308 HOW CROPS FEED. 
put a soil in contact with water, certain matters are dis- 
solved in this liquid. 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 roots 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. 
Again, it has been taught that the reserve plant-food of 
the soil is represented by the matters which acids (hydro- 
chloric or nitric acid) are capable of bringing into solu- 
tion. This is true ina certain rough sense only. The 
action of hydrochloric or nitric acid is indeed analogous 
to that of carbonic acid, which is the natural solvent; but 
between the two there are great differences, independent 
of those of degrec. 
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 Selubility.—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 statc- 
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 when the ordinary standards of solubility are re- 
ferred to, The glass bottle which may contain water for 
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