BACTERIA OF THE SOIL 239 



uniform to a great depth, as in certain of the semiarid soils 

 of the western United States, bacteria are moderately 

 abundant to considerable depths. 



Cycles of the Elements. Bacteria, yeasts and molds in 

 the soil are of primary importance because of the chemical 

 changes which they are able to bring about. The changes 

 which have been enumerated above make available elements 

 necessary for plant nutrition. Complex compounds are 

 constantly being broken down into simpler compounds and 

 simple compounds built up again into more complex forms. 

 These successive transformations and changes in complexity 

 of the various important elements may be usually grouped 

 together under the heading of the term " cycle. " The 

 elements which are important from this point of view are 

 nitrogen, sulphur, phosphorus, iron, carbon and calcium. 



THE CYCLE OF NITROGEN IN NATURE WITH SPECIAL 

 REFERENCE TO THE SOIL 



Outline. The transformations which nitrogen may 

 undergo are relatively numerous. Microorganisms play a 

 large part in bringing about these changes. Simple nitrogen 

 compounds are assimilated by higher plants and built into 

 complex compounds. These, after the death of the plant, or 

 of the animal which has eaten the plant, are returned to the 

 soil and broken down into simpler compounds. As the 

 starting point in the discussion of the cycle of nitrogen one 

 may take the more complex protein. Many microorganisms 

 may attack this, bringing about the change termed proteoly- 

 sis. Eventually the nitrogen appears in the form of am- 

 monia and the process termed ammonification is complete. 

 Various microorganisms in the soil are capable of oxidizing 

 ammonia-producing nitrities. Still other bacteria oxidize 

 the nitrites to nitrates. These are assimilated by the 

 green plant, gradually built up into compounds more 

 and more complex until finally plant proteins are de- 



