ELEMENTS NECESSARY FOR PLANT GROWTH 43 



in coal-bearing strata and in other rocks. The hydrated 

 oxide is iron rust 



39. Summary. Of the ten elements necessary for plant 

 growth, three carbon, hydrogen, and oxygen are exclu- 

 sively derived from the atmosphere; one nitrogen 

 partly from the atmosphere and partly from the soil; 

 and six phosphorus, potassium, sulphur, calcium, mag- 

 nesium, and iron are derived exclusively from the soil. 

 From the standpoint of plant requirements only three are 

 often deficient in soils phosphorus, nitrogen, and potas- 

 sium, and in many soils only one or two elements are de- 

 ficient. The condition of the soil often warrants applica- 

 tion of lime in some form. 



