242 



PLANT PHYSIOLOGY 



of the atmosphere and that organic substances of the soil are not 

 used directly for carbon nutrition. 



Knop and Sachs in their classical investigations were the first 

 to elaborate the method of water cultures and to apply it to the 

 solution of the question: Which of the elements are absolutely 

 indispensable for the nutrition of plants? They established 

 quite definitely the indispensability of the following elements: 

 K, Ca, Mg, Fe, S, P, and N. 



Of these elements, nitrogen certainly is not one of the ash 

 constituents, for it generally volatilizes during combustion of the 



Soda 

 Lime, 



Solution 



oda Lime 



Brick 

 Patticl^'s 



Water 



Fig. 72. — Culture of a plant in an atmosphere free from carbon dioxide {after 



Smith, et aL). 



plant substance. It is similar to these other elements in that it 

 cannot be obtained by the plant directly from the atmosphere 

 and must be introduced into the nutritive solution in some form. 

 Only nodule-bearing leguminous plants are capable of utilizing 

 free atmospheric nitrogen. 



To these seven principal elements, several others have been 

 added recently: manganese, zinc, boron, aluminum, copper, 

 silicon, etc. These elements were formerly regarded as super- 

 fluous, accumulating in the plant only by virtue of the fact that 

 they are dissolved in the water that is absorbed by plants from 

 the soil. Their addition to the number of necessary elements 

 was the result of water cultures conducted with salts carefully 

 freed from the smallest amounts of impurities by the exact 

 methods of modern chemistry and used in containers coated on 



