92 



BIOLOGY AND HUMAN LIFE 



separately and in combination. From such experiments we 

 learn that it is not the sandiness of the soil, or the color, or 

 merely the water in it that makes the growth of plants possible ; 

 it is something in the soil that can dissolve in water. 



68. The salts of the soil. These soluble substances in the soil 

 are the salts, of which there are many different kinds. Are all 

 or any of these salts related to plant growth? In carefully 

 planned and carefully conducted experiments plants were grown 

 in solutions of soil minerals from which now one element and 

 now another was omitted. The omission of some elements 

 seems to result in no perceptible difference, but the omission of 

 others will absolutely prevent the further growth of the plants. 

 From the results of such experiments the following table has 

 been constructed: 



This table shows whether or not a given elemicnt is found to 

 be necessary for plant life. It also shows in what particular 

 way each element is related to the life of the plant. 



