SUPERPHOSPHATES.
As was mentioned in the chapter on Bones, Liebig in the year 1840
discovered that the effect of adding oil of vitriol, or sulphuric acid,
to bones was to render the phosphate they contain soluble. This
discovery marked an epoch in the history of artificial manures, and laid
the foundation of the now enormous manufacture of superphosphate. In
1862 the juries of the London International Exhibition published an
elaborate report containing an interesting article on the manure trade
of Great Britain, in which it was stated that the annual quantity of
superphosphate then made amounted to from 150,000 to 200,000 tons. Now
it may be placed not far short of a million tons. Probably that made in
the United States is considerably more. In the first instance,
superphosphate was manufactured by Sir John Lawes from spent bone-char.
This was superseded by coprolites and Estremadura phosphorite, Suffolk
coprolites being for many years the chief material employed. This in
turn was succeeded by the richer Cambridge coprolites, but of late years
coprolites have practically ceased to be a source of superphosphate, the
other mineral phosphates mentioned in the previous chapter--such as the
South Carolina, Belgian, Somme, &c., phosphates--taking their place.
_Manufacture of Superphosphate._
The manufacture of superphosphate is of too technical a nature to permit
of discussion in a work of this kind. It is important, however, that the
general principles underlying the process of manufacture and the
chemical changes in the phosphate taking place during the process be
clearly understood. In the first place, great importance attaches in the
manufacture of the superphosphate to the fineness of division of the raw
material, and much ingenuity has been spent on apparatus designed for
this purpose. The difficulty of grinding the phosphate varies, of
course, with the nature of the material used--apatite, for example,
being much more difficult to reduce to the necessary fineness than
phosphatic guano. The finer the state of division, the more complete
will be the decomposition of the phosphate by the acid. Mr Warington
recommends that for first-class work the powder should be so fine as to
admit of it passing through a sieve of eighty wires to the inch. After
the phosphate is reduced to powder, it is mixed with acid. This takes
place in the mixer, which is generally in the form of an iron cylinder
furnished in the centre with a revolving shaft, the sulphuric acid used
being the ordinary chamber acid (sp. gr. 1.57). Whatever strength of
acid is used, there must be a certain quantity of water present to form
gypsum. It is to the formation of gypsum in the resulting product that
the dryness of the superphosphate is due. The proportion of sulphuric
acid used depends on the composition of the phosphate; and here it may
be pointed out that the presence of much carbonate of lime is a most
important factor in determining the quantity of acid required. The
reason of this is, that where carbonate and phosphate of lime are
present together, sulphuric acid first acts upon the carbonate, and it
is not till this is wholly decomposed that the phosphate can be acted
upon. Hence mineral phosphates with a large percentage of carbonate of
lime do not constitute such an economical material for the manufacture
of superphosphate as those in which the percentage of carbonate is
small.[225] A certain amount of heat is necessary for the purpose of
enabling a quick decomposition to take place. For this purpose the
sulphuric acid added has been previously heated. In the ordinary
manufacture of superphosphate, however, this is not considered
necessary, as the heat developed by the chemical action between the
phosphate and the acid is sufficiently great. The phosphate, after being
thoroughly mixed with the acid, is discharged into what is technically
known as the pit, a chamber built of brick or concrete. The mixture,
which is in a fluid state when it enters the pit, very soon hardens, and
is dug out in a day or two. It is next reduced to powder in a
disintegrator, and is then ready for use as a manure.
_Nature of the Reaction taking place._
In order to clearly understand the nature of the reaction which takes
place when sulphuric acid is added to a phosphatic material, it may be
well to say a word or two on the composition of the different compounds
of lime and phosphoric acid.
_Phosphates of Lime._
In the various phosphatic manures used in agriculture there are four
different kinds of phosphates. In the commonest form, popularly called
bone-phosphate, which is the form in which lime and phosphoric acid are
combined in bones, guano, and the ordinary mineral phosphates, the lime
and phosphoric acid are combined in the form of what is known as
tribasic phosphate of lime, or tricalcic phosphate--that is to say, for
every equivalent of phosphoric acid there are three equivalents of lime.
This may be represented as follows:--
Lime }
Lime > Phosphoric acid.
Lime }
Or we may also say that for every 142 parts by weight of phosphoric acid
there are 168 parts by weight of lime in this form of phosphate. This is
the least soluble form of phosphoric acid,[226] and is the form
generally referred to in commercial analyses as insoluble phosphate.
