Nitrogenous Fertilizers 37 



of solubility, which also measures to some degree avail- 

 ability, since it happens that soluble forms of nitrogen 

 are really more available than the insoluble forms, though 

 neither the soluble nor insoluble forms show the same rate 

 of availability ; that is, a pound of soluble nitrogen is not 

 equally available from whatever source derived, and a 

 pound of insoluble from one source may be much more 

 available than a pound from another. The form in which 

 nitrogen exists in vegetable and animal matter is called 

 the "organic form," because it is associated with other 

 constituents, as carbon, hydrogen and oxygen, which are 

 necessary to make the substances that constitute animal 

 or vegetable matter. The term "organic," as applied to 

 nitrogen, covers a whole series of substances, and does 

 not indicate a uniformity, either in content or quality of 

 the nitrogen, as is the case with distinct chemical com- 

 pounds; hence, associated with the knowledge of form 

 of nitrogen, when it exists in organic products, must be a 

 knowledge of whether the material contains a very con- 

 siderable amount of nitrogen, and whether it is likely to 

 be readily changed, and thus become available as food for 

 plants. 



Any nitrogenous vegetable or animal matter may serve 

 as a fertilizer, though organic nitrogen in commercial fer- 

 tilizers is usually obtained from products relatively rich 

 in this constituent, and it is only these that can be used 

 to advantage in making what are known as "high-grade 

 fertilizers." The leading animal substances of this class 

 are now mentioned. 



DRIED BLOOD 



One of the most important products from which organic 

 nitrogen is derived for commercial fertilizers is dried blood. 



