Section Iv.—Natural Distinction or Difference Between Animal and

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.

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.”