28 PRINCIPLES OF FARM PRACTICE 



plant use nitrogen and perhaps other food material found 

 in humus, manure and crop residue, such as straw. The 

 series of changes brought about by the action of bacteria on 

 the content of organic matter is called nitrification. There 

 are some conditions favorable to nitrification that should be 

 understood. The presence of water and oxygen is essential. 

 Any treatment of the soil that will secure plenty of water 

 and oxygen in its upper surface will be favorable to nitri- 



fication. It is also true 

 that water and oxygen 

 are essential for the best 

 development of crops. 

 Some of the methods 

 of controlling the water 

 supply and of aerating 

 the soil have already 

 been considered. 



A common example of how manure may Source of Organic mat- 

 lose much of its value as a fertilizer. (Indiana tef Qn ^ farm _ 



Agr. Exp. Station.) 



most common and most 



valuable form of organic matter that accumulates on the 

 farm is stable and barnyard manure. Straw, cornstalks, and 

 other crop residue are also valuable. Certain heavy crops, 

 such as rye and cowpeas, are sometimes grown as green 

 manure, for the express purpose of adding organic matter 

 to the soil. 



Value of manure. Barnyard and stable manure is not 

 only rich in nitrogen, but also contains a considerable amount 

 of phosphorus and potassium. The money value of these 

 three elements alone is estimated at $2.00 or more, per ton. 

 The total value of the annual production of manure in the 

 United States has been roughly estimated at $800,000,000 



