Manures and the principles of manuring
by Charles Morton Aikman
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[221] Since the discovery of the Florida deposits of phosphate, the working of the Canadian mines has been practically abandoned.

[222] See Appendix, p. 381.

[223] These phosphates are now no longer worked.

[224] These deposits were discovered a few years ago; and as they are of considerable extent and high quality, have entirely revolutionised the phosphate market. About 300,000 tons are now annually raised in Florida.


NOTE (p. 375).


- - - - - - 1885. 1886. 1887. 1888. 1889. - - - - - - Tons. Tons. Tons. Tons. Tons. United States 138,844 144,623 165,275 111,369 122,554 Canada 21,484 18,069 19,194 12,423 23,297 Dutch West Indies (Curacao, Aruba) 11,588 12,581 9,505 10,736 14,730 British West Indies (Sombrero, &c.) 7,727 3,351 6,451 11,010 1,880 Spain and Portugal 19,282 5,825 15,612 6,978 1,326 Belgium 35,405 31,551 45,322 54,261 64,643 Holland 865 2,194 4,778 4,137 2,270 France 2,276 1,503 11,140 39,059 65,490 Australia - 200 350 - 1,250 Germany 704 - - - - Hayti (San Domingo) - 2,175 3,044 6,238 4,094 Brazil - - 1,200 - - Venezuela and Guiana - - 405 - - Norway - - - - - Other countries 397 1,039 1,139 1,675 390 *Florida phosphate. - - - - - Carolina phosphate. - - - - - - - - - - -

-+ -+ + 1890. 1891. 1892. -+ -+ + Tons. Tons. Tons. United States 177,283 *131,084 *201,465 Canada 21,089 15,918 7,814 Dutch West Indies (Curacao, Aruba) 14,763 8,851 6,648 British West Indies (Sombrero, &c.) 3,970 1,960 2,473 Spain and Portugal - 320 971 Belgium 82,096 70,723 65,079 Holland 2,428 3,434 6,627 France 35,659 18,325 18,239 Australia - - - Germany - - - Hayti (San Domingo) 992 1,639 2,965 Brazil - - - Venezuela and Guiana - 540 - Norway 4,151 1,495 305 Other countries 1,070 1,483 1,594 *Florida phosphate. - 35,203 66,327 Carolina phosphate. - 96,881 135,138 -+ -+ +



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.


[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.


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}O5 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, SO3) + 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


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



In this substance we have a most important addition to our phosphatic manures. It has been in the market since 1886, and the consumption alone in Germany in 1887 amounted to nearly 300,000 tons. In this country it is only now beginning to be used to any extent.

Its Manufacture.

Thomas-slag is a bye-product obtained in the manufacture of steel by what is known as the "basic" process. In the year 1879 an improvement in the well-known "Bessemer" process was patented by Messrs Gilchrist & Thomas. It must be explained that in the manufacture of steel from pig-iron certain impurities in the raw material have to be got rid of in order to produce a good steel. Among these impurities one of the most important is phosphorus. This is owing to the fact that even a very small percentage of phosphoric acid in steel has the effect of rendering it brittle. The extraction of the phosphorus from the raw material was formerly, however, attended with very serious difficulties, and had the effect naturally of rendering steel a costly article, inasmuch as only the purer kinds of pig-iron could be used for the purpose.

By the introduction in 1879, however, of the "Thomas-Gilchrist" or "basic" process, these difficulties were very largely overcome, and the employment of even such impure irons as the Cleveland (containing comparatively a large percentage of phosphorus) was rendered possible, and the price of steel consequently generally very much reduced. The process consists of submitting the molten pig-iron to a very great heat in a pear-shaped vessel (known technically as the "converter"). This is open at the top, and is supported on hinges, which permit of its being moved so as to pour off the scum which rises to the surface at the end of the operation, and which, we may explain, consists of "basic slag." In the original process the sides of the "converter" were lined with fire-bricks, consisting largely of silica. This process was known as the "acid" process. In the "Thomas-Gilchrist" process, however, the sides of the "converter" are lined with lime (dolomitic limestone being largely used), lime being also added to the pig-iron. An air-blast is injected through the molten mass, and the impurities are burnt, or oxidised as it is chemically termed. The phosphorus in the iron is thus converted into phosphoric acid, and, uniting with the lime, forms phosphate of lime, which rises, as we have already said, to the surface in the form of a scum, and is separated from the steel by being poured off.

