COMPOSITION AND PROPERTIES OF MINERAL MANURES.
Mineral manure is a term which is now used with great laxity. In its
strict sense, it means manures which contain only, and owe their
exclusive value to the presence of, those substances which go to make up
the inorganic part or ash of plants. It has, however, been usually taken
to include all saline matters, and especially the compounds of ammonia
and nitric acid, which are indebted for their manurial effects to the
nitrogen they contain; and thus is so far incorrect. It would, however,
be manifestly impossible to arrange these compounds with any degree of
accuracy among either animal or vegetable manures, and hence the
necessity of including them amongst those which are strictly mineral.
The most important practical distinction between them and the substances
discussed in the two preceding chapters is, that the latter generally
contain the whole or the greater part of the constituents of plants.
Even bones yield a certain quantity of alkalies, magnesia, sulphuric
acid, and chlorine, and may in some sense be considered as a general
manure. But those to which the term mineral manure is applied for the
most part contain only one or two of the essential elements of plants,
and hence cannot be applied as substitutes for the substances already
discussed, although they are frequently most important additions to
them.
_Sulphate and Muriate of Ammonia._--These and other salts of ammonia
have been tried experimentally as manures, and it has been ascertained
that they may all be used with equal success; but as the sulphate is by
much cheaper, it is that which probably will always be employed to the
exclusion of every other. It contains, when pure, 25.7 per cent ammonia.
It is now manufactured of excellent quality for agricultural use, and
when good, contains from 95 to 97 per cent of actual sulphate, the
remainder consisting chiefly of moisture and a small quantity of fixed
residue; but specimens are occasionally met with containing as much as
10 per cent of impurities, which, as its price is high, makes a material
difference in its value. Inferior descriptions are also occasionally
sold, among which is a variety distinguished by containing a large
quantity of water and fixed salts, although it appears to the eye a good
article. Its composition is--
I. II.
Water 9.05 5.77
Sulphate of ammonia 79.63 85.21
Fixed salts 11.17 9.02
------ ------
100.00 100.00
Ammonia 20.55 21.94
An article called sulphomuriate of ammonia is also sold for agricultural
use. It is obtained as a refuse product in the manufacture of magnesia,
and is a mixture of sulphate and muriate of ammonia, with various
alkaline salts. It differs somewhat in quality, and is sold by analysis
at a price dependent on the ammonia it contains.
I. II.
Water 14.49 25.39
Sulphate of ammonia 62.55 47.79
Muriate of ammonia 15.3 ...
Sulphate of soda ... 9.12
Sulphate of magnesia ... 18.38
Chloride of potassium 4.75 2.94
Chloride of sodium 17.35 0.35
------ ------
100.00 100.00
Ammonia 16.50 11.28
The quality of sulphate of ammonia may generally be judged of from its
dry and uniformly crystalline appearance, and it may be tested by
heating a small quantity on a shovel over a clear fire, when it ought to
volatilize completely, or leave only a trifling residue. Some care,
however, is necessary in applying this test, as in the hands of
inexperienced persons it is sometimes fallacious. The salts of ammonia
may be applied in the same way as guano; but they are most
advantageously employed as a top-dressing, and principally to grass
lands. In this way very remarkable effects are produced, and within a
week after the application, the difference between the dressed and
undressed portions of a field is already conspicuous. Experience has
shewn that success is best insured when the salt is applied during or
immediately before rain, so that it may be at once incorporated with the
soil; as when used in dry weather little or no benefit is derived from
it. It seems also to exert a peculiarly beneficial effect upon clover;
and hence it ought to be employed only on clover-hay, as where ryegrass
or other grasses form the whole of the crop we have better manures.
_Ammoniacal Liquor of the Gas-Works, and of the Animal Charcoal
Manufacturers._--Both of these are excellent forms in which to apply
ammonia, when they can be obtained. The ammoniacal liquor of the
gas-works is very variable in quality, but contains generally from 4 to
8 ounces of dry ammonia per gallon, which corresponds in round numbers
to from 1 to 2 lb. of sulphate of ammonia. It is best applied with the
watering-cart, but must be diluted before use with three or four times
its bulk of water, as if concentrated it burns up the grass, and it is
also advisable to use it during wet weather. The ammoniacal liquor of
the ivory-black works contains about 12 per cent of ammonia, or about
four or five times as much as gas liquor. It has been used in some parts
of England, made into a compost, and applied to the turnip and other
crops, and, it is said, with good effect. _Bone oil_, which distils over
along with it, has also been used in the form of a compost; it contains
a large quantity of ammonia and of nitrogen in other forms of
combination; the total quantity of nitrogen it contains being 9.04 per
cent, which is equivalent to 10.98 of ammonia. Only part of this
nitrogen is actually in the state of ammonia; and some circumstances
connected with the chemical relations of the other nitrogenous compounds
in this substance render it probable that they may pass very slowly into
ammonia, and may therefore be of inferior value; but the substance
deserves a trial, as it is very cheap. It must be carefully composted
with peat, and turned over several times before being used.
