50 MAINIJ AGRICULTURAIv EXPERIMENT STATION. I906. 



The fertility of the soil would remain practically unchanged if 

 all the ingredients removed in the various farm products were 

 restored to the land. This may be accomplished by feeding the 

 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 stufifs 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 oe 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 fer- 

 tilizers, 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 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. 



* Farmers' Bulletin 44 of the U. S. Dept. of Agriculture, "Commercial 

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

 Congressman. 



