234 SOILS AND FERTILIZERS 



Materials as sodium nitrate which supply only one ele- 

 ment are called amendments. It frequently happens 

 that a soil requires only one element in order to produce 

 good crops, and in such cases only the one element 

 needed should be supplied. 



Complete fertilizers are often used when the soil is 

 in need of an amendment only. 



281. Variable Composition of Commercial Fertilizers. 

 Since commercial fertilizers are made by mixing 

 various materials which contain different amounts of 

 nitrogen, phosphoric acid, and potash, it follows they 

 are extremely variable in composition and value. No 

 two samples are the same, hence the importance of 

 knowing the composition of every brand purchased. 

 The composition of fertilizers is varied to meet the 

 requirements of different soils and crops. Some ferti- 

 lizers are made rich in phosphoric acid, while others are 

 rich in nitrogen and potash. 



282. How a Fertilizer is Made. The most common 

 materials used in making complete fertilizers are : ni- 

 trate of soda, kainit, and dissolved phosphate rock. 

 These materials have about the following composition : 



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 ingre- 

 dient. Many fertilizers contain about twice as much 