_Nitrates of Potash and Soda._--Nitrate of potash has long been used as
a manure, but its high price has prevented its general application, and
its place has now been almost entirely taken by nitrate of soda, which
is much cheaper and contains weight for weight a larger quantity of
nitrogen. Both these salts are employed as sources of nitrogen; but
nitrate of potash owes also a certain proportion of its value to the
potash it contains. Nitrate of soda, on the other hand, must be
considered to owe its entire value to its nitric acid, as soda is of
little value to the plant; and, moreover, can be obtained in common salt
at a price so low, as to make it a matter of no moment in the valuation
of the nitrate. In its ordinary state, as imported from Peru, nitrate of
soda contains from 5 to 10 per cent of impurities, and it bears a price
proportionate to the quantity of the pure salt present in it. When of
good quality it contains about 15 per cent of nitrogen, equivalent to 18
of ammonia, and is, therefore, richer in that constituent of plants than
Peruvian guano. It is essentially a rapidly acting manure, and produces
a marked effect within a very few days after its application; but owing
to the fact that nitric acid cannot be absorbed and retained by the soil
in the same manner as ammonia, it is liable to be lost unless it can be
at once assimilated by the plant. For this reason it acts best when
applied in small quantity as a top-dressing to grass-land, and to young
corn. A large application has no advantages, and there can be no doubt
that the best effect would be produced by several very small quantities,
applied at intervals. In one experiment, Mr. Pusey found 42 lb. per acre
to increase the produce of barley by 7 bushels, and very favourable
results have been obtained by other experimenters. The beneficial
effects of nitrate of soda appear to be almost entirely confined to the
grasses and cereals. At least experience here has shewn that it produces
little or no effect on clover; and one farmer has stated, that having
recently adopted the practice of sowing clover with a very small
proportion of ryegrass only, he has been led to abandon the use of
nitrate of soda, which he formerly employed abundantly, when ryegrass
formed a principal part of his crop. The action of nitrate of soda is
very remarkable, not only in this respect, but also because a given
quantity of nitrogen in it _appears_ to produce a greater effect than
the same quantity in sulphate of ammonia or guano. At the same time this
statement must be taken as very general, definite experiments being
still too few to admit of its being stated as an absolute fact. The
probability is, that the same quantity of nitrogen, in the form either
of ammonia or nitrate of soda, will produce the same effect, although
the conditions necessary for its successful action may not be the same
with the two manures. It is alleged that nitrate of soda is
advantageously conjoined with common salt, which is said to check its
tendency to make the grain crops run to straw, and to prevent their
lodging, as they are apt to do, when it is employed alone. But
considerable difference of opinion exists in this point, many farmers
believing that salt produces no effect. When employed for hay,
especially when mixed with clover, it is advisable to use it along with
an equal quantity of sulphate of ammonia, which gives a better result
than either separately.
_Salts of Potash and Soda._--The substances just mentioned must be
considered to owe their chief manurial value to nitric acid; but other
salts have been used as manures in which the effect is undoubtedly due
to the alkalies themselves. With the exception of common salt, most of
the alkaline salts have only been used to a limited extent; and it is
remarkable that, so far as our present experience goes, there is no
class of substances from which more uncertain results are obtained.
_Muriate and Sulphate of Potash_ have both been used, and the former
has in some cases, and in particular seasons, produced a very remarkable
effect in the potato; but in other instances it has proved quite
useless. The cause of this difference has not been ascertained. Sulphate
of soda has also been used to some extent, but apparently without much
benefit; and there is no reason to expect that it should act better than
common salt, which can be obtained at a much lower price.
_Chloride of Sodium, or Common Salt_, has at different times been
employed as a manure, but its effects are so variable and uncertain,
that its use, in place of increasing, has of late years rather
diminished, it having frequently been found that on soils in all
respects similar, or even on the same soil, in different years, it
sometimes proves advantageous, at others positively injurious. Its use
as an addition to nitrate of soda has been already alluded to, and it is
said that it produces the same effect when mixed with guano and salts of
ammonia. The accuracy of this statement is doubted by many persons, and
the explanation which has been given of the cause of its action is more
than dubious. It is supposed to enable the plant to absorb more silica
from the soil; but this is a speculative explanation of its action, and
has not been supported by definite experiment. Although little effect
has been observed from salt, it deserves a more accurate investigation,
as not withstanding the extent to which it has been employed, we are
singularly deficient in definite experiments with it.
_Carbonates of Potash and Soda_ have only been tried experimentally, and
that to a small extent, nor is it likely that they will ever come into
use, owing to their high price. The remarks we have made in the section
on the ashes of plants regarding the subordinate value of soda, will
enable the reader to see that greater effects are to be anticipated
from the former than from the latter of these salts. They _may_,
however, exert a chemical action on the soil, altogether independent of
their absorption by the plant, but its nature and amount are still to
determine.
_Silicates of Potash and Soda_ have been employed with the view of
supplying silica to the plant, but the results have been far from
satisfactory. This may perhaps have been due to the doubtful nature of
the commercial article, but now that silicate of soda can be obtained of
good quality, it is desirable that the experiments should be repeated.
