VEGETABLE MANURES, AND THE CAUSE OF THIS DIFFERENCE.
In what do animal manures differ from vegetable manures,—what is the
cause of this difference,—how does the digestion of vegetable matter
improve its value as a manure?
1. The characteristic distinction between animal and vegetable manures
is this,—that the former contain a much larger proportion of nitrogen
than the latter. This will be seen at once, by comparing together
the tables given in the two preceding sections, in which the numbers
represent the relative agricultural values of certain animal and
vegetable substances compared with farm-yard manure. The lowest numbers
represent the highest value, and the largest amount of nitrogen, and
these low numbers are always opposite to the purest animal substances.
2. In consequence of containing so much nitrogen, animal substances
are further distinguished by the rapidity with which, when moist, they
putrify or run to decay. During this decay the nitrogen they contain
gradually assumes the form of ammonia, which is perceptible by the
smell, and which, when proper precautions are not taken, is apt in
great part to escape into the air. Hence the loss by fermenting manure
too completely,—or without proper precautions to prevent the escape of
volatile substances. And as animal manure, when thus over-fermented, or
permitted to lose its ammonia into the air, is found much less active
upon vegetation than before; it is reasonably concluded, that to this
ammonia, chiefly, their peculiar virtue, when rightly prepared, is in a
great measure to be ascribed.
Vegetable substances do not decay so rapidly,—do not emit the odour of
ammonia when fermenting,—nor, when prepared in the most careful way,
does vegetable manure exhibit the same remarkable action upon vegetable
life as is displayed by almost every substance of animal origin.
3. Whence do animal substances derive all this nitrogen? Animals live
only upon vegetable productions containing little nitrogen; can they
then procure all they require from this source alone? Again, does the
act of digestion produce any chemical alteration upon the food of
animals, that their excretions should be a better manure,—should be
richer in nitrogen than the substances on which they feed? Does theory
throw any light upon the opinion generally entertained among practical
men upon this point?
These two apparently distinct questions will be explained by a brief
reference to one common natural principle.
Animals have two necessary vital functions to perform,—to breathe and
to digest. Both are of equal importance to the health and general
welfare of the animal. The digester (the stomach) receives the food,
melts it down, extracts from it what is best suited to its purposes,
and conveys it into the blood. The breathers (the lungs) sift the blood
thus mixed up with the newly digested food, combine oxygen with it,
and extract carbon,—which carbon, in the form of carbonic acid, they
discharge by the mouth and nostrils into the air.
Such is a general description of these two great processes,—their
effect upon the food that remains in the body and has to be rejected
from it, is not difficult to perceive.
Suppose an animal to be full grown. Take a full grown man. All that he
eats as food is intended merely to renovate or replenish his system,
to restore that which is daily removed from every part of his body by
natural causes. _In the full grown state, every thing that enters the
body must come out of the body_ in one form or another. The first part
of the food that escapes is that portion of its carbon that passes off
from the lungs during respiration. This quantity varies in different
individuals—chiefly according to the quantity of exercise they take.
From 5 to 9 ounces a day is the average quantity, though in periods of
violent bodily exertion 13 to 15 ounces of carbon are breathed out in
the form of carbonic acid.
Suppose a man to eat a pound and a half of bread and a pound of beef in
24 hours, and that he gives off by respiration 8 ounces of carbon (3500
grains) during the same time. Then he has
Carbon. Nitrogen.
Taken, in his food, about 4500 grains, and 500 grs. while
He has given off in }
respiration, } 3500 and little or no nitrogen,
Leaving to be converted }
into food, or to } 1000 grs. and 500 grs.
be rejected, }
Our two conclusions, therefore, are clear. The vegetable food, by
respiration, is freed from a large portion of its carbon, which is
discharged into the air,—nearly the whole of the nitrogen remaining
behind. In the food consumed the carbon was to the nitrogen as 9 to 1;
in that which remains, after respiration has done its work, the carbon
is to the nitrogen in the proportion of only 2 to 1.
It is out of this residue, rich in nitrogen, that the several parts of
animal bodies are built up. Hence the reason why they can be formed
from food poor in nitrogen, and yet be themselves rich in the same
element.
It is this same residue also which, after it has performed its
functions within the body, is discharged again in the form of solid and
liquid excretions. Hence the greater richness in nitrogen,—the greater
fertilizing power of the dung of animals than of the food on which they
live.
Two other remarks I shall add for the benefit of the practical man.
1. The manure of the cow, taking it mixed, is not so rich in nitrogen
as that of man,—because the cow in the stall, large though it be, and
great the bulk of food it consumes, does not give off much more carbon
by respiration than an active full grown man. Hence the proportion of
carbon in the excretions of this animal is greater than in those of
man. The dry manure is richer than the dry food, weight for weight, but
not in the same proportion as if the cow respired a quantity of carbon
more nearly corresponding to its bulk, when compared with the weight of
carbon thrown off from the lungs of man.
2. Since the parts of animals—their blood, muscles, tendons, and
the gelatinous portion of the bones—contain much nitrogen, young
beasts which are growing, must appropriate to their own use, and
work up into flesh and bone, a portion of the nitrogen contained
in the _non-respired_ part of their food. But the more they thus
appropriate, the less will pass off into the fold-yard; and hence it
is natural to suppose that the manure, either liquid or solid, which
is prepared where many growing cattle are fed, will not be so rich
as that which is yielded by full grown animals. I am not aware how
far this deterioration has been observed in practice, but it may with
some degree of certainty be expected to take place,—unless by giving
a richer food to the young cattle, the difference to the farm-yard be
made up.[18]
[18] Though I have dwelt as long upon these interesting and, I believe,
novel considerations, as the limits of this little work will permit,
yet I must refer the reader for fuller details, and to perhaps a
clearer exposition of the principles above advanced than I have here
been able to give, to my “LECTURES _on Agricultural Chemistry and
Geology_.”