COMMERCIAL FERTILIZERS 1 97 



cases only the one element needed should be supplied. 

 Complete fertilizers are sometimes used when the 

 soil is only in need of an amendment. 



267. Variable Composition of Commercial Ferti- 

 lizers. Since commercial fertilizers are made by mix- 

 ing various materials which contain different amounts 

 of nitrogen, phosphoric acid, and potash, it follows 

 that they are extremely variable in composition and 

 value. No two samples are the same, hence the im- 

 portance of knowing the composition of every separate 

 brand purchased. The composition of fertilizers is 

 varied to meet the requirements of different soils and 

 crops. Some fertilizers are made rich in phosphoric 

 acid, while others are rich in nitrogen and potash. 



268. 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 

 much potash as nitrogen and five times as much phos- 

 phoric acid as potash. In order to make a ton of such 

 a fertilizer it would be necessary to take : 



Pounds. 



Nitrate of soda 225 



Kainit 425 



Phosphate 1350 