It is said to have produced good effects on the potato.
_Sulphate of Magnesia_ can be obtained at a low cost, and has been used
as a manure in some instances with very marked success. It has been
chiefly applied as a top-dressing to clover hay, but it seems probable
that it might prove a useful application to the cereals, the ash of
which is peculiarly rich in magnesia.
Many other saline substances have been tried as manures; but in most
instances on too limited a scale to permit any definite conclusions as
to their value. The experiments have also been too frequently performed
without the precautions necessary to exclude fallacy, so that the
results already arrived at must not be accepted as established facts,
but rather as indications of the direction in which further
investigation would be valuable. There is little doubt that many of
these substances might be usefully employed, if the conditions necessary
for their successful application were eliminated; and no subject is at
present more deserving of elucidation by careful and well-devised field
experiments.
_Phosphate of Lime._--The use of bones in their natural state as a
manure has been already adverted to, and it was stated, that though
their value depended mainly on the phosphates, the animal matters and
other substances contained in them were not without effect. The action
of phosphates is greatly promoted by solution in sulphuric acid, and the
application of the acid has brought into use many varieties of
phosphates of purely mineral origin, or which have been deprived of
their organic matters by artificial processes. Of these, the spent
animal charcoal of the sugar-refiners, usually containing about 70 per
cent of phosphates, and South American bone ash, are the most important.
The latter is now imported in very large quantity, and has the
composition shewn in the following analyses:--
I. II. III.
Water 6.10 6.28 3.03
Charcoal 5.05 2.19 2.02
Phosphates 79.20 71.10 88.55
Carbonate of lime 4.05 3.55 5.60
Alkaline salts 0.15 traces ...
Sand 5.45 16.90 0.80
------ ------ ------
100.00 100.00 100.00
Bone ash has hitherto been almost entirely consumed as a raw material
for the manufacture of superphosphates; but as it is sold at from L4:
10s. to L5: 10s. per ton when containing 70 per cent of phosphates, it
is, in reality, a very cheap source of these substances, and merits the
attention of the farmer as an application in its ordinary state.
Of strictly mineral phosphates, a considerable variety is now in use,
but they are employed exclusively in the manufacture of superphosphates,
as in their natural state they are so hard and insoluble, that the plant
is incapable of availing itself of them.
_Coprolites._--This name was originally applied by Dr. Buckland to
substances found in many geological strata, and which he believed to be
the dung of fossil animals. It has since been given to phosphatic
concretions found chiefly in the greensand in Suffolk and
Cambridgeshire, which are certainly not the same as those described by
Dr. Buckland, but consist of fragments of bones, ammonites, and other
fossils. Coprolites are now collected in very large quantities, and
about 43,000 tons are annually employed. They are extremely hard, and
require powerful machinery to reduce them to powder, and hence their
price is considerable, being about L2: 10s. per ton. Their composition
varies somewhat according to the care taken in selecting them, and the
locality from which they have been obtained. A general idea of their
composition may be derived from the subjoined analyses:--
Water 1.95 1.90
Organic matter 2.59 6.85
Phosphate of lime 55.21} 61.15
Phosphate of iron 3.84}
Carbonate of lime 26.70 16.20
Sulphate of lime 1.97 "
Alkaline salts 1.85 3.21
Sand 5.89 11.65
------ ------
100.00 100.00
Within the last two or three years, coprolites have been found in great
abundance in France, but they are of inferior quality, and rarely
contain more than 40 per cent of phosphates.
_Apatite_, or mineral phosphate of lime, is found in large deposits in
different places. It is particularly abundant in Spain, and occurs also
in America and Norway. From the latter country it has been imported to
some extent; and during the last year considerable quantities have been
brought from Spain, and the importations will undoubtedly increase very
largely as the means of transport improve in that country. Spanish
apatite contains--
Water 0.80
Phosphate of lime 93.30
Carbonate of lime 0.50
Chlorine, etc. traces
Sand 4.70
-----
99.30
Several other varieties of mineral phosphates have been imported under
the name of guano. The most important is Sombrero Island guano, which is
found on a small island in the Gulf of Mexico, where it occurs in a
layer said to be forty feet thick. It contains--
Water 8.96
Phosphate of lime 37.71
Phosphates of alumina and iron 44.21
Phosphate of magnesia 4.20
Sulphate of lime 0.86
Carbonate of lime 3.36
Sand 0.70
------
100.00
A somewhat similar substance, but in hard crusts, has been imported,
under the names of Maracaybo guano, Pyroguanite, etc., which contains--
Water 1.03
Organic matter 6.78
Phosphates 75.69
Alkaline salts 4.91
Sand 11.64
------
100.00
Phosphoric acid in the alkaline } 0.78
salts = 1.68 phosphate of lime }
These substances are all excellent sources of phosphates, but they are
so hard that the plants cannot extract phosphoric acid from them, and
they are only useful when made soluble by chemical processes.