When this phosphate is acted upon with sulphuric acid, a soluble
phosphate is formed, as Liebig first showed, to which the name
superphosphate has been given, and which is also known as monobasic
phosphate of lime, or monocalcic phosphate. This compound may be
represented as containing, instead of three equivalents of lime, only
one, the other two equivalents being replaced by water. This compound
may be represented as follows:--
Lime }
Water > Phosphoric acid.
Water }
In it, for every 142 parts of phosphoric acid, there are only 56 parts
of lime. It is soluble in water, and gives to the commercial article
known as superphosphate of lime its value. Intermediate in composition
between these two phosphates there is another known as precipitated
phosphate of lime, or dicalcic phosphate (the same as reverted
phosphate), which contains two equivalents of lime and one equivalent of
water as follows:--
Lime }
Lime > Phosphoric acid.
Water }
This compound contains, for every 142 parts of phosphoric acid, 112
parts of lime; and in solubility occupies an intermediate position.
Lastly, there is a fourth compound of lime and phosphoric acid, which
only occurs in one phosphatic manure--viz., phosphatic slag, in which
indeed it was first discovered--which consists of four equivalents of
lime to one of phosphoric acid, to which the name tetrabasic phosphate
of lime or tetracalcic phosphate has been given. Its composition may be
illustrated as follows:--
Lime }
Lime > Phosphoric acid.
Lime >
Lime }
Or, for every 142 parts of phosphoric acid, there are 224 parts of lime.
Contrary to what we might expect, this phosphate is less insoluble than
the ordinary tribasic or bone phosphate. This may be owing to the fact
that, in the tetrabasic phosphate, there is more lime present than that
which the phosphoric acid can retain with strong chemical affinity.[227]
In the manufacture of superphosphate the tribasic phosphate is converted
into the soluble phosphate--the lime, which was formerly in combination
with the phosphoric acid, uniting with the sulphuric acid, and forming
gypsum.[228] It was till recently supposed that soluble phosphate and
gypsum were the only two resulting products of this decomposition. It
has been recently shown, however, by Ruffle and others, that this is
not, strictly speaking, the case, and that probably a large proportion
of free phosphoric acid is formed; in fact, it seems probable that in
the first stage of the reaction, only phosphoric acid is produced, and
that this subsequently acts upon the undecomposed phosphate, with the
production of monocalcic phosphate.[229] The amount of sulphuric acid
which experience has shown it is necessary to add for the successful and
economical manufacture of superphosphate, depends on the composition of
the raw material employed. The larger the percentage of tribasic
phosphate, the larger the quantity of sulphuric acid required for its
decomposition; but sometimes even a poor phosphate consumes a large
amount of sulphuric acid. This is the case where much calcium carbonate
or fluoride is present in the raw phosphate, as both of these compounds
require a quantity of acid for their decomposition, which takes place
before the decomposition of the phosphate. Hence phosphates rich in
carbonate of lime are not well suited as economical materials from which
to manufacture superphosphate.
_Reverted Phosphates._
A change which is apt to take place in superphosphate after its
manufacture is what is known as reversion of the soluble phosphate. Thus
it is found that on keeping superphosphate for a long time the
percentage of soluble phosphate becomes less than it was at first. The
rate at which this deterioration of the superphosphate goes on varies in
different samples. In a well-made article it is practically
inappreciable, whereas in some superphosphates, made from unsuitable
materials, it may amount to a considerable percentage. The causes of
this reversion are twofold. For one thing, the presence of undecomposed
phosphate of lime may cause it. This source of reversion, however, is
very much less important than the other, which is the presence of iron
and alumina in the raw material. When a soluble phosphate reverts, what
takes place is the conversion of the monocalcic phosphate into the
dicalcic. Now in the first case, where reversion is due to the presence
of undecomposed phosphate, the action taking place may be represented as
follows:--
Lime } } { lime } }
Lime } phosphoric acid } { water} phosphoric acid }
Lime } } + { water} } =
(One molecule of insoluble } { (One molecule of soluble}
phosphate) } { phosphate) }
Lime } } { lime }
Lime } phosphoric acid } { lime } phosphoric acid.
Water} } + { water}
(One molecule of reverted } { (One molecule of reverted
phosphate) } { phosphate.)
It may be mentioned, however, that reversion from this cause probably
takes place to a very slight extent in practice.[230] Where reversion is
due to the presence of iron and alumina in the raw material, the nature
of the reaction is not well understood, and is consequently not so
easily demonstrated as in the former case. Where iron is present in the
form of pyrites, or ferrous silicate, it does not seem to cause
reversion. It is only when it is present in the form of oxide--and in
most raw phosphatic materials it is generally in this latter
form[231]--that it causes reversion in the phosphate.
