6 MAINE AGRICULTURAI, EXPE;RIMENT STATION. IQII. 



is extremely soluble in water, it is not readily removed from the 

 soil by leaching, as it is held by the organic compounds of the 

 soil. 



Nitrogen as nitrate exists in commercial products chiefly as 

 nitrate of soda. Nitrogen in this form is directly and imme- 

 diately available, no further changes being necessary. It is 

 completely soluble in water, and diffuses readily throughout the 

 soil. It differs from the ammonia compounds in forming no 

 insoluble compounds with soil constituents and may be lost by 

 leaching. 



Phosphoric acid is derived from materials called phosphates, 

 in which it may exist in combination with Hme, iron, or alumina 

 as phosphates of lime, iron or alumina. Phosphate of lime is 

 the form most largely used as a source of phosphoric acid. 

 Phosphoric acid occurs in fertilizers in three forms : That solu- 

 ble in water and readily taken up by plants; that insoluble 

 in water but still readily used by plants and known as "re- 

 verted" ; and that soluble only in strong acids and consequently 

 very slowly used by the plant. The "soluble" and "reverted" 

 together constitute the "available" phosphoric acid. The phos- 

 phoric acid in natural or untreated phosphates is insoluble in 

 water, and not readily available to plants. If it is combined 

 with organic substances as in animal bone, the rate of decay is 

 more rapid than if with purely mineral substances. The insol- 

 uble phosphates may be converted into soluble forms by treat- 

 ment with strong acids. Such phosphates are known as acid 

 phosphates or superphosphates. The "insoluble phosphoric 

 acid" of a high cost commercial fertilizer has little or no value 

 to the purchaser because at the usual rate of application the 

 quantity is too small to make any perceptible effect upon the 

 crop, and because its presence in the fertilizer excludes an equal 

 amount of more needful and valuable constituents. 



Potash in commercial fertilizers exists chiefly as muriates and 

 sulphates. With potash the form does not exert so great an 

 influence upon availability as is the case with nitrogen and 

 phosphoric acid. All ordinary forms are freely soluble in water, 

 and are believed to be nearly if not quite equally available as 

 food. The form of the potash has an important influence upon 

 the quality of certain crops. For example, the results of experi- 

 ments seem to indicate that the quality of tobacco, and certain 