_Superphosphate; Dissolved Bones._--These names were at first applied to
bones which had been treated with sulphuric acid; but superphosphates
are now rarely made from bones alone, but bone ash and some of the
mineral phosphates just described are employed, either along with them,
or very frequently alone. The manufacture of superphosphates depends on
the existence of two different compounds of phosphoric acid and lime,
one of which contains three times as much lime as the other. That which
contains the larger quantity of lime is found in the bones and all other
natural phosphates, and is quite insoluble in water; but when two-thirds
of its lime are removed, it is converted into the other compound, which
is exceedingly soluble. This change is effected by the use of sulphuric
acid, which combines with two-thirds of the lime of the ordinary
insoluble phosphate of lime, and converts it into the _biphosphate of
lime_, which is soluble. When, therefore, we add to 100 lbs. of common
phosphate of lime the necessary quantity of sulphuric acid, it yields 64
lbs. of biphosphate, containing the whole of the phosphoric acid, which
is the valuable constituent, the diminution in weight being due to the
removal of the valueless lime. Hence it follows, also, that as the lime
so removed is converted into sulphate, there must, for every 100 lbs. of
phosphate of lime converted into biphosphate, be produced 87 lbs. of dry
sulphate of lime, or 110 of the ordinary sulphate called gypsum. This is
the minimum quantity which can be present, but in actual practice it is
liable to be greatly exceeded, more especially where coprolites are
used, owing to the large amount of carbonate of lime they contain, which
is also converted into sulphate by the action of the acid, so that it is
far from uncommon to find the gypsum twice as great as it would be if
materials free from carbonates could be obtained. By employing a
sufficiency of sulphuric acid, the whole quantity of phosphoric acid in
the bones may be thus brought into a soluble state, but in actual
practice it is found preferable to leave part of it in the insoluble
condition; as where it is entirely soluble, its effect is too great
during the early part of the season, and deficient at its end. In order
to dissolve bones, bone ash, or mineral phosphates, they are mixed with
from a third to half their weight of sulphuric acid, of specific gravity
1.70 or 140 deg. Twaddell. When mineral phosphates, and particularly
coprolites, are used, the quantity of sulphuric acid must be increased
so as to compensate for the loss of that which is consumed in
decomposing the carbonate of lime they contain. When operating on the
small scale, the materials are put into a vessel of wood, stone, or lead
(iron is to be avoided, as it is rapidly corroded by the acid), and
mixed with from a sixth to a fourth of their weight of water, which may
with advantage be used hot. The sulphuric acid is then added, and mixed
as uniformly as possible with the bones. Considerable effervescence
takes place, and the mass becomes extremely hot. At the end of two or
three days it is turned over with the spade, and after standing for some
days longer, generally becomes pretty dry. Should it still be too moist
to be sown, it must be again turned over, and mixed with some dry
substance to absorb the moisture. For this purpose everything containing
lime or its carbonate must be carefully avoided, as they bring back the
phosphates into the insoluble state, and undo what the sulphuric acid
has done. Peat, saw-dust, sand, decaying leaves, or similar substances,
will answer the purpose, and they should all be made thoroughly dry
before being used. An excellent plan is to sift the bones before
dissolving, to apply the acid to the coarser part, and afterwards to mix
in the fine dust which has passed through the sieve, to dry up the mass;
or a small quantity of bone ash, of good quality, or Peruvian guano, may
be used. On the large scale, mechanical arrangements are employed for
mixing the materials, so as to economise labour, and mineral phosphates,
such as apatite, can then be used with advantage. In such cases, blood,
sulphate of ammonia, soot, and other refuse matters, are occasionally
used to supply the requisite quantity of nitrogenous substances, but
large quantities are also made from bone ash, etc., without these
additions.
The composition of superphosphates must necessarily vary to a great
extent, and depends not only on the materials, but on the proportion of
acid used for solution. The following analysis illustrates the
composition of good samples made from different substances--
+-----------------------------------+------------------+------------------+
| | | |
| | Bones alone. | Bone-Ash. |
| | | |
+-----------------------------------+------------------+------------------+
| Water, | 7.74 ... 7.79 | 5.33 ... 10.40 |
| Organic matters and ammoniacal | | |
| salts, | 17.83 ... 21.69 | 6.94 ... 4.92 |
| Biphosphate of lime | 13.18 ... 9.87 | 21.35 ... 23.09 |
| Equivalent to soluble phosphates, |(20.57)...(15.39) |(33.33)...(36.02) |
| Insoluble phosphates | 10.31 ... 21.17 | 5.92 ... 6.08 |
| Sulphate of lime, | 46.00 ... 35.30 | 56.16 ... 47.78 |
| Alkaline salts, | 1.46 ... 0.94 | trace. |
| Sand, | 3.48 ... 3.00 | 4.23 ... 4.30 |
| +------------------+------------------+
| |100.00 ...100.00 |100.00 ...100.00 |
| Ammonia, | 2.11 ... 3.01 | 0.23 ... 0.31 |
+-----------------------------------+------------------+------------------+
+-----------------------------------+-------------------+-------------------+
| | | Mixtures |
| |Chiefly Coprolites.|containing Salts of|
| | | Ammonia, etc. |
