needs to be richer in available
phosphoric acid, to produce even a fair crop of turnips, than to produce
a large crop of wheat. And the experience of farmers everywhere tends in
the same direction. England is the greatest turnip-growing country in
the world, and you will find that where one farmer applies potash to
turnips, or superphosphate to wheat, a hundred farmers use
superphosphate as a special manure for the turnip crop."

"And we are certainly warranted in saying," continued the Doctor, "_that
the composition of a plant affords_, in practical agriculture, and on
ordinary cultivated soils, _no sort of indication as to the composition
of the manure it is best to apply to the crop_."

"Again," continued the Doctor, "if the theory was a correct one, it
would follow that those crops which contained the most nitrogen, would
require the most nitrogen in the manure. Beans, peas, and clover would
require a soil or a manure richer in available nitrogen than wheat,
barley, or oats. We know that the _very reverse_ is true--know it from
actual, and repeated, and long-continued experiments like those of Lawes
and Gilbert, and from the common experience of farmers everywhere."

"You need not get excited," said the Deacon, "the theory is a very
plausible one, and while I cannot dispute your facts, I must confess I
cannot see _why_ it is not reasonable to suppose that a plant which
contains a large amount of nitrogen should not want a manure specially
rich in nitrogen; or why turnips which contain so much potash should not
want a soil or manure specially rich in potash."

"Do you recollect," said I, "that crop of turnips I raised on a poor
blowing-sand?"

"Yes," said the Deacon, "it was the best crop of turnips I ever saw
grow."

"That crop of turnips," said I, "was due to a dressing of superphosphate
of lime, with little or no potash in it."

"I know all that," said the Deacon. "I admit the fact that
superphosphate is a good manure for turnips. What I want to know is the
reason why superphosphate is better for turnips than for wheat?"

"Many reasons might be given," said the Doctor; "Prof. Voelcker
attributes it to the limited feeding range of the roots of turnips, as
compared to wheat. 'The roots of wheat,' says Prof. Voelcker, 'as is
well known, penetrate the soil to a much greater depth than the more
delicate feeding fibres of the roots of turnips. Wheat, remaining on the
ground two or three months longer than turn