Voelcker illustrate this in
a striking manner. Two lots of guano--one pure and one mixed with
salt--were exposed to the action of the air for a month, and were then
tested as to the amount of water they contained, when it was found that
the lot containing the salt had absorbed 2 per cent more water than the
other.

Much stress has been laid on the importance of having the guano buried a
certain depth in the soil; and many experiments have been carried out to
prove how much better it acts when so applied. This is probably due to
the prevention of any loss of volatile ammonia, and the mixture of the
manure with the soil-particles before it comes in contact with the
plant-roots. This last precaution is an important one, for it has been
found that the raw material is apt to have a bad effect on the seed or
the plant's roots. This has been found to be especially the case in
regard to potatoes, the quality of which has been found to suffer when
the guano is brought into direct contact with the tubers. As guano is a
manure which is speedily available, it is desirable to apply it as
shortly before it is required by the plant as possible. It is therefore
generally best applied in spring, shortly before seed-time, or indeed at
the same time. Where farmyard manure is used, the guano has been
recommended to be used as a top-dressing in small quantities. In the
majority of cases it will be advisable, however, not to apply it as a
top-dressing, for the various reasons above-mentioned.

_Quantity to be used._

As to the quantity to be used, this of course will depend on the soil,
the crop, and the amount and nature of the other manures employed: 1 to
4 cwt. per acre have been the usual limits, but even heavier dressings
have been commonly resorted to, especially in Scotland, where 6 to 8 or
even 9 cwt. for turnips are often used. Sir J. B. Lawes and Sir James
Caird long ago, shortly after the introduction of guano, estimated, from
the experiments they carried out, that the application of 2 cwt. per
acre to the wheat crop gave an increase of 8 to 9 bushels in grain, and
added a fourth to the quantity of straw. The former authority recommends
2 to 3 cwt. per acre for wheat, to be sown broadcast and harrowed into
the land before sowing the seed. We have already stated that it may be
used in all soils and for all kinds of crops. While this is so, it has
been found to have specially favourable results when applied to the
turnip crop,