from bones,
apatite, or fossil excrements (the coprolithes).

The principal problem for agriculture is, how to replace those
substances which have been taken from the soil, and which cannot be
furnished by the atmosphere. If the manure supplies an imperfect
compensation for this loss, the fertility of a field or of a country
decreases; if, on the contrary, more are given to the fields, their
fertility increases.

An importation of urine, or of solid excrements, from a foreign
country, is equivalent to an importation of grain and cattle. In a
certain time, the elements of those substances assume the form of
grain, or of fodder, then become flesh and bones, enter into the
human body, and return again day by day to the form they originally
possessed.

The only real loss of elements we are unable to prevent is of the
phosphates, and these, in accordance with the customs of all modern
nations, are deposited in the grave. For the rest, every part of
that enormous quantity of food which a man consumes during his
lifetime ( say in sixty or seventy years), which was derived from
the fields, can be obtained and returned to them. We know with
absolute certainty, that in the blood of a young or growing animal
there remains a certain quantity of phosphate of lime and of the
alkaline phosphates, to be stored up and to minister to the growth
of the bones and general bulk of the body, and that, with the
exception of this very small quantity, we receive back, in the solid
and fluid excrements, all the salts and alkaline bases, all the
phosphate of lime and magnesia, and consequently all the inorganic
elements which the animal consumes in its food.

We can thus ascertain precisely the quantity, quality, and
composition of animal excrements, without the trouble of analysing
them. If we give a horse daily 4 1/2 pounds' weight of oats, and 15
pounds of hay, and knowing that oats give 4 per cent. and hay 9 per
cent. of ashes, we can calculate that the daily excrements of the
horse will contain 21 ounces of inorganic matter which was drawn
from the fields. By analysis we can determine the exact relative
amount of silica, of phosphates, and of alkalies, contained in the
ashes of the oats and of the hay.

You will now understand that the constituents of the solid parts of
animal excrements, and therefore their qualities as manure, must
vary with the nature of the creature's food. If we feed a cow upon
beetroot, or potatoes, without hay