Lawes
and Gilbert that different plants require and consume very different
amounts of inorganic matter. (12/10. 'Journal of the Royal Agricultural
Society of England' volume 24 part 1.) But the amount in the soil would
probably not make so great a difference to the several individuals of
any particular species as might at first be expected; for the
surrounding species with different requirements would tend, from
existing in greater or lesser numbers, to keep each species in a sort of
equilibrium, with respect to what it could obtain from the soil. So it
would be even with respect to moisture during dry seasons; and how
powerful is the influence of a little more or less moisture in the soil
on the presence and distribution of plants, is often well shown in old
pasture fields which still retain traces of former ridges and furrows.
Nevertheless, as the proportional numbers of the surrounding plants in
two neighbouring places is rarely exactly the same, the individuals of
the same species will be subjected to somewhat different conditions with
respect to what they can absorb from the soil. It is surprising how the
free growth of one set of plants affects others growing mingled with
them; I allowed the plants on rather more than a square yard of turf
which had been closely mown for several years, to grow up; and nine
species out of twenty were thus exterminated; but whether this was
altogether due to the kinds which grew up robbing the others of
nutriment, I do not know.
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