102 FIRST PBINCIPLES OF AGRICULTURE. 



Incomplete and Complete Fertilizers. Fertilizing 

 materials may contain but one or two of the essential con- 

 stituents, nitrogen, phosphoric acid, and potash. Hence 

 the name " incomplete fertilizer " is sometimes applied to 

 them, signifying that they do not serve in all cases to 

 supply the probable needs of the crop. 



The fertilizers manufactured from raw materials usually 

 contain all three of these essential constituents ; hence 

 they are called " complete fertilizers," signifying that they 

 completely meet the needs of the crop in reference to the 

 number of the constituents that are liable to be lacking. 



Methods of Buying. In buying a fertilizer, that 

 which gives direct value is the fertilizing constituent, 

 nitrogen, phosphoric acid, or potash ; hence the transac- 

 tion is virtually the buying of one or more of these con- 

 stituents. It is readily seen, therefore, that the more 

 concentrated the product, the less will be the actual cost 

 of the constituent desired. 



Again, fertilizers may be bought and used either as 

 " incomplete," raw materials, or as " complete," 

 manufactured products or mixtures, the process of man- 

 ufacture consisting chiefly in mixing, grinding, and pre- 

 paring the various materials described. There are cer- 

 tain advantages and disadvantages in both methods of 

 buying. The advantages in the purchase and use of raw 

 materials are : 



1. A better knowledge of the kind and quality of 

 plant-food obtained ; that is, these products as a rule pos- 

 sess characteristics which distinguish them from others 

 and from each other, and they are more liable to be 

 uniform in composition than mixtures. 



