on, as is seen in the
detailed analysis given in page 205.

In other guanos which have undergone more complete decomposition, and
from which the soluble matters have been more or less completely
exhausted by rain, the alkaline salts, or at least the potash they
originally contained, have almost entirely disappeared. Hence an
important difference between Peruvian guano and most other varieties.
The former can be used as a complete substitute for farm-yard manure,
and excellent crops of turnips and potatoes can be raised by means of it
alone, and at a less cost than with ordinary dung. But though this may
be done, and in many cases is attended with great economic advantages,
it is a practice that cannot be recommended for general use, because the
quantity of valuable matters contained in the usual application of guano
is much smaller than in farm-yard manure, and the probability is that it
would not, if used alone during a succession of years, be sufficient to
maintain the soil permanently in a high state of fertility. Five cwt. of
Peruvian guano, which is a liberal application per acre, contains about
95 lbs. of ammonia, and 130 of phosphates, while 20 tons of good
farm-yard manure contain 312 of ammonia, and about the same quantity of
phosphates, and when the other constituents, such as potash and soda,
are compared with those in guano, the difference is still more striking.
On the other hand, guano is a rapidly acting manure; its constituents
are in a condition in which they are more immediately accessible by the
plant, and its immediate effect is far more marked, as it is chiefly
expended on the crop to which it is applied. It has indeed been alleged
that it produces no effects on the subsequent crops, but this opinion
can scarcely be considered as well founded. In no case does the crop
raised by means of it contain the whole of the ammonia or phosphates
present in the manure, and the unappropriated quantity, though it may,
and probably does, escape from the lighter soils, must be retained and
preserved for the use of subsequent crops by heavy and retentive clay
soils. The general inference is, that though guano may at an emergency
be used as an entire substitute for farm-yard manure, the practice is
one to be generally avoided. When, however, as occasionally happens
after a long continued use of farm-yard manure, organic matters have
accumulated in the soil, and passed into an inert condition, then
Peruvian guano may