INDIRECT MANURES--GYPSUM, SALT, ETC.
GYPSUM.
In the previous chapter mention was made of gypsum as a compound of
lime, but no reference to its action as a manure was made. In the past,
gypsum was used extensively and highly valued. It was found to be of
especial value for clover; and there is a story told of Benjamin
Franklin which illustrates the very striking nature of its action on
this crop. It is related that he once printed with gypsum the words
"This has been plastered" on a field of clover, and that for a long time
afterwards the legend was plainly discernible on account of the
luxuriance of the clover on the parts of the field which had been thus
treated.
_Mode in which gypsum acts._
Despite the fact that gypsum is a most ancient manure, it is only of
late years that we have come to understand the true nature of its
action. For long it was believed that the reason of its striking effect
in promoting clover was due to the fact that, as clover was a
lime-loving plant, the action of gypsum was owing to the lime it
contained. That, however, the action of gypsum is not due to the fact
that it supplies lime to the plant, seems evident when it is stated that
were this so, any other form of lime would have the same beneficial
effect. It is well known, however, that this is not so. Besides, as we
have already pointed out, lime is not a constituent which most soils
lack, so far as the needs of the crop are concerned. There is a certain
amount of truth in the old belief that gypsum enriches the soil in
ammonia by fixing it from the air. The power that gypsum has as a fixer
of ammonia has already been referred to in the chapter on Farmyard
Manure; but in this case the gypsum is brought in contact with the
ammonia. The origin of this old belief was due to a misconception as to
the amount of ammonia in the atmosphere. No doubt gypsum greatly
increases the power of a soil to absorb ammonia from the air; but the
quantity of ammonia in the air is so very trifling, that its action in
this respect is hardly worth considering. The true explanation of the
action of gypsum is to be found in its effect on the double silicates,
which it decomposes, the potash being set free. Its action is similar to
that of other lime compounds, only more characteristic. As a manure,
therefore, its action is indirect, and its true function is to oust the
potash from its compounds. Its peculiarly favourable action on clover is
due to the fact that clover specially benefits by potash, and that
adding gypsum practically amounts to adding potash. Of course it should
be borne in mind that the soil must contain potash compounds if gypsum
is to have its full effect. Now, however, that potash salts suitable for
manuring purposes are abundant, it may well be doubted whether it is not
better to apply potash directly. Further, it must be borne in mind that
gypsum is applied to the soil whenever it receives a dressing of
superphosphate of lime, as gypsum is one of the products formed by
treating insoluble phosphate of lime with sulphuric acid.
It is possible that gypsum may act as an oxidising agent in the soil,
just as iron in the ferric condition does. It has a large quantity of
oxygen in its composition, and under certain conditions may act as a
carrier of oxygen to the lower layers of the soil. When it is used, it
should be applied some months before the crop is sown.
Gypsum, therefore, although it contains two necessary
plant-constituents, lime and sulphuric acid, cannot be regarded as a
direct manure; and as its action comes to be more fully understood, its
use, which was never very abundant in this country, will probably
decrease. We have already, in the chapter on Nitrification, referred to
the action of gypsum in promoting nitrification.
SALT.
The action of salt as a manure presents a problem which is at once of
the highest interest and surrounded with the greatest difficulties. In
view of the large quantities now used for agricultural purposes, a
somewhat detailed examination of the nature of its action is not out of
place in a work such as the present.
_Antiquity of the Use of Salt._
The recognition of the manurial functions of salt dates back to the very
earliest times. Its use among the ancients is testified by numerous
allusions in the Old Testament; while, according to Pliny, it was a
well-known manure in Italy. The Persians and the Chinese seem also to
have used it from time immemorial, the former more especially for
date-trees.
_Nature of its Action._
Despite, however, the great antiquity of its use, much difference of
opinion seems always to have existed as to the exact method of its
action, and as to its merits as a manure in promoting vegetable growth.
It furnishes, in fact, a good example of the difficulty which exists in
the case of many manures, whose action is chiefly indirect, of fully
understanding their influence on the soil and on the crop. In fact, the
action of salt is probably more complicated than that of any other
manurial substance.
