/O AGRICULTURAL BACTERIOLOGY. 



portance are chiefly the apparent inadequacy of the other two, 

 and the fact that positive experiments have shown that bacteria 

 are certainly able to fix considerable amounts of nitrogen in the 

 soil. At present it is a general belief that we must look to the 

 action of microorganisms for an explanation of the primitive 

 soil nitrates. This subject will be better understood in a later 

 chapter and will, therefore, be postponed. 



In addition to the nitrates the plant absorbs from the soil 

 various minerals dissolved in water. With these we are not 

 concerned. Under the agency of plant life, these various foods 

 are combined to form complex compounds which we call 

 organic. The energy needed for the construction is furnished 

 the plant by sunlight. The plants thus produce large num- 

 bers of complex compounds out of its simple foods, but these 

 compounds consist of a few well-known types of substances. 

 By far the greatest proportion of them consist of starches, 

 sugars, fats and proteids. So far as the nitrogen is concerned 

 it is practically all in the proteids and similar bodies, while the 

 carbon is contained in all of these products. 



. RESTORATION OF THE CARBON TO THE ATMOSPHERE. 



The compounds thus built up have different destinies. Part 

 of them are appropriated by the animal kingdom and to these 

 we shall presently turn. Another large part is not appropri- 

 ated by animals but begins at once to undergo destructive 

 changes which bring its ingredients back again to their starting 

 point. In regard to some of them the processes of chemical 

 destruction are comparatively simple. The star dies, the sugars 

 and the fats are subject to chemical changes which take place, 

 to a limited extent, under the direct influence of chemical 

 forces, since they may be directly oxidized. All forms of 

 active combustion in fires produce such oxidation, the result 

 of which is that the carbon in the compounds is united with 

 oxygen and liberated in the form of C( ). the hydrogen at the 



