316 MAN'S USE AND CONSERVATION OF SOILS 



by the growing and harvesting of crops. These are phos- 

 phorus and potassium. Wheat crops, for example, rapidly 

 exhaust soluble phosphorus compounds from the soil; and 

 generous supplies of potassium compounds are necessary 

 for the successful raising of cotton. 



Substances that contain elements needed for the life and 

 growth of plants are called fertilizers. The most common 



SOUTHERN COTTON FIELD 



fertilizers are manures. They contain nitrogen, potassium, 

 and phosphorus, in about the proportions needed for the 

 raising of ordinary crops. 



Commercial fertilizers generally contain one or more of 

 the three elements mentioned, in proportions adapted to 

 the needs of varying crops. Saltpeter is a compound rich 

 in nitrogen, and is therefore a good fertilizer. The most 

 common way in which phosphorus is obtained for fertilizing 

 is in the form of phosphoric acid. Much of this is prepared 

 at stockyards from by-products, formerly wasted. Phos- 