_Salt not a necessary Plant-food._
We have already seen that neither sodium nor chlorine--the two
constituent elements of salt--are in all probability absolutely
necessary plant-foods. If they are necessary, the plant only requires
them in minute quantities. Despite this fact, soda is an ash-constituent
of nearly every plant, and in many cases one of the most abundant. In
amount it is one of the most variable of all the ash-constituents, being
present in some plants only in minute quantities, while in others it
occurs in large quantities. Mangel and plants of the cabbage tribe may
be cited as examples of plants containing large amounts of soda in their
composition. But the plants which contain it in largest quantity are
those which thrive on the sea-coast, and it has been thought that for
them at least salt is a necessary manure. This, however, does not seem
to be the case. In fact, the amount of soda in a plant seems to be
largely a matter of accident. It may be added that the succulent
portions of a plant are generally richest in soda.
_Can Soda replace Potash?_
Again, it has been believed that soda is capable of replacing potash in
the plant; but this does not seem to be the case to any extent. The view
that soda is able to replace potash, it has been thought, is supported
by the variation which exists in the proportion of soda and potash in
different plants. It must be remembered, however, that it is highly
probable that most plants contain a larger quantity of ash-constituents
than is absolutely necessary for their healthy growth. Especially is
this the case with such a necessary plant-food as potash, of which there
is generally present, in all likelihood, an excess. The variation in the
quantity of potash and soda present in many plants under different
circumstances can scarcely, therefore, be regarded as furnishing a proof
of the replacement of potash by soda. Incidentally we may mention, as a
fact worthy of notice, that cultivated plants have more potash and less
soda in their composition than wild plants. What has been said of soda
may be held to apply equally to chlorine, as it seems to be chiefly in
the form of common salt that soda enters the plant. The amount of salt,
therefore, present in plants must be regarded as largely accidental and
dependent on external circumstances, such as the nature of the soil, &c.
_Salt of universal Occurrence._
But even were salt a necessary plant-food, its occurrence in the soil is
already of sufficient abundance to obviate any necessity for its
application. It may be said to be of almost universal occurrence. Even
the air contains it in traces. That this is the case in the
neighbourhood of the sea-coast is well known; but even in air far
inland, accurate analysis of the air would probably demonstrate its
presence in greater quantity than is commonly believed. It is a wise
provision that plants absorb salt, for it increases their efficiency as
food,--the function of salt as a constituent of animal food being of the
very highest importance. It is an indispensable food-ingredient for
animal life. With regard to ordinary farm-stock, the amount of salt
which naturally occurs in their food is quite sufficient. In the case,
however, of pastures in countries far removed from the sea, the custom
of specially supplying stock with salt is common. This is done by
placing a piece of rock-salt in the fields.
_Special Sources of Salt._
The salt of commerce is obtained from various sources. Besides the sea,
we have ample sources of salt in the large saline deposits found in many
parts of Europe, especially in Austria, and in England in Cheshire.
_The Action of Salt indirect._
From what has been said above, it is clear that the action of salt as a
manure is indirect and not direct. What the nature of that indirect
action is we shall now proceed to discuss.
In considering the evidence of the manurial value of salt, we are at
once brought face to face with the fact that the experience of its
action in the past has as often been unfavourable as favourable. Salt,
it is well known, is both an antiseptic and a germicide. It is, indeed,
one of the most commonly used of preservatives. When applied in large
quantities to the soil, it has a most deleterious action on vegetation.
This hurtful action of salt has long been known; and it is as often
mentioned in the writings of antiquity on account of its unfavourable as
on account of its favourable action. Thus, for example, among the
ancient Jews it was customary, after the conquest of a hostile town, to
strew salt on the enemy's fields, for the purpose of rendering them
barren and unfertile. And again, among the Romans, for the same purpose,
salt was often spread on a spot where some great crime had been
committed.
While, therefore, its unfavourable action has long been known, the fact
that there are circumstances under which its action is, on the contrary,
favourable for promoting vegetable growth has also been long recognised.
The difficulty for the agricultural student is to reconcile these two
seemingly contradictory experiences. For the English agriculturist the
subject possesses especial interest, since in England it has been in the
past most generally used and its action most discussed since the time of
Lord Bacon, who discusses in his writings the action of solutions of it
on different plants.
The true explanation of salt being so different in its action is to be
found in the quantity applied, the nature of the soil, the crop to which
it is applied, and the conditions under which it is applied--_i.e._,
whether it is applied alone or along with other manures.