Not at first used.

This, then, is how the Thomas-slag is obtained. It did not seem, however, for some years after the introduction of this ingenious process, to have struck any one that this rich phosphatic bye-product might prove a valuable addition to our artificial fertilisers. The result was, that the Thomas-slag was treated as another of the only too numerous valueless bye-products which seem to be necessarily incidental to most of our chemical and other manufactures, and was allowed to accumulate in large quantities without being used for any purpose.

Discovery of its Value.

In 1883 some short articles published in Germany on the subject were the means of first drawing the attention of the public to its importance as a manure. During the years 1884 and 1885 numerous experiments were carried out on the subject in the same country; and from then up till the present hour it has become more and more extensively used in Germany, till in 1887, as already stated, its consumption amounted to nearly 300,000 tons.


It consists mainly of phosphate of lime, silicate of lime, free lime, free magnesia, and oxides of iron and manganese. Its composition, of course, naturally varies; but the following may be taken as an average analysis:[233]—

Per cent. *Phosphoric acid 17 Lime in combination with phosphoric, silicic, sulphuric, and carbonic acids 40 Free lime 15 Oxides of iron 12 *Equal to tricalcic phosphate 37

As a rule, the phosphoric acid varies considerably, ranging from 10 to 20 per cent—that is, from 22 to 44 per cent tricalcic phosphate. This is owing to the difference in the percentage of phosphorus in the raw material and the quantity of lime added. Attempts have been made in Germany during the last two or three years to obtain a slag richer in phosphoric acid than that obtained heretofore, and a process for this purpose has been patented by Professor Scheibler. This consists of a slight modification in the ordinary process. Instead of treating the pig-iron with an excessive quantity of lime, the amount added is not sufficient to effect the complete dephosphorisation of the iron. The resulting slag is very rich in phosphoric acid, and is correspondingly poor in iron. The iron is then again treated with fresh lime, and the phosphorus completely removed, while the same lime may be used over again. Such slag forms a very much more concentrated phosphatic manure than the ordinary slag, and is known as patent phosphate meal.

A point which not only renders the slag a product of peculiar interest from a chemical point of view, but has a most important bearing on its value as a manure, is the nature of the compound formed by the union of the lime with the phosphoric acid.

In the ordinary so-called raw phosphates, such as bone-meal, bone-ash, coprolites, &c., the lime and phosphoric acid are combined in the form of what is known, in chemical phraseology, as tribasic phosphate of lime. That is to say, that for every equivalent of phosphoric acid there are three equivalents of lime. Now it was naturally concluded at first that the tribasic phosphate was the form in which these two substances existed in the slag. This, however, was found out not to be the case, in the following way. On allowing the slag to cool, it was found that small but perfectly defined crystals were formed. These crystals, by careful analysis, were shown, first by Hilgenstock, to consist of a form of phosphate of lime hitherto unknown, in which four equivalents of lime were combined with one equivalent of phosphoric acid, and which was therefore called "tetrabasic phosphate."

Processes for preparing Slag.

As soon as the idea of utilising the slag as a manure was suggested, various plans for extracting its phosphoric acid, and rendering it available as plant-food, were devised. These were deemed necessary, it was thought, by the very insoluble nature of the phosphates in the slag, as well as by the supposed injurious action which would be exerted on plant-life by the protoxide of iron it contained. Accordingly, a large number of patents were taken out, "covering almost every conceivable method for treating the slag, whether practicable or not. They all in the main are combinations or variations of the following processes:—

"1. Preliminary preparation of the Slag.

(a) By treating molten, or otherwise, with superheated steam, or cooling when hot with water, to reduce it to small pieces or to a fragile state.

(b) Grinding.

(c) Treating with water to wash out free lime, or with sugar solution.

(d) Roasting in the air, or with some oxidising agent.

"2. Solution of the Slag.

(a) Completely in weak or strong acids (hydrochloric, sulphuric, &c.)

(b) Partially, so as to dissolve the phosphates and silicates of lime, and leave most of the iron and manganese oxides.

