CHAPTER XL 



CHEMICAL CHANGES. 



The soil is not inert, but a great number of changes take place 

 in it. some purely chemical, and others brought about by the 

 action of bacteria and other forms of life. The most important 

 changes have to do with organic matter and nitrogen. The bulk 

 of the nitrogen in organic forms in soils is useless to plants, and 

 must be changed before it can be taken up by them. Phosphoric 

 acid and potash are fixed by the soil, and their compounds under- 

 go various changes. 



Changes of Nitrogen in the Soil. Many changes take place in 

 the nitrogen of the soil, all brought about by bacteria. There is, 

 first, the transformation of organic nitrogen into ammonia, 

 termed ammonification. Next is the change of organic matter, 

 ammonia, and nitrites, to nitrates, a change called nitrification. 

 Another change is 'the destruction of nitrates, either nitrites, 

 ammonia, protein, or free nitrogen being formed, a change called 

 dentrification. A further change is the production of organic 

 compounds from the elementary nitrogen of the air, a change 

 caled nitrogen fixation. As these changes are brought about by 

 the agency of bacteria, it is necessary to give a little space to the 

 study of soil bacteria. 



Soil Bacteria. 1 Bacteria are organisms so small as to be seen 

 only under the highest power of the microscope. They are 

 grown either in liquids or on the surface of slices of potatoes, 

 solidified plates of gelatin, agar, or similar material, the proper 

 nourishment being supplied. The plate method is used for isolat- 

 ing and studying bacteria, since many kinds of bacteria form 

 characteristic colonies, and a pure culture can easily be secured 

 therefrom. Unfortunately many important soil bacteria do not 

 grow well on such plates. This is especially true of the nitrate 

 bacteria. Since bacteria abound everywhere, it is necessary 

 in the study of bacteria to destroy all those which are originally 



1 Review of Investigations of Soil Bacteriology, Voorhees and Lipman, 

 Bulletin 94, Office of Kxp. Sta. 