_Value of reverted Phosphate._
The value of reverted phosphate is a subject which has given rise to
much dispute among chemists. That it has a higher value than the
ordinary insoluble phosphate is now admitted; but in this country, in
the manure trade, this is not as yet recognised. At first it was thought
that it was impossible to estimate its quantity by chemical analysis.
This difficulty, however, has been overcome, and it is generally
admitted that the ammonium citrate process furnishes an accurate means
of determining its amount. Both on the Continent and in the United
States reverted phosphate is recognised as possessing a monetary value
in excess of that possessed by the ordinary insoluble phosphate. The
result is, that raw phosphates containing iron and alumina to any
appreciable extent are not used in this country, although they do find a
limited application in America and on the Continent.
_Composition of Superphosphates._
Superphosphates as manufactured may be divided, generally speaking, into
three classes--viz., low class, medium, and high class. The ordinary or
medium class contains from 25 to 27 per cent of soluble phosphate; and
here it may be pointed out that by soluble phosphate is meant the
percentage of tribasic phosphate which has been dissolved--not, as might
at first sight be supposed, the percentage of monocalcic phosphate. The
lower-class superphosphates are those containing less than 25 per cent,
generally 23 to 25 per cent, of soluble phosphate; while the high-class
superphosphate may contain from 30 to 45 per cent. For the manufacture
of high-class superphosphate only a certain number of raw phosphates are
available, such as Curacao and Somme phosphates, phosphatic guanos,
bone-char, &c. Certain processes have been patented for the manufacture
of even more concentrated superphosphates, and by them phosphates
containing as much as 40 per cent of soluble phosphoric acid--_i.e._,
equal to 87 per cent of soluble phosphate--have been prepared. To this
class belongs the so-called double superphosphate, manufactured at
Wetzlar in Germany. Such a concentrated form of manure is naturally very
expensive to manufacture, and is hardly to be recommended for home
consumption. Where, however, manures have to be conveyed long distances,
and the freight is consequently very high, such a concentrated article
may be found most economical.
_Action of Superphosphates._
When superphosphate is applied to the soil it is converted into an
insoluble state. In short, the process of reversion is carried on on a
wholesale scale. This is due to the lime, iron, and alumina salts which
the soil contains. In all probability the phosphate is finally converted
into a hydrated ferric or aluminic phosphate, in which form it is
gradually acted upon by the sap of the plant-roots as required. This
being the case, it may be asked, Why is superphosphate so much more
rapid in its action than insoluble phosphate; or why should we be at the
trouble and expense of dissolving the phosphate if it has to become
insoluble again in the soil? This question is one of very great
importance, for the answer to it furnishes, in our opinion, the key to
the whole phosphate question. When superphosphate is added to the soil,
being soluble in water, it is soon dissolved and carried down by the
rain into its pores, and becomes thoroughly mixed with the
soil-particles. It is thus soon fixed in the soil, beyond the risk of
being washed away. The result is, that the phosphate is obtained in a
state of division infinitely more minute than could ever be obtained by
mechanical grinding, and is, further, most intimately mixed with the
particles of the soil. It is this intimate mixture of the phosphate with
the particles of the soil, and its minute state of division, that
constitute the only reason for rendering superphosphate superior in its
action to even the most finely ground insoluble phosphates. This opinion
is supported by the fact, that although the chemist has imitated nature
in this matter so far as to manufacture precipitated phosphate, he has
failed, as a rule, in getting as favourable results with it as with
superphosphate. Although the mechanical state of division of the
manufactured precipitated phosphate is probably as fine as that
obtained by nature from the superphosphate, it is impossible to obtain
so intimate a mixture with the soil-particles, and hence the results
obtained are different. For these reasons it will be easily seen that
the rate of action of the superphosphate must always be quicker than
that of any other form of phosphatic manure. The phosphate is everywhere
distributed in the soil. The plant-roots are thus furnished with a
continuous supply throughout their growth, and micro-organisms, which
require for their development a supply of this necessary plant-food, are
propagated. A regularity in the plant's growth is thus secured, which is
of great importance. But while admitting this, there are many cases in
which this greater quickness of action does not render soluble phosphate
the most economical form. The nature of the crop, as well as the nature
of the soil, may in many cases be such as to render the application of
the cheaper insoluble phosphate more economical. It is imperative that
the early growth of some crops be hastened as much as possible by a
ready supply of easily assimilable plant-food, in order to enable them
to successfully sustain the attack of certain pests to which they are
liable to succumb. This, for example, is notably the case with turnips.
In such a case there can be no doubt that the value of soluble phosphate
to the young plants is very great, as it enables them to survive this
critical period.
