MAINTAINING THE FERTILITY OF THE LAND 115 



fertilizer is barnyard manure. Growing plants have also 

 been plowed under for many centuries. Usually these 

 plants have been weeds, but sometimes crops are sown 

 for the purpose of green-manuring. 



The Indians taught the first settlers in America how to 

 grow corn and to use fish as a fertilizer. 



"According to the manner of the Indians, we manured 

 our ground with herrings, or rather shads, which we have 

 in great abundance and take with ease at our doors. 



"You may see in one township a hundred acres together 

 set with these fish, every acre taking a thousand of them, 

 and an acre thus dressed will produce and yield as much 

 corn as three acres without fish." 



Fish are still in common use along the Atlantic coast, 

 and dried fish and fish scraps are sold as fertilizers. 



Salt (NaCl) was sometimes applied to land, but this 

 is not considered to be a wise practice because it does not 

 contain the elements that are likely to be deficient. Any 

 good effects of salt are probably due to chemical action 

 or to the action of salt in helping to dissolve other ele- 

 .ments in the soil. Many such substances have been used, 

 but they are not now used so much as formerly. 



The purchased fertilizers that are most commonly 

 used are mixtures containing nitrogen, phosphoric acid, 

 and potash. The materials for making these fertilizers 

 are usually obtained from slaughter-houses, or are mined 

 from the earth. 



The use of fertilizers in the United States has rapidly 

 increased, and the area on which they are used is con- 

 stantly extending westward. Little is yet used west of the 

 Mississippi river. In 1879, farmers in the United States 



