casion to draw the student's attention to further on in
discussing the nature of farmyard manure--and that is, that the urine of
the common farm animals is practically devoid of phosphoric acid.

_Sources of Loss of Phosphoric Acid in Agriculture._

As we have already done in the case of nitrogen, we may now attempt to
form some conception of the sources of loss and gain of phosphoric acid
in the soil. The sources of loss may be divided into natural and
artificial. Of natural sources of loss we have only one, and that is
loss by drainage.

_Loss of Phosphoric Acid by Drainage._

We have already seen that the condition in which phosphoric acid is
present in the soil is as insoluble phosphate. In drainage-water it
occurs in mere traces. Minute though the amount seems when stated as
percentage, and small as it appears beside the loss (from the same
source) of nitrogen, it is yet, if considered for large areas,
sufficiently striking. Thus it has been estimated that in the river Elbe
there is carried off by drainage from the fields of Bohemia 2-3/4
million pounds (1200 tons) of phosphoric acid annually. This, it is
true, is a very trifling amount compared with the annual loss of
nitrogen from an equal area; but then it must be remembered, on the
other hand, the sources of gain to the soil of this ingredient are not
so numerous as are those of nitrogen, the only sources of phosphoric
acid being in the manure applied to the soil, and that coming from the
gradual disintegration of phosphatic minerals.

_Artificial Sources of Loss._

The other sources of loss may be classed under the term artificial, and
are connected with agricultural practice. Just as we have seen that in
the case of nitrogen enormous quantities of that substance are
constantly being removed from the soil in those crops which are
consumed off the farm, so, too, enormous quantities of phosphoric acid
are being removed in the same way. As illustrating this fact, it may be
mentioned that Professor Grandeau has recently estimated that in the
entire crops grown in France in one year there are about 298,200 tons of
phosphoric acid; while the amount returned in the dung of farm animals
is only 157,200, or only about one-half of what is removed in the crops,
leaving a deficit of 147,000 tons to be made good by the addition of
artificial phosphatic manures, if the fertility of the soil is to be
maintained. The same authority has calculated that in the bones