CHAPTER XXI. 



AMMONIFICATION, NITRIFICATION AND DENIT- 

 RIFICATION. 



The green plant takes the carbon that it needs for its 

 growth from the air in the form of carbon dioxide. 

 During each growing period there is an immense amount 

 of this compound removed from the air, but which how- 

 ever is constantly replenished by various factors among 

 which the bacteria are very important. Through the 

 work of micro-organisms and the respiration of plants 

 and animals the carbon bound up in the organic matter 

 is again made ready for the use of the plant. 



The nitrogen that the plant demands for its growth 

 is taken from the soil in the form of nitrates. The 

 amount of combined nitrogen in the soil is small, and 

 unless there are factors at work that shall restore the 

 nitrogen found in the bodies of plants and animals to a 

 form in which it can again be used by the plant, the soil 

 would soon be depleted of this element. The immense 

 v amount of free nitrogen in the air, amounting to at least 

 35,000 tons over each acre, is not available for the green 

 plant, except under special conditions that will be dis- 

 cussed in a subsequent chapter. 



The nitrogen of the soil is in the humus and is not 

 available to the plant but must be worked over and 

 changed to a form in which the green plant can use it, 

 just as the nitrogen added to the soil in the green crop 

 or in manure must pass through a complex series of 

 changes to be of use to the plant again. 



