ON THE METHOD OF APPLICATION AND ON THE MIXING OF MANURES.
Having considered the manuring of the different crops, we may now pass
on to the consideration of some points in the method of application and
on the mixing of manures.
_Equal Distribution of Manures._
A most important object in applying manures is to effect equal
distribution of the manure in the soil. This is often, however,
unusually difficult to do, especially in the case of artificial manures,
where the quantity to be spread over a large area of the soil is
extremely small. The difficulty in the case of farmyard or other very
bulky manure is not so great. In order to overcome this difficulty in
the case of artificial manures, it is often advisable to mix them with
some such substance as sand, ashes, loam, peat, or salt. The manure is
thus diluted in strength, and a very much larger bulk of substance is
obtained to work with. Circumstances must decide which of these
substances to use. If the soil be a heavy clay, the addition of sand or
ashes may have an important mechanical effect in improving its texture;
while, on the other hand, if it be a light soil, the addition of peat
may improve its mechanical condition. It must also be remembered that
peat itself contains a large amount of nitrogen, and thus forms a manure
of some value. In using loam or peat to mix with artificial manures,
they should be first dried and then riddled; while if ashes be used,
they should be previously reduced to a fine state. Wood-ashes, however,
must be used with caution, and ought not to be mixed with ammoniacal
manures, as they are apt to contain caustic alkali, which would tend to
drive off the ammonia in a volatile state.
It has been recommended, in order to save trouble and effect equal
distribution, that the manure to be applied should always be made up to
the same amount, so that the farmer by experience may ascertain the rate
at which to apply it. And here it may be well to say a word or two on
the subject of mixing manures--a subject with which the farmer is not
always so conversant as it is desirable in the interests of his own
pocket he should be.
_Mixing Manures._
It is to be feared that not unfrequently indiscriminate mixing may cause
very serious loss in the most valuable constituent of a manure. It may
be well, therefore, to point out one or two of the causes of the loss
that is apt to ensue on the mixing of different kinds of manures
together.
As the subject depends for its clear comprehension on certain chemical
elementary principles, it may be well for the benefit of non-chemical
readers to state these pretty fully.
_Risks of Loss in Mixtures._
The risks of loss which may occur from the mixing of artificial manures
together may be of different kinds. One is the risk of actual loss of a
valuable ingredient through volatilisation; another is the risk of the
deterioration of the value of a mixture through change of the chemical
state of a valuable ingredient. Undoubtedly the most common and most
serious source of loss is the former. Of the three valuable manurial
ingredients--nitrogen, phosphoric acid, and potash--only the first is
liable to loss by volatilisation, and this generally only when the
nitrogen is either in the form of ammonia or nitric acid.
_Loss of Ammonia._
Ammonia, when uncombined, is a very volatile gas with a pungent smell, a
property which enables its escape from a manure mixture to be very
easily detected. It belongs to a class of substances which are known
chemically as bases, and which have the power of combining with acids
and forming salts. Sulphate of ammonia is a salt formed--as its name
indicates--by the union of the base, ammonia, with the acid, sulphuric
acid. Now when ammonia unites with sulphuric acid and forms sulphate of
ammonia, it is no longer volatile and liable to escape as a gas, but
becomes "fixed," as it is called.
Although most salts are more or less stable bodies--not liable to
change--if left alone, and not submitted to a high temperature or
chemical action, they can be easily decomposed if they are heated or
brought into contact with some other substance which will give rise to
chemical action. Sulphate of ammonia is a salt that is very easily
decomposed. This is due to the fact that its base, ammonia, is very
volatile, and not capable of being held very firmly by an acid, even by
sulphuric, which is among the least volatile of all the common acids.
If, therefore, sulphate of ammonia be heated above the boiling-point of
water, or brought in contact with any other substance which will give
rise to chemical action, it is easily decomposed. Now a salt may be
acted upon by a base or an acid or another salt. When it is brought in
contact with a base, if the base with which it is brought in contact be
a stronger base than the base of the salt, the salt is decomposed, and a
new salt is formed. The acid, in short, exchanges its old base for the
new one.
