated by growing plants, 

 can be absorbed by microorganisms, in the presence of available 

 carbohydrates, and changed again to microbial protein. It can 

 be washed out from the soil by drainage waters. Under certain 

 conditions, such as hmited aeration, it can be reduced to gases 

 of nitrogen. The nitrogen thus becomes lost from the soil and 

 returns to the atmosphere. 



LITERATURE 



1. Buchanan, R. E., and Fulmer, E. I. Physiology and biochemistry of 



bacteria. Volume 3, Chapter 17. The Wilhams & Wilkins Co. 

 Baltimore, 1930. 



2. Greaves, J. E. Agricultural bacteriology. Chapters 21 and 22. Lea 



and Febiger. Philadelphia, 1922. 



3. Stephenson, M. Bacterial metabolism. Chapter 9. Longmans, Green 



& Co. London, 1930. 



4. Waksman, S. a. Principles of soil microbiology. Chapters 3, 7, 20, 21. 



The Wilhams & Wilkins Co. Baltimore, 1927. 