_Action of Superphosphate sometimes unfavourable._
But even in this case there may be other conditions which render
insoluble phosphate a preferable manure. Such a case is where the soil
is of a very light nature and is deficient in lime. In this case the
acid superphosphate, not having the necessary base to combine with, may
prove even hurtful to the young plants. According to the late Dr
Voelcker, a concentrated superphosphate may produce a smaller crop than
a fertiliser containing only a quarter as much soluble phosphoric acid,
when applied to root-crops on sandy soils, greatly deficient in lime.
Cases such as the above, however, are extremely rare; and we may say
that, in the case of root-crops generally, superphosphate must be
regarded as of special value.
_Application of Superphosphate._
In any case, superphosphate ought to be applied to a soil some time
before it is likely to be assimilated by the plant, in order to allow
neutralisation of its acid character to be fully effected before the
plant's roots come in contact with it. Thus Professor S. W. Johnson, one
of the greatest living American authorities, states it as his opinion
that recent investigations tend to show that soluble and reverted (or
precipitated) phosphates are, upon the whole, about equally valuable as
plant-food, and of nearly equal commercial value. But as Sir John Lawes,
in quoting Professor Johnson to the above effect, remarks, this opinion
is based on an experience of American agriculture, in which country
soluble phosphate is chiefly applied to cereal crops, while in this
country it is chiefly applied to turnips. In the case of cereal crops,
the importance of a speedy early growth is not so great, as we have
already pointed out, as it is in turnips, where the danger to the young
plants from the ravages of the turnip-fly is such that the growth of
even a day or two may make a very considerable difference.
_Value of Insoluble Phosphate._
A consideration of the action of superphosphate, then, throws a good
deal of light on the conditions which determine the value of insoluble
phosphates when applied to the soil, and shows that the state of
division, intimacy of mixture with soil-particles, and the nature of the
soil, are the determining factors. Insoluble phosphates, as we shall
have occasion to see when discussing basic slag, have their best action
on soils poor in lime and rich in organic matter. Tables have been drawn
up with a view to furnishing a guide for the value of phosphoric acid in
different manures. In the Appendix[232] we give those of Wolff for 1893,
and an American table, drawn up for 1892. The comparative values of
mineral phosphates, as well as Peruvian guano and bone-dust, will be
further referred to in the following chapter.
_Rate at which Superphosphate is applied._
The rate at which superphosphate is applied to the soil varies in
different parts of the country. In England 2 to 3 cwt. per acre is
considered an average dressing; whereas in many parts of Scotland it is
applied in as large quantities as 6 to 8 cwt. per acre to the turnip
crop. The reason why so much heavier dressings can be advantageously
given in northern parts of this country is owing to the much longer
period of unchecked growth. In the more southern districts, where the
rainfall is less, mildew is almost certain to appear when the sowing is
as early as required for a maximum crop. With it, as with other manures,
the quantity must be determined by the conditions of its application,
and the amount of other manure applied.
FOOTNOTES:
[225] This holds true, it may be mentioned, with regard to the
application of certain manures, such as bone-char, to the soil.
Bone-char was for a long time used in France as a manure without being
dissolved. The action of such a manure, containing a considerable
percentage of carbonate of lime, is slower than its action would be were
it pure phosphate of lime, as the carbonate of lime is first acted upon
(as in the case of superphosphate manufacture) by the soil acids.
[226] The solubility of tribasic phosphate, of course, is not always
equal in different manures. For example, the phosphate in apatite, owing
to the crystalline structure of that mineral, is not nearly so soluble
as the phosphate in phosphatic guanos, although in both cases its
chemical composition is practically the same.
[227] For formulae of the different phosphates, see Appendix, Note I., p.
398.
[228] For chemical formulae, showing reaction, see Appendix, Note II., p.
398.
[229] Of course it is well known that free phosphoric acid is obtained
by acting upon phosphate of lime with an excess of sulphuric acid; but
the point above referred to as having been recently discovered is, that
when phosphate of lime is acted upon, even by a small quantity of
sulphuric acid, free phosphoric acid is formed.
[230] For chemical formulae showing this reversion, see Appendix, Note
III., p. 399.
[231] For chemical theories on reversion of soluble phosphate by iron
and alumina, see Appendix, Note IV., p. 399.
[232] See Appendix, Note V., p. 400.
APPENDIX TO CHAPTER XIII.
NOTE I. (p. 388).
The formulae, and molecular and percentage composition, of the different
phosphates, are given in the following table:--
----------------------------------------------------------------------------
| Composition in terms of--
+--------------------------+-------------------
| Molecular weight. | Per cent.
