n price, the value
attached to each of their constituents ought to be varied with the state
of the market; but it is obviously impossible for the farmer to watch
the changes in price with such minuteness as to enable him to do this,
and it is much more convenient, as well as safer, to adopt a fixed
average, which can be used with reasonable accuracy at all times. The
fact is, that this system of valuation is only an approximation to the
truth; and if absolute accuracy were aimed at, it would be necessary to
vary the estimates, not only at different times, but at different
localities at the same time, and to some extent also according to the
kind of manure. The price of soluble phosphates more especially,
fluctuates to a great extent, being practically fixed by each
manufacturer according to the facilities which his position or command
of raw material offer for producing them at a low rate. We thus find
that when made from bones alone, the cost of that substance is not
unfrequently as high as L40 per ton, and when bone-ash alone is used it
is sometimes as low as L20. Such extreme differences, of course, cannot
be taken into account in the system of valuation adopted, where all that
can be done is to take average values, which, when applied to average
samples, ought to bring out their value.

The data which have already been given regarding the price of the
individual constituents of manures can be applied to the determination
of the value of any mixture in two different ways by means of the
subjoined table:--

+----------------------------+-----------------+-------------------+
| | Price per Ton. | Per cent per Ton. |
+----------------------------+-----------------+-------------------+
| Ammonia | L60 0 0 | L0 12 0 |
| Insoluble phosphates | 7 0 0 | 0 1 5 |
| Do. in phosphatic guanos | 10 0 0 | 0 2 0 |
| Soluble phosphates | 30 0 0 | 0 6 0 |
| Biphosphate of lime | 46 16 0 | 0 9 4-1/2 |
| Alkaline salts | 1 0 0 | 0 0 2-4/10 |
| Sulphate of lime | 1 0 0 | 0 0 2-4/10 |
| Potash | 20 0 0 | 0 4 0 |
| Nitrate of soda | 16 0 0 | 0 3 2-1/2 |
| Organic matter | 0 10 0 | 0 0 1-1/4 |
+----------------------------