+-----------------------------------+-------------------+-------------------+
| Water, | 5.90 ... 10.17 | 7.07 ... 15.82 |
| Organic matters and ammoniacal | | |
| salts, | 5.10 ... 4.13 | 9.87 ... 13.96 |
| Biphosphate of lime | 12.24 ... 13.75 | 17.63 ... 12.67 |
| Equivalent to soluble phosphates, |(19.10)...(21.43) |(27.50)...(19.77) |
| Insoluble phosphates | 16.90 ... 0.17 | 12.60 ... 8.40 |
| Sulphate of lime, | 52.39 ... 62.62 | 49.77 ... 45.14 |
| Alkaline salts, | 2.47 ... 0.96 | 0.06 ... 1.07 |
| Sand, | 6.00 ... 8.20 | 3.00 ... 2.94 |
| +-------------------+-------------------+
| |100.00 ...100.00 |100.00 ...100.00 |
| Ammonia, | 0.11 ... 0.57 | 1.28 ... 1.55 |
+-----------------------------------+-------------------+-------------------+
Superphosphates made from bones alone are generally distinguished by a
large quantity of ammonia, and a rather low per centage of biphosphate
of lime. This is owing to the difficulty experienced in making the acid
react in a satisfactory manner on bones, the phosphates being protected
from its action by the large quantity of animal matter which, when
moistened, swells up, fills the pores, and prevents the ready access of
the acid to the interior of the fragments. Superphosphates from
bone-ash, on the other hand, contain a mere trifle of ammonia, and when
well made a very large quantity of biphosphate of lime. Their quality
differs very greatly, and depends, of course, on that of the bone-ash
employed, which can rarely be obtained of quality sufficient to yield
more than 30 or 35 per cent of soluble phosphates. Coprolites are seldom
used alone for the manufacture of superphosphates, but are generally
mixed with bone-ash and bone dust. Mixtures containing salts of ammonia,
flesh, blood, etc., are also largely manufactured, and some are now
produced containing as much as four or five per cent of ammonia, and the
consumption of such articles is largely increasing.
The analyses above given are all those of good superphosphates, in which
abundance of acid has been used so as to convert a large proportion of
insoluble into soluble phosphates; but there are many samples of very
inferior quality to be met with in the market, in which the proportion
of acid has been reduced, and the quantity of phosphates made soluble is
consequently much lower than it ought to be. The following analyses
illustrate the composition of such manures, which are all very inferior
and generally worth much less than the price asked for them.
Water 21.60 5.37 7.19
Organic matter and ammoniacal
salts, 11.62 13.91 8.80
Biphosphate of lime 2.98 2.02 6.42
Equivalent to soluble
phosphates (4.65) (3.15) (10.02)
Insoluble phosphates 25.70 15.80 14.03
Sulphate of lime 23.66 47.52 51.93
Alkaline salts 10.70 3.73 3.43
Sand 3.80 11.65 8.20
------ ------ ------
100.00 100.00 100.00
Ammonia, 1.32 0.59 0.33
The deliberate adulteration of superphosphate, that is, the addition to
it of sand or similar worthless materials, I believe to be but little
practised. The most common fraud consists in selling as pure dissolved
bones, articles made in part, and sometimes almost entirely, from
coprolites. Occasionally refuse matters are used, but less with the
intention of actually diminishing the value of the manure as for the
purpose of acting as driers. It is said that sulphate of lime is
sometimes employed for this purpose, but this is rarely done, because
that substance is always a necessary constituent of superphosphate in
very large quantities; and as farmers look upon it with great suspicion,
all the efforts of the manufacturers are directed towards reducing its
quantity as much as possible. It is very commonly supposed by farmers
that the sulphate of lime found in so large quantity in all
superphosphates, and often amounting to as much as fifty per cent, has
been added to the materials in the process of manufacture, but this is a
mistake; it is a necessary and inevitable product of the chemical action
by which the phosphates are rendered soluble, although its quantity
depends on the materials from which the manure is made. When pure bones
are used its quantity is small, and it does not greatly exceed twice
that of the biphosphate of lime; but in a manure made from coprolites,
or other substances containing a large proportion of carbonate of lime,
which must in the process of manufacture be converted into sulphate, it
may be four or five times as much.
Although there is no manure which varies more in quality, or requires
greater vigilance on the part of the purchaser, in order to obtain a
good article, there is no doubt that superphosphates, owing to the
process of manufacture being better understood, and to increased
competition, have considerably improved in quality. Six or eight years
since a manure containing thirty per cent of phosphates, of which twelve
or fifteen had been converted into biphosphate, was considered a fair
sample, but now the proportion rendered soluble is greatly increased;
and where bone ash alone is employed, as much as thirty and even forty
per cent of soluble phosphates is occasionally found. This, of course,
is an exceptional case, and great attention and care in the selection of
materials are necessary to obtain so large a proportion. The analyses
already given will shew the farmer what he has to expect in good
superphosphates, but it is very necessary that he should take care to
obtain from the manufacturer a manure equal to the guarantee; and he
ought to bear in mind that, owing to the difficulty of getting materials
of constant composition, variations often take place to a considerable
extent in manures which are supposed to be made in exactly the same
manner.
