fertilizer inspection. 39 



Constituents of Fertilizers.* 



The only ingredients of plant-food which we ordinarily need 

 to consider in fertilizers are potash, lime, sulphuric acid, phos- 

 phoric acid, and nitrogen. The available supply of sulphuric 

 acid and lime is often insufficient ; hence one reason for the good 

 effect so often observed from the application of lime, and of 

 plaster, which is a compound of lime and sulphuric acid. The 

 remaining substances, nitrogen, phosphoric acid and potash, are 

 the most important ingredients of our common commercial fertil- 

 izers, both because of their scarcity in the soil and their high 

 cost. It is in supplying these that phosphates, bone manures, 

 potash salts, guano, nitrate of soda, and most other commercial 

 fertilizers are chiefly useful. 



The term "form" as applied to a fertilizing constituent has 

 reference to its combination or association with other constit- 

 uents which may be useful, though not necessarily so. The 

 form of the constituent, too, has an important bearing upon its 

 availability, and hence upon its usefulness as plant food. Many 

 materials containing the essential elements are practically worth- 

 less as sources of plant food because the form is not ri^ht ; the 

 plants are unable to extract them from their combinations ; they 

 are "unavailable." In many of these materials the forms may 

 be changed by proper treatment, in which case they become val- 

 uable not because the element itself is changed, but because it 

 then exists in such form as readily to feed the plant. 



Nitrogen is the most expensive of the three essential fertiliz- 

 ing elements. It exists in three dififerent forms, organic nitro- 

 gen, ammonia and nitrate. 



Organic nitrogen exists in combination with other elements 

 either as vegetable or animal matter. All materials containing 

 organic nitrogen are valuable in proportion to their rapidity of 

 decay, because change of form must take place before the nitro- 

 gen can serve as food. Organic nitrogen differs in availability 

 not only according to the kind of material which supplies it, but 

 according to the treatment it receives. The nitrogen in the 

 tables of analyses marked "insoluble in water" is organic nitro- 

 gen. 



•Farmers Bulletin 44 of the U. S. Dept. of Agriculture, "Commercial Fertilizers, 

 Composition and Use," can be had free by applying to your Congressman. 



