rds
dryness, complete reduction to the pulverulent state, and the like. A
certain allowance ought always to be made for careful manufacture; and,
on the other hand, where the manure is damp or ill reduced, a small
deduction (the amount of which must be decided by the experience of the
valuator) ought to be made on account of the risk which the farmer runs
of loss from unequal distribution, and the extra cost of carriage of an
unnecessary quantity of water.

It is also necessary to take into account the particular element
required by the soil. Thus, a farmer who finds his soil wants
phosphates, will look to the manure containing the largest quantity of
that substance, and possibly not requiring ammonia, will not care to
estimate at its full value any quantity of that substance which he may
be compelled to take along with the former, but will look only to the
source from which he can obtain it most cheaply. It may be well,
therefore, to point out that ammonia is most cheaply purchased in
Peruvian guano; insoluble phosphates in coprolites; and soluble
phosphates in superphosphates, made from bone-ash alone. In general,
however, it will be found most advantageous to select manures in which
the constituents are properly adjusted to one another, so that neither
ammonia, soluble nor insoluble phosphates, preponderate; but, of course,
it must frequently happen that it will prove more economical to buy the
substances separately and to make the mixture, than to take the manure
in which they are ready mixed.

In judging of the value of any manure, it is also important to make sure
that the analysis which forms the basis of the calculation is that of a
fair sample, which correctly represents the bulk actually delivered to
the purchaser, and not one which has been made to do duty for an
unlimited quantity of manure, which is supposed to be all of equal
quality, as often happens in the hands of careless manufacturers, and
too great attention cannot be devoted to the selection of the sample,
which is very often done in an exceedingly slovenly manner.

CHAPTER XIII.

THE ROTATION OF CROPS.

Reference has already been more than once made to the fact that a crop
growing in any soil must necessarily exhaust it to a greater or less
extent by withdrawing from it a certain quantity of the elements to
which its fertility is due. That this is the case has been long admitted
in practice, and it has also been established that the e