What it gives up it loses, and to the extent
of its loss it is exhausted. It is no more untrue to say that the great
cities of the world have not, in their building, exhausted the forests
and the mines to any extent, than to say that the annual abundant
harvests of corn and wheat have not, in any degree, exhausted the
prairies and bottom lands of the West. Some lands may be exhausted for
particular crops in a single year; others in five years, others in ten,
while others may yield undiminished returns for twenty, fifty, or even a
hundred years. But it is plain that annual cropping without rotation,
and without compensation by nature or art, must finally deprive the soil
of the required elements. Nor should we deceive ourselves by considering
only those exceptions whose existence is due to the fact that nature
makes compensation for the loss. Annual or occasional irrigation with
rich deposits,--as upon the Nile and the Connecticut,--allowing the
land to lie fallow, rotation of crops and the growth of wood, are so
many expedients and provisions by which nature increases the
productiveness of the earth. Nor is a great depth of soil, as two, five,
ten, or twenty feet, any security against its ultimate impoverishment.
Only a certain portion is available. It has been found in the case of
coal-mines which lie at great depths, that they are, for the present,
valueless; and we cannot attach much importance to soil that is twenty
feet below the surface.
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