204 ELEMENTS OF GENERAL SCIENCE 



does not hold other injurious substances, as plant poisons, 

 which do sometimes exist in the soil. Very little is known 

 about plant poisons in the soil, but recent investigations show 

 that some plants excrete substances which may be injurious to 

 them, and possibly to other plants. Much attention is now 

 being given to problems of soil fertility. 



Water is one of the most important substances of the soil. 

 Plants use water directly as a raw material for food manufac- 

 ture, and they also use other materials which are secured from 

 the soil only when they are dissolved in water. If water were 

 absent from the soil, plants would starve ; and if there is too 

 much water, some plants will drown from lack of air. 