_Effect of Lime on Ammonia Salt._
This is exactly what takes place when the base lime comes in contact
with an ammonium salt, such as sulphate of ammonia. The sulphuric acid
exchanges its old base, ammonia, for the stronger base, lime, and
sulphate of lime is formed, and ammonia is set free as a gas, and
escapes and is lost. Sulphate of ammonia, or any substance in which
there is an ammonia salt, must never be brought in contact with free
lime, otherwise the ammonia will be lost, and should be harrowed in on
chalky soils for this reason.
It is different entirely with gypsum--which is sulphate of lime--or
phosphate of lime, both of which may be safely mixed with sulphate of
ammonia without any danger of escape of ammonia. It follows from the
above that a mixture which must on no account be tried is slag phosphate
and sulphate of ammonia. This is because the slag phosphate contains a
large percentage of free lime, which would at once, on being brought in
contact with the sulphate of ammonia, decompose it, and cause the
ammonia to be lost. For this same reason guano must not be mixed with
slag. It is perhaps unnecessary, however, to warn one against so doing,
as it is not likely such a mixture would be made, as the ratio of
phosphoric acid to nitrogen in guanos is generally greater than is
required. If it be desired to mix the slag with a quickly available form
of nitrogen, nitrate of soda is not liable to loss; although for other
reasons it is not desirable to apply nitrate of soda along with the
slag, as the former manure should be applied almost always as a
top-dressing.
_Loss of Nitric Acid._
The risks of the loss of nitrogen in the form of nitric acid, although
not so great as they are in the case of ammonia, are still considerable.
As nitric acid is not a base but an acid, what is to be avoided in
mixing nitrates is bringing them in contact with any other manure which
contains another free and stronger acid--as, for example,
superphosphate. The free acid present in superphosphate has the tendency
to drive out the nitric acid from the nitrate and usurp its place. The
risk of loss of expulsion in the above cases is always augmented by the
rise of temperature which invariably accompanies chemical action of any
kind; and although the loss of nitrogen, in the form of nitric acid,
caused by mixing superphosphate and nitrate of soda, might, under
ordinary circumstances, amount to very little, yet, if the mixture were
to be allowed to stand any time, and the temperature of the mass to be
heightened, the loss which would undoubtedly then ensue would be
considerable.
The nitrogen salt which it is safe to mix with superphosphate is
sulphate of ammonia.
_Reversion of Phosphates._
But, as has already been mentioned, there is another loss which may
result from the mixing of manures. This is the deterioration of the
value of an ingredient by reason of change of chemical condition. This
is a source of loss that was little suspected a number of years ago, but
it is now well known that superphosphate of lime, under certain
conditions, is changed from its soluble to an insoluble form. We have
already referred to the reversion of phosphate in the chapter on the
Manufacture of Superphosphates.[247] It was there pointed out that
reversion is often caused by the presence of iron and alumina or
undissolved phosphate, and that the risk of reversion is therefore very
much less in a well-made article, made from pure raw material, than in
one made from a raw phosphate containing much iron and alumina.
Superphosphates containing a large percentage of insoluble phosphates
ought not to be kept too long before being used as a manure, otherwise
much of the labour and expense involved in their manufacture will be
lost by the reversion of their soluble phosphate. Further, it is highly
inadvisable to mix superphosphates with basic slag, which contains a
large percentage of both iron and free lime. Lastly, if it is desired to
mix superphosphate with insoluble phosphate, the mixture ought to be
made just previous to application.
_Manurial Ingredients should be applied separately._
The question of applying manure in mixtures is one on which considerable
difference of opinion may exist. For many reasons manures are often
better applied in the unmixed condition. For example, a mixture of a
quickly acting nitrogenous manure with a slowly acting phosphatic manure
is not suitable. In such a case either the nitrogenous manure will be
applied too long before it is required by the plant, and thus suffer
from risk of loss, or the phosphatic manure will not be applied long
enough before it is likely to be used. By applying manures in an unmixed
condition the chances are that a more economical use of them is made
than would otherwise be the case. On the other hand, while the
application of the separate constituents may be desirable from the
scientific point of view, it involves a considerable amount of extra
trouble. Of course a further consideration is the desirability in many
cases of having a complete manure. The above hints, therefore, on the
risks of loss which exist in mixing manures, may be of service to the
agricultural student.
FOOTNOTES:
[247] See p. 389.