LIFE IN THE SOIL 99 



avoided. Probably we could not live were it not for the 

 activities of the useful bacteria and yeast plants. 



"The different chemical changes produced by soil- 

 bacteria are quite numerous. Some kinds are specialized 

 for one series of changes, others for changes of a different 

 sort. Some will attack by preference carbohydrates like 

 starch or sugar, some will decompose woody tissue, some 

 will cause the decay of proteins, some of fats, etc. This 

 division of labor allows an effective decomposition of 

 humus. Various gases and acids are produced in the 

 course of decay, and help to decompose the rock particles 

 in the soil and to render the mineral plant-food contained 

 in them available. The insoluble protein compounds in 

 the roots and stubble are broken down and their nitrogen 

 changed partly to ammonia. The particles of ammonia, 

 as they are thus generated by bacteria of many kinds, 

 are at once pounced upon by a special class of germs 

 whose function it is to change the ammonia into nitrate. 

 Thanks, therefore, to the activities of many species of 

 bacteria, the nitrogen locked up in the humus and in 

 green manure is transformed gradually into nitrate, and 

 is then quite suitable for the building of roots, stems, 

 leaves and fruits." 1 



An equally important function of soil-bacteria is the 

 fixation of free nitrogen from the air. This subject will 

 be treated under Nitrogen in the next chapter. 



J New Jersey, Bulletin No. 211, p. 19. 



