244 AGRICULTURE 



years, the land will finally become exhausted under such 

 treatment. This is for the simple reason that the crops re- 

 move from the soil each year more of the elements neces- 

 sary to their growth than are returned to it. The deficiency 

 must finally be made up if permanent fertility is to be main- 

 tained. This is accomplished by supplementing manuring 

 and rotation with what are called the commercial fertilizers. 



Increased use of commercial fertilizers. Commercial 

 fertilizers have long been used in a small way, but it is only 

 recently that they are coming to be employed on a large 

 scale. The farmers of the United States are now paying 

 out considerably more than $100,000,000 a year for such 

 fertilizers. There are at present more than five hundred 

 manufacturers selling the various fertilizing products. 



Since, as we have seen, only three of the elements neces- 

 sary to plant production are likely to run short, commercial 

 fertilizers are commonly limited to these three nitrogen, 

 phosphorus and potassium. 



Commercial nitrogen fertilizers. Nitrogen is the 

 most expensive of the three essential fertilizers. Commer- 

 cial nitrogen fertilizer is sold chiefly in three different 

 forms: (1) nitrate of soda (Chile saltpeter) ; (2) sulphate of 

 ammonia, which is a by-product of the manufacture of coke 

 and gas; and (3) dried blood, ground or steamed bone, or 

 other animal products from packing houses. 



The nitrogen fertilizers cost from fifteen to fifty cents 

 a pound. Instead of depending on them the intelligent 

 farmer will therefore use every effort to maintain the supply 

 of nitrogen in his soil through the nitrogen-fixing legumes, 

 occasionally plowing under a crop of clover, alfalfa, cow- 

 peas, vetches, or soy-beans. Where these can be grown 

 successfully, there will be little need to buy nitrogen ; it can 

 be obtained from the seventy million pounds of free nitro- 

 gen in the atmosphere above each acre. 



