CONSTITUTION OF TIE SOIT. 307 
before. We gather, then, that there arc three items to be 
regarded in the simplest view of the chemical compo- 
sition of the soil, viz., the inert mechanical basis, the 
presently available nutritive ingredients, and the reserve 
matters from which the available ingredients are supplied 
as needed, 
In a previous chapter we have traced the formation of 
the soil from rocks by the eonjoint agencies of mechanical 
and chemical disintegration. It isthe perpetual operation 
of these agencies, especially those of the chemical kind, 
which serves to maintain fertility. The fragments of rock, 
and the insoluble matters generally that exist in the soil, 
are constantly suffering decomposition, whereby the elc- 
ments that feed vegetation become available. What, 
therefore, we have designated as the inert basis of the soil, 
is inert for the moment only. From it, by perpetual 
change, is preparing the available food of crops. Various 
attempts have been made to distinguish in fact between 
these three classes or conditions of soil-ingredients; but 
the distinction is to us one of idea only. We cannot realize 
their separation, nor can we even define their peculiar con- 
ditions. Weare ignorant in great degree of the power 
of the roots of plants to imbibe their food; we are equally 
ignorant of the mode in which the elements of the soil are 
associated and combined; we have, too, a very imperfect 
knowledge of the chemical transformations and decomposi- 
tions thet occur within it. We cannot, therefore, dissect 
the soil and decide what and how much is immediately 
available, and what is not. Furthermore, the soil is chem- 
ically so complex, and its relations to the plant are so com- 
plicated by physical and physiological conditions, that we 
may, perhaps, never arrive at a clear and unconfused idea 
of the mode by which it nourishes a crop. Nevertheless, 
what we have attained of knowledge and insight in this 
direction is full of value and encouragement. 
Deportment of the Soil towards Solvents.—When we 
