70 FIRST PRINCIPLES OF AGRICULTURE. 



A large number of materials, differing widely in their 

 character and composition, contain nitrogen in this form. 



Plants that derive their nitrogen from the soil absorb 

 it chiefly in the form of a nitrate; when nitrogen is 

 applied in this form, no changes are required to enable 

 it to serve as a direct food. Materials furnishing nitrates 

 are, therefore, regarded as of the greatest importance in 

 the manufacture of commercial manures. 



Ammonia, while it does nourish plants directly, usually 

 undergoes a change into nitrate first, though this change 

 proceeds rapidly when the conditions mentioned as favor- 

 able for nitrification are present. As a rule, therefore, 

 an appreciable time does elapse before all the nitrogen 

 in ammonia serves as plant-food. 



Nitrogen in organic forms is first changed by the 

 decay or rotting of the substance into ammonia, and the 

 ammonia is then changed into a nitrate. The rapidity 

 of this decay depends both upon the character of the 

 substance itself, and upon its physical form and its 

 mechanical conditions or fineness of division. The 

 tougher and more dense the substance, and the coarser 

 the particles, the longer the time required to rot, and 

 the more slowly available as nitrogenous food. The rapid- 

 ity with which nitrogen may become useful as food to 

 plants is, therefore, determined by its chemical form. 



Nitrates, since they are immediately useful to the 

 plant, may all be absorbed by the crop upon which they 

 are applied, while ammonia salts and organic nitrogen 

 may be only partially used, because the necessary changes 

 for them to undergo may not take place completely 

 before the plant is fully matured. 



