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.