206 SOILS AND FERTILIZERS 



which supply only one element are called amend- 

 ments. It frequently happens that a soil requires only 

 one element in order to produce good crops. In such 

 cases only the one element needed should be sup- 

 plied. Complete fertilizers are sometimes used when 

 the soil is only in need of an amendment. 



264. Variable Composition of Commercial Fer- 

 tilizers. — Since commercial fertilizers are made by 

 mixing various materials which contain different 

 amounts of nitrogen, phosphoric acid, and potash, it 

 follows that they are extremely variable in composi- 

 tion and value. No two samples are the same, 

 hence the importance of knowing the com- 

 position of every separate brand purchased. The 

 composition of fertilizers is varied to meet the require- 

 ments of different soils and crops. Some fertilizers 

 are made rich in phosphoric acid, while others are 

 rich in nitrogen and potash. 



265. How a Fertilizer is Made. — The most com- 

 mon materials used in making complete fertilizers 

 are : Nitrate of soda, kainit, and dissolved phosphate 

 rock. These materials have about the following com- 

 position : 



Nitrate of soda 15.5 per cent, nitrogen. 



Kainit 12.5 per cent, potash. 



Dissolved phosphate • • • 14.0 per cent, phosphoric acid. 



The fertilizer may be made rich or poor in any one 

 element. Many fertilizers contain about twice as 



