OFFICIAL INSPECTIONS 29. 5 



exchanged in the market for feeding stuffs of high fertilizing 

 value, the loss of soil fertility may be reduced to a minimum, or 

 there may be an actual gain in fertility. 



Constituents of Fertilizers. 



The only ingredients of plant-food which we ordinarily need 

 to consider in fertilizers are potash, lime, phosphoric acid, and 

 nitrogen. The available supply of 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, phos- 

 phoric acid and potash, are the most important ingredients of 

 our common commercial fertilizers, 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 constitu- 

 ents which may be useful, though not necessarily so. The form 

 of the constituent, too, has an important bearing upon its avail- 

 ability, 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 right ; 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 different 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 plant food. Organic nitrogen differs in availa- 

 bility not only according to the kind of material which supplies 

 it, but according to the treatment it receives. 



Nitrogen as ammonia usually exists in commercial manures in 

 the form of sulphate of ammonia and is more readily available 

 than organic nitrogen. While nitrogen in the form of ammonia 



