74 FIRST PRINCIPLES OF AGRICULTURE. 



also in their agricultural value or usefulness to plants, 

 largely according to their method of preparation for 

 market. 



Dried blood, for instance, is rich in nitrogen when 

 carefully prepared, though commercial samples show 

 wide variations in composition, and in the quality of 

 the nitrogen. A pound of nitrogen, therefore, may have 

 a very different value in one sample than in another, 

 under uniform conditions of use, while under the same 

 conditions of use a pound of nitrogen in the form of 

 nitrate is equally valuable from whatever source de- 

 rived. 



Materials containing organic nitrogen are not soluble 

 in water, and the nitrogen is not immediately available 

 to plants. Their value as a source of this element is, 

 therefore, measured by conditions which favor the rapid 

 change of their nitrogen into soluble and available forms. 

 The first condition is fineness of division, which permits 

 of a more even distribution, and the exposing of a 

 larger surface area to the action of the agencies in the 

 soil and air; and the second is the physical character 

 of the material itself. If it is hard and dense, decay 

 will be slower, since the processes which cause it are 

 resisted; if soft and porous, the rotting is more rapid, 

 because the agencies which cause it are encouraged to 

 act. 



Dried Blood. — Dried blood is one of the most valua- 

 ble sources of organic nitrogen. Its fineness of division 

 permits of its easy and uniform distribution, and its 

 physical character is such as to encourage rapid decay 

 under suitable conditions. 



