Keeping in mind, then, the fact we have just stated—viz., that the composition of urine will vary according to different conditions—we may obtain an approximate idea of what its composition is from the following results of analyses by Stoeckhardt. In 1000 parts the following quantities of water, nitrogen, phosphoric acid, and alkalies were found to be present.
From the following table it will be seen that the urine of swine (containing 97 per cent of water) is much poorer in nitrogen and alkalies than is the case with the urine of the sheep, horse, or cow.[134] While this is the case, the amount of phosphoric acid it contains is greater than that contained in the sheep's urine.
| Phosphoric | ||||||||
| Water. | Nitrogen. | Acid. | Alkalies. | |||||
| Per | Per | Per | Per | |||||
| 1000 | Per | 1000 | Per | 1000 | Per | 1000 | Per | |
| parts. | cent. | parts. | cent. | parts | cent. | parts. | cent. | |
| Sheep (2 lb. hay per diem) | 865 | 86.5 | 14 | 1.4 | .5 | .050 | 20 | 2.0 |
| Swine (winter food) | 975 | 97.5 | 3 | .3 | 1.25 | .125 | 2 | .2 |
| Horses (hay and oats) | 890 | 89.0 | 12 | 1.2 | — | — | 15 | 1.5 |
| Cows (hay and potatoes) | 920 | 92.0 | 8 | .8 | — | — | 14 | 1.4 |
Phosphoric acid is present in the urine of the farm animals in the most minute traces: practically, it may be considered to be wanting in the urine of the horse and the cow, and is present only in small quantities in sheep's urine. The pig's urine, indeed, contains it in larger quantities; but the percentage is still so small as to justify the statement that the urine of the common farm animals is not a complete manure, and must be supplemented by phosphates, if it is to be used alone. The incomplete nature of urine as a manure constitutes a strong argument in favour of its being applied along with the solid excreta, which contain, as we have seen, considerable quantities of phosphoric acid. It is on this account that the drainings of rotten manure-heaps are more valuable, from a manurial point of view, than urine itself, since these contain the soluble portion of the phosphates in the solid excreta.[135] The urine of all animals, however, is not equally poor in phosphates. In the case of flesh-eating animals, such as the dog, the urine is found to contain them in considerable quantities.
The above tables show that the most valuable urine, weight for weight, is that of the sheep, as it contains the largest amount of alkalies (including potash) and nitrogen; that the urine of the horse comes next; then that of the cow; while, as has already been pointed out, that of the pig is the poorest.
In order to make our survey of the composition of urine uniform with that of the dung, let us see how the urine of the common farm animals compares in the matter of the composition of its dry substance. The following results (basing our calculations on Stoeckhardt's figures, previously given) show this:—
| Nitrogen, | Phosphoric acid, | Alkalies, | |
| per cent. | per cent. | per cent. | |
| Pig | 12.0 | 5 | 8 |
| Horse | 10.9 | trace | 13.6 |
| Sheep | 10.4 | 3.7 | 14.9 |
| Cow | 10.0 | trace | 17.5 |
From these figures we see that the dry substance of the urine of the pig is richest in nitrogen and phosphoric acid, but poorest in alkalies, of the four common farm animals; that of the horse comes next in the amount of nitrogen it contains, but that, on the whole, there is very little difference between the horse, cow, and sheep in this respect.[136]
As in the case of the dung, this subject is best studied in relation to the food consumed. We are again indebted to Wolff's investigations for valuable information on this point. He has found that the following percentages of organic matter, nitrogen, and mineral substances, originally present in the dry matter of the food, are voided in the urine:—
| Cow. | Ox. | Sheep. | Horse. | Average. | |
| Organic matter | 4.0 | 4.4 | 2.0 | 3.3 | 3.4 |
| Nitrogen | 31.0 | 54.8 | 42.3 | 60.7 | 47.2 |
| Mineral substances | 43.1 | 34.3 | 41.0 | 37.5 | 39.0[137] |