CHAP. IX. 
FOOD OF PLANTS. 
309 
tion of nitrate of strontian, the other in one containing pure 
distilled water, after the lapse of a week the water in the 
second glass was tested, but no strontian could be discovered 
in it, although a single grain in one pint would have been 
readily detected. Hence it appears, " that plants do possess, 
to a certain extent at least, a power of selection by their roots, 
and that tlie earthy constituents which form the basis of their 
solid parts are determined as to quantity by some primary 
law of nature, although their amount may depend upon the 
more or less abundant supply of the principles presented to 
them from without." Linn. Trans, xvii. 266. 
It must be obvious that the exhaustion of soil by plants 
means their having consumed all the nutritive particles that 
it contains. Whether this means all particles that are ca- 
pable of forming carbonic acid, is, however, not so certain : 
it is highly probable that other matters are equally indispen- 
sable to the health of particular plants ; as, for example, of corn. 
Corn cannot remain in health unless it has the power of at- 
tracting fluid silex from the earth, and of consolidating it in 
its cuticle. It is to be supposed that the presence of alkaline 
principles in the soil is necessary to render the siliceous mat- 
ter soluble ; therefore, to exhaust a soil of alkaline principles 
would be to render it unfit for the support of corn ; and, con- 
sequently, alkaline principles may be considered nutritive 
in regard to corn : and so of other things. 
Hence arises the very complicated nature of the theory of 
manures, and the seeming impossibility of reducing it to any 
fixed and intelligible laws. Ignorant as we are of most of the 
more obscure phenomena that are attendant upon vegetable 
life, unacquainted with the action of a large proportion of the 
principles that the chemist discovers among the tissue of plants, 
and incapacitated by our limited means of observation from 
watching any except the most obvious and general properties 
of living vegetable matter, we cannot expect, in such a state of 
things, to arrive at any precise ideas as to what kind of food 
or stimulants exercises the most energetic and wholesome in- 
fluence upon plants. I accordingly feel no surprise at the 
statement of a friend of mine, well known alike for his agri- 
cultural skill, his chemical knowledge, and his remarkable 
X 3 
