POTASSIC MANURES.
_Relative Importance._
In Chapter VI. we pointed out that of the three manurial ingredients
potash was the one most abundantly occurring, and that, consequently,
the necessity of adding it in the form of an artificial manure existed
less frequently than in the case of nitrogen or phosphoric acid. It was
further pointed out that, under the ordinary conditions of agriculture,
a greater restoration to the soil of the potash removed in the crops was
made in the straw used in farmyard manure than was the case with regard
to the other two ingredients. Despite these facts, there are many cases
where the addition of potassic manures is of the highest importance in
increasing plant-growth. It will be well, therefore, to devote a little
space to considering our different potassic manures and their respective
action.
_Scottish Soils supplied with Potash._
Potassic manures are not so valuable in this country since experience
has shown that most Scottish soils are abundantly supplied with this
manurial ingredient. Moreover, under the conditions of most European
farming, there seems to be a steady gain to the soil of potash. In
America, however, the action of potash as a manure seems to be more
strikingly illustrated. Indeed, wherever forage crops or straw are sold
off the farm in large quantities, or where beets, cabbages, carrots,
potatoes, onions, &c., are also grown in large quantities, the necessity
for potash manuring generally arises.
_Sources of Potassic Manures._
The value of potash as a manure first came to be recognised from the
favourable action of wood-ashes. Of course their favourable action is
not due solely to potash, as they contain, in addition to the other ash
ingredients of the plant, phosphates; and their value as a manure may
also be said to depend not a little on their indirect action. They
contain a certain percentage of caustic alkali, which promotes the
decomposition of the nitrogenous matter of the soil. But making due
allowance for these other valuable properties, the chief value of
wood-ashes is undoubtedly due to the potash they contain. Hence the use
of the commercial article called _potash_, which is a mixture of
potassium carbonate and hydrate, and which is obtained from wood-ashes,
was formerly common to a considerable extent as a manure, especially for
clover. _Barilla_, a rich potassic manure prepared by burning certain
strand plants, especially the saltwort, was also in the past largely
exported from Sicily and Spain. _Kelp_, a product got by burning
sea-weed in Scotland, is also a rich potassic manure. Since, however,
the discovery of the Stassfurt mines, all potassic manures have come
from these.
_Stassfurt Potash Salts._
Huge salt deposits exist at Stassfurt in Germany. They have been formed
by the evaporation of an inland sea. Salt was first discovered in these
deposits in 1839, but for long the presence of potash salts was little
suspected, and it was not until 1862 that the potash salts were worked.
We have already, in the Appendix to Chapter VI., given a list of the
chief potash minerals occurring in the Stassfurt deposits. These
minerals are found in layers, the lowest layer consisting of almost pure
salt; while immediately above this we have a layer of salt mixed with
the mineral polyhallite (containing potassium sulphate) of about 100
feet thick. Above this last layer there is a layer of about 90 feet,
containing kieserite (magnesium sulphate) mixed with potassium and
magnesium chlorides; and above this again is a layer (90 feet) of
carnallite, which furnishes the chief source of the potash salts used
for manurial purposes.
At first the crude salts, as obtained direct from the deposits, were
sold as manures under the name of _Abraum_ salts. Now, however, they are
purified. Of potash salts in 1888 some 25,000 tons were exported from
Stassfurt for manurial purposes. Of these salts there may be mentioned,
viz., kainit, an impure form of the sulphate, containing on an average
about 12 per cent of potash, and the muriate and the sulphate--both
salts, in a more or less pure form, being used. A word or two may be
added on the effect of the two forms of potash--viz., as the sulphate
and as the muriate.
_Relative Merits of Sulphate and Muriate of Potash._
It is a well-known fact that muriate of potash, far from having a
beneficial effect on certain crops, is actually harmful. Of these,
sugar-beets, potatoes, and tobacco may be mentioned. In the case of
beets it seems to have an effect in lessening the percentage of
crystallisable sugar, while potatoes are rendered waxy. With regard to
the tobacco-plant, it seems to impair the value of the leaf from the
smoker's point of view. That this deleterious action is due to the form
in which the potash is present, and not to the potash itself, seems to
be pretty clear, since potash in the form of sulphate has not this
deleterious effect on these plants. Another objection which has been
urged against muriate of potash is that, when applied as a manure, it
is apt to give rise to the formation of calcium chloride,--a compound
which is distinctly hurtful to many plants. A similar charge cannot be
brought against sulphate of potash, since gypsum, which is the chief
compound it is likely to give rise to, is of much value, as we have
already pointed out, as an indirect manure. On the whole, therefore,
sulphate of potash seems to be the safest form in which to add potash.
Unfortunately, however, most of the commercial sulphates are very
impure, and contain generally considerable quantities of muriate. In
favour of the muriate, it may be said that it is the more concentrated
manure, and that it diffuses better in the soil than the sulphate--a
point of great importance. It has, moreover, been used without any bad
effect for clover, corn, grass, and some root crops.
_Application of Potash Manures._
The extreme tenacity with which the soil-particles fix potash salts,
when applied as manures, is a point which ought to be borne in mind in
their application. This, as we have just noticed, is greater in the case
of the sulphate than in the case of muriate, and it has been observed
that certain other fertilisers seem to exercise a considerable influence
in hindering their fixation. Among these may be mentioned bone-meal and
farmyard manure. Nitrate of soda also seems to increase the
diffusibility of potash salts. Conversely, potash salts seem to help to
fix ammonia.
For the above reasons potash manures ought to be applied to the soil a
considerable period before they are likely to be used by the crop. There
is little risk of any serious loss taking place owing to rain. Autumn
application is generally recommended. Even in very light soils it has
been proved in the Norfolk experiments that autumn application has an
immense advantage over spring application. It has been found that where
potash is applied as sulphate, little sulphuric acid is absorbed by the
plant.
_Soils and Crops suited for Potash Manures._
Of soils best suited for potash manures, it has been found that light
soils, and those largely charged with peaty organic matter (such as the
moorland soils of Germany), are most benefited; while on heavy clayey
soils the percentage of potash which these latter contain is already
sufficiently abundant for the needs of plants. At Flitcham the value of
potash on chalk soils has been strikingly demonstrated. Of crops, it is
now pretty generally acknowledged that those of the leguminous order are
most benefited by potash. Especially in the case of clover has potash
always proved itself a manure worth applying.
_Rate of Application._
Potash is best applied in small quantities. From 1 to 2 cwt. of the
muriate or sulphate is a common amount, and from 6 to 8 cwt. of kainit.