308 HOW CROPS FEED. 



put a soil in contact with water, certain matters are dis- 

 solved in this liqui'l. It has been thought that the sub- 

 stances taken up by water at any moment are those which 

 at that time represent the available plant-food. This no- 

 tion was based upon the supposition that the plant cannot 

 feed itself at the I'oots save by matters in solution. Since 

 Liebig has brought into prominence the doctrine that roots 

 are able to attack and dissolve the insoluble ingredients 

 of the soil, this idea is generally regarded as no longer 

 tenable, 



Agam, it has been taught that the reserve plant-food of 

 the soil is represented by the matters whicli acids (hydro- 

 chloric or nitric acid) arc c;ipable of bringing into solu- 

 tion. This is true in a certain rough sense oidy. The 

 action of hydrochloric or nitric acid is indeed analogous 

 to that of carbonic acid, Avhich is the natural solvent ; but 

 between the two there are great diiferences, independent 

 of those of degree. 



Although we have no means of learning with positive 

 accuracy what is the condition of the insoluble ingredients 

 of the soil as to present or remote availability, the deport- 

 ment of the soil towards water and acids is highly in- 

 structive, and by its study -we make some approach to the 

 solution of this question. 



Standards of Solubility. — Before proceeding to details, 

 some words upon the limits of solubility and upon what 

 is meant by soluble in water or in acids will be appropri- 

 ate. The terms soluble and insoluble are to a great de- 

 gree relative as applied to the ingredients of the soil. 

 When it is affirmed that salt is soluble in water, and that 

 glass is insoluble in that liquid, the meaning of the state- 

 ment is plain ; it is simply that salt is readily recognized 

 to be soluble and that glass is not ordinarily perceived to 

 dissolve. The statement that glass is insoluble is, however, 

 only true Avhen the ordinary standards of solubility arc re- 

 ferred to. The glass bottle vrhich mav contain water f^r 



