SPECIAL MANURES.
Twenty five to thirty years ago, much was said in regard to special
manures. Fertilizers were prepared for the different crops with special
reference to the composition of the plants.
“But it was known then, as now,” said the Doctor, “that all our
agricultural plants were composed of the same elements.”
“True, but what was claimed was this: Some crops contain, for instance,
more phosphoric acid than other crops, and for these a manure rich in
phosphoric acid was provided. Others contained a large proportion of
potash, and these were called ‘potash crops,’ and the manure prescribed
for them was rich in potash. And so with the other ingredients of
plants.”
“I recollect it well,” said the Doctor, “and, in truth, for several
years I had much faith in the idea. It was advocated with consummate
ability by the lamented Liebig, and in fact a patent was taken out by
the Musgraves, of Liverpool, for the manufacture of Liebig’s Special
Manures, based on this theory. But the manures, though extensively used
by the leading farmers of England, and endorsed by the highest
authorities, did not in the end stand the test of actual farm practice,
and their manufacture was abandoned. And I do not know of any
experienced agricultural chemist who now advocates this doctrine of
special manures.
“Dr. Vœlcker says: ‘The ash-analyses of plants do not afford a
sufficiently trustworthy guide to the practical farmer in selecting the
kind of manure which is best applied to each crop.’”
“Never mind the authorities,” said the Deacon; “what we want are facts.”
“Well,” replied the Doctor, “take the wheat and turnip crop as an
illustration.
“We will suppose that there is twice the weight of wheat-straw as of
grain; and that to 10 tons of bulbs there is 3 tons of turnip-tops. Now,
100 lbs. each of the ash of these two crops contain:
_Wheat crop._ _Turnip crop._
Phosphoric acid 11.44 7.33
Potash 15.44 32.75
Sulphuric acid 2.44 11.25
Lime 5.09 19.28
Magnesia 3.33 1.56
“There are other ingredients,” continued the Doctor, “but these are the
most important.
“Now, if you were going to compound a manure for wheat, say 100 lbs.,
consisting of potash and phosphoric acid, what would be the
proportions?”
The Deacon figured for a few moments, and then produced the following
table:
100 Lbs. Special Manure for Wheat and Turnips.
_Wheat manure._ _Turnip manure._
Phosphoric acid 42½ lbs. 18⅓ lbs.
Potash 57½ ” 81⅔ ”
------------ ------------
100 lbs. 100 lbs.
“Exactly,” said the Doctor, “and yet the experiments of Lawes and
Gilbert clearly prove that a soil 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. Vœlcker
attributes it to the limited feeding range of the roots of turnips, as
compared to wheat. ‘The roots of wheat,’ says Prof. Vœlcker, ‘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 turnips, 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 raise a good crop of cabbage,
I should not think of consulting a chemical analysis of the cabbage. If
I set out cabbage on an acre of land, which, without manure, would
produce 16 tons of cabbage, does any one mean to tell me that if I put
the amount of nitrogen, phosphoric acid and potash which 10 tons of
cabbage contain, on an adjoining acre, that it would produce an extra
growth of 10 tons of cabbage. I can not believe it. The facts are all
the other way. Plant growth is not such a simple matter as the advocates
of this theory, if there be any at this late day, would have us
believe.”