_Mechanical Action on Soils._
In the first place, it must be noted that salt exerts a mechanical
action on the soil of a very similar kind to that exercised by lime.
When applied to clay soils it causes a flocculation or coagulation of
the fine clay-particles, and thus prevents the soil from puddling to the
same extent as would otherwise be the case. In fact, an example of this
action of salt when in solution causing the precipitation of fine
suspended clayey matter, is afforded by the formation of deltas at the
mouths of rivers. The power of clarifying muddy water is common indeed
to saline solutions. Schloesing attributes the clarifying power of a
soil to the presence of the saline matters it contains; and from this
point of view it would appear that manures containing any saline
substance may exert an important mechanical influence on the soil.
_Solvent Action._
But a much more important property of salt is its solvent action on the
plant-food present in the soil. Its action in decomposing the minerals
containing lime, magnesia, potash, &c., is similar to the action of
gypsum. By acting upon the double silicates it liberates these necessary
plant-foods. It is not only on the basic substances upon which it acts,
but also on the phosphoric and silicic acids, which it sets free. Its
power of dissolving ammonia from the soil is considerable. Experiments
with a weak solution of salt on a soil by Peters and Eichhorn to test
its solvent power, showed that the salt solution dissolved more than
twice as much potash and nearly thirty times as much ammonia as an equal
quantity of pure water did. When applied to the soil, it seems chiefly
to liberate lime and magnesia. The exact nature of the chemical action
taking place is a point of some dubiety. According to some, it is
changed into nitrate of soda; according to others, into carbonate of
soda. The latter theory seems to be the more probable one. Its action on
the lime and magnesia compounds is to convert them into chlorides; and
this chemical reaction explains the action that salt has in increasing
the water-retaining and water-absorbing power of the soil; for the
chlorides of magnesia and lime are salts which have a great power of
attracting water from the air.
Again, the very fact that salt acts as an antiseptic may serve to
explain its beneficial action in certain cases where it prevents
rankness of growth. No doubt this was its function when applied along
with Peruvian guano. This it might do by preventing too rapid
fermentation (nitrification) of the manure, or by actually weakening the
plant. Its action when applied with farmyard manure may also be similar.
But while its effect in many cases may be towards retarding
fermentation, on the other hand its action, when applied along with lime
to compost-heaps, is towards promoting more rapid decomposition.
Probably a reaction takes place between the lime and the salt, the
result of which is the formation of caustic soda.
Such are some of the ways in which salt may act. It must at once be seen
how its action in one case will be favourable and in another case
unfavourable. There must be fertilising matter present in the soil if it
is to act favourably. Again, it will only be under such circumstances,
where rankness of growth is likely to ensue, that its antiseptic
properties will act favourably and not unfavourably.
_Best used in small Quantities along with Manures._
Probably it is for these reasons that its action has been found to be
most favourable when applied along with other manures and not alone.
Applied along with nitrate of soda, as is commonly done, it doubtless
increases the efficiency of the nitrate. Some plants seem to be
undoubtedly benefited by salt: of these flax may be mentioned. The
application of salt to plants of the cabbage tribe seems also to be
highly beneficial. On mangels, along with other manures, it has also
been found to have a very favourable effect. But with many crops its
action has been proved to be less favourable.
_Affects Quality of Crop._
Although salt has often been found to increase the quantity of a crop,
the quality of the crop has been made to suffer. Its action on beetroot
has been more especially studied. The effect of its application is to
lessen the total quantity of dry matter and sugar in the plant. This has
been found to be the case both when the salt was applied alone and along
with nitrate of soda and other manures. On potatoes, again, its action
has been found to be deleterious, lessening their percentage of starch.
The deleterious action of chlorides on the quality of potatoes is also
seen when potassium chloride is applied. It is for this reason that
potash should never be applied to the potato crop in the form of
chloride.
In the late Dr Voelcker's opinion, the conditions under which salt had
the most favourable action on the mangel crop was in the case of a light
sandy soil, and applied at the rate of 4 to 5 cwt. per acre. Its action
when applied to clay soils was not so favourable.
_Rate of Application._
Lastly, the rate at which it may be applied will naturally vary. From 1
cwt. and even less, up to 6 cwt. or even more, has been the rate at
which it has been commonly applied in the past. From what has been said,
it will be seen that it is more likely to exert a favourable influence
when applied only in small quantities.