---------------+-------------+-----+------+------+------+-----+------+------
| | | |Phos- | | | |Phos-
Name. | Symbol. |Lime.|Water.|phoric|Total.|Lime.|Water.|phoric
| | | |acid. | | | |acid.
---------------+-------------+-----+------+------+------+-----+------+------
Tri- or bone- |3CaO, | | | | | | |
phosphate. | P_{2}O_{5} | 168 | 0 | 142 | 310 |54.19| 0.00| 45.81
Bi- or di- |2CaO, H_{2}O,| | | | | | |
phosphate. | P_{2}O_{5} | 112 | 18 | 142 | 272 |41.18| 6.61| 52.21
Mono- or super-|CaO, 2H_{2}O,| | | | | | |
phosphate. | P_{2}O_5 | 56 | 36 | 142 | 234 |23.93| 15.39| 60.68
---------------+-------------+-----+------+------+------+-----+------+------
NOTE II. (p. 388).
When sulphuric acid is added to tricalcic phosphate, the following
reaction takes place:--
(1.) 3CaO, P_{2}O_{5} + 2(H_{2}O, SO_{3})
(Tricalcic phosphate), (Sulphuric acid),
= 2(CaO, SO_3) + CaO, 2H_{2}O, P_{2}O_{5}
(Gypsum), (Monocalcic phosphate).
(2.) 3CaO, P_{2}O_{5} + 3(H_{2}O, SO_{3}) = 3CaO, SO_{3} +
3H_{2}O, P_{2}O_{5}, or 2H_{3}PO_{4}.
NOTE III. (p. 390).
This equation gives the chemical reaction taking place when soluble
phosphate is reverted, owing to the presence of undissolved
phosphate:----
3CaO, P_{2}O_{5} + CaO, 2H_{2}O, P_{2}O_{5}
(Tricalcic phosphate), Monocalcic phosphate,
= 2CaO, H_{2}O, P_{2}O_{5} + 2CaO, H_{2}O, P_{2}O_{5}
(Dicalcic phosphate), (Dicalcic phosphate).
NOTE IV. (p. 390).
"Just what the reactions are which are produced by the iron and alumina
compounds has never been made out very clearly. But some idea of them
may be gained from the following suggestions, which were thrown out by
the English chemist Patterson. Suppose the sulphuric acid has dissolved
a quantity of iron or alumina, then we may have the reaction:----
Fe_{2}O_{3}, 3SO_{3} + CaO, 2H_{2}O, P_{2}O_{5} = Fe_{2}O_{3},
P_{2}O_{5} + CaO, SO_{3} + 2(H_{2}O, SO_{3}),
and the free acid thus formed would proceed to dissolve more iron or
alumina from the rock that had previously escaped decomposition, and the
reaction here formulated would occur again and again. Here we have a
cumulative process continually increasing the quantity of insoluble
Fe_{2}O_{3}, P_{2}O_{5}, and diminishing in the same proportion the
soluble P_{2}O_{5}. Again, we may have simply----
2Fe_{2}O_{3} + 3(CaO, 2H_{2}O, P_{2}O_{5}) = 2(Fe_{2}O_{3}, P_{2}O_{5})
+ 3CaO, P_{2}O_{5};
where three molecules of the soluble phosphoric acid are made to revert
to the insoluble state at one blow.
"In case the iron in the original rock were in the state of ferrous
oxide, perhaps the following reaction might occur:----
4(FeO, SO_{3}) + 2O + CaO, 2H_{2}O, P_{2}O_{5} + 3CaO, P_{2}O_{5} =
2(Fe_{2}O_{3}, P_{2}O_{5}) + 4(CaO, SO_{3}).
In all these equations, except the last, alumina would serve as well as
oxide of iron."--(_Vide_ Storer's 'Agricultural Chemistry,' vol. i. pp.
276, 277.)
NOTE V. (p. 396).
The following table shows the relative trade values of phosphoric acid
in different manures:--
I.--WOLFF, 1893.
Phosphate soluble in water (as in super) 100
Precipitated phosphate, Peruvian guano 92
Reverted phosphate, finest steamed bone-dust
fish-guano, poudrette 83
Phosphatic guanos (Baker Island), wood-ashes 75
Coarser bone-dust, powdered animal charcoal, bone-ash 67
Coarse fragments of bone, powdered phosphorite and
coprolite, Thomas-slag, farmyard manure 33
II.--AMERICAN, 1892.
Phosphate soluble in water 100
Phosphate soluble in ammonium citrate 94
Fine bone-dust, powdered fish 94
Fine medium bone 74
Medium bone 60
Coarse bone 40