_Phospho-Peruvian Guano._--Under this name a kind of superphosphate,
which is understood to be made by dissolving a native "rock guano," has
recently attracted considerable attention, and is used to a large
extent. Its composition is--
Water 9.54
Organic matter 21.38
Biphosphate of lime, equivalent to 25.22 soluble
phosphates 16.81
Insoluble phosphates 10.88
Sulphate of lime 37.21
Alkaline salts, containing 1.32 of phosphoric acid,
and equivalent to 2.86 soluble phosphates 2.22
Sand 1.81
------
100.00
Ammonia, 3.50
It is chiefly distinguished by the large proportion of valuable
ingredients it contains, and the care taken to secure uniformity of
composition.
A variety of substances are sold under the name of nitrophosphate,
potato manure, cereal manure, etc. etc., which are all superphosphates,
differing only in the proportion of their ingredients, and in the
addition of small quantities of alkaline salts, sulphate of magnesia,
and other substances, but they present little difference from ordinary
superphosphates in their effects.
The use of superphosphate has greatly extended of late years, and its
consumption has increased in a greatly more rapid ratio than that of
guano or any other manure. Ten or twelve years since it was
comparatively little known, but it has now come to be used in many cases
in which Peruvian guano was formerly employed. It produces a better
effect than that manure on light soils, although in general a mixture of
the two answers better than either separately. When Peruvian guano is
to be applied along with it, the farmer will naturally select a
superphosphate made from bone ash, and containing the largest obtainable
quantity of soluble phosphates; but when it is to be used alone, it is
advisable to take one made from bones, or at all events one containing a
considerable quantity of nitrogenous matter or ammonia. The kind to be
selected must, however, be greatly dependent on the particular soil, and
the situation in which it is to be used.
_Lime._--Lime is by far the most important of the mineral manures, and
an almost indispensable agent of agricultural improvement. It has been
used as chalk, marl, shell and coral sand, ground limestone, and as
quick and slaked lime, and its action varies according as it is applied
in any of its natural forms, or after being burnt. In all of its native
forms the lime is combined with carbonic acid in the proportion of
fifty-six parts of lime to forty-four of carbonic acid, and the
carbonate is generally mixed with variable quantities of earthy
ingredients, which in some instances are important additions to it, and
affect its utility as a manure.
_Chalk_ is a very pure form of carbonate of lime, and where it abounds
has been largely employed as an application on the soil. It is dug out
of pits and exposed to the action of the winter's frost, by which it is
thoroughly disintegrated, and in spring it is applied in quantities,
which, in many instances, are only limited by the question of cost.
_Marl_ is a name given to a mixture of finely-divided carbonate of lime,
with variable proportions of clay and siliceous matters, which is found
at the bottom of valleys and in hollow places in beds often of
considerable extent and thickness, where it is deposited from the waters
of lakes holding lime in solution, fed by streams passing over
limestone, or rocks rich in lime. The composition of marls differs
greatly in different districts, and they have been divided into true
marls, and clay marls, according as the carbonate of lime or clay is the
preponderating ingredient. The following table illustrates the
composition of different varieties:--
+---------------------------+------------+-----------+-----------+-------------
| | Barbadoes. | Luneburg. | Ayrshire. | Wesermarsh.
+---------------------------+------------+-----------+-----------+-------------
| Carbonate of lime | 93.2 | 85.4 | 8.4 | 8.2
| Carbonate of magnesia | ... | 1.3 | ... | 3.0
| Sulphate of lime | ... | 0.1 | ... | 0.5
| Phosphate of lime | 0.1 | 2.3 | ... | 1.2
| Alumina and oxide of iron | 1.6 | 4.6 | 2.2 | 7.2
| Alkaline salts | ... | 0.1 | ... | 1.0
| Silica and clay | 4.6 | 5.6 | 84.9 | 78.9
| Organic matter | 0.5 | 0.6 | 2.8 | ...
| Water | ... | ... | 1.4 | ...
+---------------------------+------------+-----------+-----------+-------------
| | 100.00 | 100.00 | 99.7 | 100.00
+---------------------------+------------+-----------+-----------+-------------
The true marls, that is those in which carbonate of lime abounds, are
greatly preferable to clay marls, the latter, indeed, operate chiefly
mechanically, by altering the texture of the soil--the lime they contain
being frequently too small to exercise much appreciable effect.
Shell and coral sands consist chiefly of fragments of shells and coral
disintegrated by the action of the waves, and mixed with more or less
siliceous sand, and containing small quantities of phosphate of lime.
They occur to a considerable extent both on our own coasts and those of
France, and have been used with good effect on some descriptions of
soil.
