The manurial value of the solid excreta of animals—i.e., the proportion they contain of nitrogen, phosphoric acid, and potash—depends on a variety of conditions.
The solid excreta of horses, sheep, cows, and pigs, are well known to possess different properties, as well as to vary in their composition.
What, however, has a still greater influence is the nature of the food. This is owing to the fact that the solid excreta are made up of undigested food. We can scarcely expect the same quality of solid excreta from an animal fed on poor diet as from an animal fed on very much richer diet. Again, the percentage of the food voided in the solid excreta varies in the case of different animals.[131]
Another consideration which enters into the question is the age, as well as the treatment, of the animal. A young animal, during the period of its growth, absorbs from its food into its system a larger quantity of the three fertilising substances, nitrogen, phosphoric acid, and potash, than is the case with an adult animal whose weight is neither increasing nor diminishing. A working horse, similarly, will return more of the nitrogen, phosphates, and potash in its dung than one not at work and which is permitted to gain in weight. The nature of the composition of the solid excreta, therefore, will depend on the nature of the food, age, breed, condition, and treatment of the animal.
Let us now investigate shortly the influence of the above considerations. The solid excrements of the common farm animals are generally distinguished from one another according to the rate at which they decompose or ferment on keeping. Thus horse-dung is generally known as a "hot" dung; while cow-dung, on the other hand, is known as "cool." Why this should be so is not absolutely clear. Probably it is owing to the fact that the former contains less water, as well as to the fact (and this probably has more to do with it) that it contains a larger percentage of fertilising matter, especially nitrogen, thus affording conditions more favourable for rapid fermentation than in the case of the more moist and less rich cow-dung.
The composition of the solid excreta of various animals, as we have just said, varies with the nature of their food; so that it is impossible to take any analyses as absolutely representing its composition. It may be interesting, however, to compare the analyses of samples of horse-dung with those of some other of the commoner farm animals, with a view to obtaining an approximate idea of this difference.
Stoeckhardt has found that in 1000 lb. of the fresh solid excreta of the animals below mentioned, there were the following amounts of nitrogen, phosphoric acid, and alkalies:—
| Phosphoric | ||||||||
| Water. | Nitrogen. | Acid. | Alkalies. | |||||
| Reduced | Reduced | Reduced | ||||||
| to | to | to | ||||||
| lb. | per cent. | lb. | per cent. | lb. | per cent. | lb. | per cent. | |
| Horses (winter food) | 760 | 76 | 5 | .50 | 3-1/2 | .35 | 3 | .30 |
| Cows (winter food) | 840 | 84 | 3 | .30 | 2-1/2 | .25 | 1 | .10 |
| Swine (winter food) | 800 | 80 | 6 | .60 | 4-1/2 | .45 | 5 | .50 |
| Sheep (2 lb. hay per diem) | 580 | 58 | 7-1/2 | .75 | 6 | .6 | 3 | .30 |
From the above table it will be seen that the sheep's dung contains the least percentage of water, and is richer in nitrogen and phosphoric acid than any of the other three. The percentage of alkalies, of which the most important is potash, is, however, not so large. This may be accounted for by the interesting and well-known fact that a large percentage of potash is to be found in the wool of sheep.[132]
The solid excrement of the sheep is, therefore, weight for weight, the most valuable as a manure, as it contains more nitrogen and phosphates than the others, and at the same time is much drier.