122 MAINE AGRICULTURAI, EXPERIMElNT STATION. I912. 



crops grown on the farm to animals, carefully saving the 

 manure and returning it to the soil. If it is practicable to 

 pursue a system of stock feeding in which those products of the 

 farm which are comparatively poor in fertilizing constituents 

 are exchanged in the market for feeding stuffs of high fertiliz- 

 ing value, the loss of soil fertility may be reduced to a mini- 

 mum, or there may be an actual gain in fertility. 



Constituents oi^ 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 ferti- 

 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 



*The availability of nitrogen from different sources is discussed on 

 page 127. 