The general composition of limestones has been already adverted to,
when treating of the origin of soils, and a distinction drawn between
the common limestones and dolomite or magnesium limestone. Few
limestones can be considered as even approaching to purity, and they
almost all contain a small quantity of carbonate of magnesia as well as
earthy matters, and occasionally a little phosphate of lime. In good
specimens the quantities of these substances are generally small, and
they usually contain about half their weight of lime. When limestone is
burnt in the kiln, the change which ensues consists in the expulsion of
the carbonic acid, and the consequent conversion of the lime into the
uncombined or quick state. If water be thrown upon it when in this
condition, it becomes hot, swells up, and falls to a fine soft powder,
and has then entered into combination with water. If it be exposed to
the air, the same action takes place, although, of course, more slowly;
and if it be left for a sufficient time, it at length absorbs carbonic
acid, and reverts to its original form of carbonate of lime, although
now in a state of very fine division.
While lime may be applied in the state of carbonate, either as chalk,
marl, or pounded limestone, and with a certain amount of advantage, much
greater effects are obtained from the use of lime itself in the quick or
slaked state. These advantages are dependent partly on the mechanical
effect of the burning and slaking, which enable us to reduce the lime to
a much more minute state of division, and consequently to incorporate it
more uniformly and thoroughly with the soil, and partly on the more
powerful chemical action which it exists when in the quick or caustic
state. Other minor advantages are also secured, such as the production
of a certain quantity of sulphate of lime, produced by the oxidation of
the sulphur of the coal used in burning, etc., which, though
comparatively trifling, may, under particular circumstances and in some
soils, be of considerable importance.
The action of lime is of a complicated character. Where the soil is
deficient in lime, it must necessarily act by supplying that substance
to the plants growing in it. But this is manifestly a very subordinate
part of its action,--_1st_, Because no soil exists which does not
contain lime in sufficient quantity to supply that element to the
plants. _2d_, Because its effects are not restricted to those soils in
which it exists naturally in small quantity; and, _3d_, Because it is
found that a small application, such as would suffice for the wants of
the crops, is not sufficient to produce its best effects.
It is a familiar fact that the quantity of lime applied to the soil for
agricultural purposes is very large, as much as ten, and even twenty
tons per acre having been used, while the smallest application is
exceedingly large when compared with the mere requirements of the crops.
Of late years the very large applications once in use have become less
common, as it has been found preferable to employ smaller doses more
frequently repeated. The quantity used depends, however, to a great
extent, on the nature and condition of the soil, heavy clays, especially
if undrained, and soils of a peaty nature, requiring a large
application; while on well drained and light soils a smaller quantity
suffices. Thin soils also require only a small application. The
geological origin of the soil is also not without its influence, and its
beneficial effect is peculiarly seen on granite, porphyry, and gneiss
soils, both because these are naturally deficient in lime, and because
the decompositions by which their valuable constituents are liberated
take place with extreme slowness.
The greater part of the action of lime is unquestionably dependent on
its exerting a chemical decomposition on the soil; and it acts equally
on both the great divisions of its constituents, the inorganic and the
organic. On the former, it operates by decomposing the silicates, which
form the main part of the soil, and the alkalies they contain being thus
set free, a larger supply becomes available to the plant. On the organic
constituents its effects are principally expended in promoting the
decomposition which converts their nitrogen into ammonia; and thus a
supply of food, which might remain for a long period locked up, is set
free in a state in which the plant can at once absorb it. But these
chemical decompositions are attended by a corresponding change in the
mechanical characters of the soil. Heavy clays are observed to become
lighter and more open in their texture; and those which are too rich in
organic matter have it rapidly reduced in quantity, and the excessive
lightness which it occasions diminished.
The effects of an application of lime are not generally observed
immediately, but become apparent in the course of one or two years, when
it has had time to exert its chemical influence on the soil; but from
that time its effects are seen gradually to diminish and finally to
cease entirely. The period within which this occurs necessarily varies
with the amount of the application and the nature of the soil, but it
may be said generally that lime will last from ten to fifteen years. The
cessation of its effects is due to several circumstances, partly of
course to the absorption of lime by the plants, partly to its being
washed out of the soil by the rains, and partly to its tendency to sink
to a lower level, a tendency which most practical men have had
opportunities of observing. In the latter case, deep-ploughing often
produces a marked effect, and sometimes makes it possible to postpone
for a year or two the reapplication of lime. All these circumstances
have their influence in bringing its action to an end, but the most
important is, that after a time it has exhausted its decomposing effect
on the soil, having destroyed all the organic matter, or liberated all
the insoluble mineral substances which the quantity added is competent
to do, and so the soil passes back to its old state. It does even more,
for unless active measures are taken to sustain it by other means, it is
found that the fertility of the soil is apt to become less than it was
before the use of lime. And that it should be so is manifest, if we
consider that the lime added has liberated a quantity of inorganic
matter, which, in the natural state of the soil, would have become
slowly available to the plant, and that it must have acted chiefly in
those very portions which, from having already undergone a partial
decomposition, were ready to pass into a state fitted for absorption,
and thus as it were must have anticipated the supplies of future years.
This effect has been frequently observed by farmers, and is indeed so
common, that it has passed into a proverbial saying, that "lime enriches
the fathers and impoverishes the sons." But this is true only when the
soil is stinted of other manures, for when it is well manured the
exhausting effect of lime is not observed; and it must be laid down as a
practical rule, that its use necessitates a liberal treatment of the
soil in all other respects. But when lime has been once employed it
becomes almost necessary to resort to it again; and generally so soon as
its effects are exhausted a new quantity is applied, not so large as
that which is used when the soil is first limed, but still considerable.
