ips, can avail itself for a
longer period of the resources of the soil; therefore in most cases the
phosphoric acid disseminated through the soil is amply sufficient to
meet the requirements of the wheat crop; whilst turnips, depending on a
thinner depth of soil during their shorter period of growth, cannot
assimilate sufficient phosphoric acid, to come to perfection.' This is,
I believe, the main reason why the direct supply of readily available
phosphates is so beneficial to root-crops, and not to wheat."

"This reason," said I, "has never been entirely satisfactory to me. If
the roots of the turnip have such a limited range, how are they able to
get such a large amount of potash?

"It is probable that the turnip, containing such a large relative amount
of potash and so little phosphoric acid, has roots capable of absorbing
potash from a very weak solution, but not so in regard to phosphoric
acid."

"There is another way of looking at this matter," said the Doctor.
"You must recollect that, if turnips and wheat were growing in the same
field, both plants get their food from the same solution. And instead
of supposing that the wheat-plant has the power of taking up more
phosphoric acid than the turnip-plant, we may suppose that the turnip
has the power of rejecting or excluding a portion of phosphoric acid. It
takes up no more potash than the wheat-plant, but it takes _less_
phosphoric acid."

But it is not necessary to speculate on this matter. For the present we
may accept the fact, that the proportion of potash, phosphoric acid, and
nitrogen in the crop is no indication of the proper proportion in which
these ingredients should be applied to the soil for these crops in
manure.

It may well be that we should use special manures for special crops; but
we must ascertain what these manures should be, not from analyses of the
crops to be grown, but from experiment and experience.

So far as present facts throw light on this subject, we should conclude
that those crops which contain the _least_ nitrogen are the most likely
to be benefited by its artificial application; and the crops containing
the most phosphoric acid, are the crops to which, in ordinary practical
agriculture, it will be unprofitable to apply superphosphate of lime.

"That," said the Doctor, "may be stating the case a little too strong."

"Perhaps so," said I, "but you must recollect I am now speaking of
practical agriculture. If I wanted to rais