When this is done very frequently, however, bad effects ensue; the soil
gets into a particular state, in which it is so open that the grain
crops become uncertain, and such land is said, in practical language, to
be overlimed. The explanation of this state of matters commonly assumed
by those unacquainted with chemistry is, that the land has become too
full of lime; but a moment's consideration of the very small fraction of
the soil which even the largest application of lime forms, will serve to
shew that this cannot be the cause. Ten tons of lime per acre amounts to
only one per cent of the soil, and as a considerable part of the lime is
carried off by drainage in the course of years, it is obvious that even
very large and frequently repeated doses are not likely to produce any
great accumulation of that substance. In point of fact, analyses of
overlimed soils have proved that the lime does not exceed the ordinary
quantity found in fertile land. The explanation of the phenomenon is
probably to be found in the rapid decomposition of organic matter by the
lime, and its escape as carbonic acid, by which the soil is left in that
curious porous condition so well known in practice. The cure for
overliming is found to be the employment of such means as consolidate
the soil, such as eating off with sheep, rolling, or laying down to
permanent pasture.
The immediate effect of lime on the vegetation of the land to which it
is applied is very striking. It immediately destroys all sorts of moss,
makes a tender herbage spring up, and eradicates a number of weeds. It
improves the quantity and quality of most crops, and causes them to
arrive more rapidly at maturity. The extent to which it produces these
effects is dependent on the form in which it is applied. When the lime
is used hot, that is, immediately after it has been slaked, they are
produced most rapidly and effectually; but if it has been so long
exposed to the air as to absorb much of the carbonic acid it lost in
burning, and has got into what is commonly called the mild state, it
operates more slowly; and when it is applied as chalk, marl, or pounded
limestone, its action is still more tardy. Various circumstances, which
must depend upon very different considerations, must necessarily
influence the farmer in the selection of one or other of these different
forms of lime; but on the whole, it will be found that the greatest
advantages are on the side of the well-burned and freshly slaked lime.
The consideration of all the minutiae to be attended to, however, would
carry us far beyond the limits of this work, and trench to some extent
on the subject of practical agriculture.
Various kinds of refuse matters containing lime have been used in
agriculture, but they are generally inferior to good lime, and not
generally more economical. The most important of these is gas lime, or
lime which has been used for purifying coal gas. In going through this
process it absorbs carbonic acid from the gas, and consequently passes
back, more or less, completely into the form of carbonate of lime. But
it also takes up sulphur, which remains in it in the form of sulphuret
of calcium. It is well known that all sulphurets are prejudicial to
vegetable life, and hence, when fresh gas lime is used, its effects are
often injurious rather than beneficial. But if it be exposed for some
time to the air, oxygen is absorbed, the sulphur is converted into
sulphuric acid, gypsum is produced to the extent of some per cent, and
the lime then becomes innocuous. When composted with dry soil, the
admission of air into the interior of the lime is facilitated, and this
change takes place with greater rapidity. The waste lime from
bleach-works, tanneries, and other manufactories, is occasionally used
by farmers; but unless obtained at a nominal price, it cannot compete
with good quick lime, owing to the large amount of water it contains,
and the consequent increase in the cost of carriage.
_Sulphate of Lime or Gypsum._--Gypsum has been extensively used as a
manure, and is found to exert a very remarkable influence upon clover,
and leguminous crops generally. It is employed in quantities varying
from two cwt. per acre up to a very large quantity, and almost
invariably with good results, in some instances even with the production
of double crops. Much speculation has taken place as to the cause of
this action which is so specific in its character, and from Sir Humphrey
Davy down to the present time, many chemists and agriculturists have
considered the matter. Sir Humphrey Davy attributed its action to its
supplying sulphur to those plants which, according to him, contain an
unusually large quantity of that element. That opinion has been since
entertained by others, but it can scarcely be considered as well
founded, for the more accurate experiments recently made do not point to
any conspicuous differences between the quantities of sulphur contained
in these and other plants. It is, moreover, to gypsum alone that these
effects are due, and if it were merely as a source of sulphur that it
was employed, there are other salts which could be equally, perhaps more
advantageously, used; such, for instance, as sulphate of soda. Others
have attributed its action to its power of fixing ammonia, but this
explanation is certainly untenable, for the soil itself possesses this
property very powerfully, and it is inconceivable that the addition of a
few hundred weights of gypsum should have any effect in promoting this
action. The experiments which have been made with gypsum leave no doubt
as to its effect, more especially on leguminous plants, but they do not
afford an explanation of its mode of action, for which further
inquiries, directed especially to that object, are required.
The application of gypsum to the soil appears to have diminished of late
years, and this is probably due to the large consumption of
superphosphates, and other manufactured manures, which contain it in
abundance. In an ordinary application of these substances, there are
contained from one to two hundredweight of gypsum; and it is not likely
that when they have been extensively used, much benefit will be derived
from a further application of it by itself.