"3. Precipitation of the phosphoric acid, with lime or iron salts: or,

"Processes in which the slag is smelted with charcoal, to reduce phosphates to phosphides, treated with acid, and the phosphuretted hydrogen burnt to phosphoric acid; and,

"Processes in which the slag is fused with soda or potash salts,—caustic, chlorides, sulphates, carbonates,—with or without steam being forced through, to form soluble alkaline phosphates."[234]

Many of these processes were tried; but it was found by experiment that the best and most economical way was by applying the slag direct to the ground in a state of very fine powder. Experiments further showed that it had not the injurious effect on vegetation which it was feared it would have from the protoxide of iron it contained. The discovery that its phosphoric acid existed, as has been already explained, as a tetrabasic phosphate of lime, has strengthened the opinion that this is the best method of application.

A good deal has been found to depend upon the fineness of the ground slag, with the result that it is now commonly sold on a mechanical as well as a chemical analysis—i.e., the slag is guaranteed to pass through a sieve of a certain fineness.

Solubility of Slag.

Professor Wagner of Darmstadt has carried out some extremely interesting experiments on the solubility of slag. He found that very finely powdered slag was dissolved in carbonic acid water to the extent of 36 per cent, while, similarly treated, phosphorite only dissolved to the extent of 8 per cent.[235] Another very important solvent is citrate of ammonia. Reverted (or precipitated) phosphate is entirely soluble in it, and phosphate soluble in it ought to be valued as worth more than that which is not. Now, the solubility of Thomas-slag in citrate of ammonia was found by Professor Wagner to be no less than 74 per cent, while that of phosphorite only amounted to 4 per cent. These results were corroborated by Professor S. W. Johnson, who found that of the 19.87 per cent of phosphoric acid contained in a sample of basic slag, no less than 19.57 per cent was soluble in ammonium citrate, while a finely ground sample of phosphatic rock yielded, on analysis, only 1.81 per cent soluble in citrate of ammonia, of a total of 29.49 per cent phosphoric acid which it contained. Professor Fleischer has also tested the comparative solubility of basic slag and phosphorite, by boiling them in a solution of acetic acid. The former was found to have been dissolved to the extent of 19 per cent, while the latter to only 5 per cent. A highly interesting and most important experiment was performed by Mr Heinrich Albert, of Biebrich. One gramme of basic slag and 100 grammes of peat were mixed together in a litre of water, and it was found that, after standing for fourteen days, 79 per cent of the phosphoric acid contained in the slag was rendered soluble.

In the above experiments it was found that the fineness of grinding had a marked effect on the solubility of the slag, and that the finer it was ground the greater was its solubility. This has been further demonstrated in Professor Wagner's practical experiments. From these it was found that finely ground slag has an action four times as quick as coarse slag; but that, as far as practical results were concerned, there seemed to be a limit to the fineness to which it was advisable to grind the slag, as slag above a certain fineness did not give better results than a coarser slag. At any rate, he found that slag of a fineness so great that it all passed through a gauze sieve, gave no better results in his experiments than slag which left 17 per cent behind. We may say, however, that the finer the slag is ground, the greater will its activity as a manure be; and that a certain degree of fineness is absolutely necessary to constitute it an active fertiliser. As Professor Wagner's experiments are among the most valuable and complete carried out on basic slag, we shall give a somewhat detailed account of them.

Darmstadt Experiments.

Professor Wagner's experiments were carried out on such different kinds of crops as flax, rape, wheat, rye, barley, peas, and white mustard, and the object of the experiments was to ascertain the comparative activity as fertilisers of superphosphate, basic slag of different degrees of fineness, Peruvian guano, damped bone-meal, and very finely ground coprolites. In order to obtain a correct estimate of the relative value of these different forms of phosphatic manures, it was necessary to render the nitrogen in the bone-meal and the nitrogen and potash contained by the Peruvian guano inactive—i.e., to limit the test strictly to phosphoric acid. This was done by adding to the super, basic slag, and coprolites, quantities of nitrogen and potash equal to those contained by the other manures. There was further added to all the experiments (the unmanured ones, of course, as well) an excess of nitrogen and potash. In this way the increase in returns could only be due to the phosphoric acid.

The general results obtained from these experiments may be summed up as follows: Taking the activity of "super" to be represented by 100, then the relative activity of—

Basic slag of No. 1[236] fineness is 61 Basic slag, No. 2[237] 58 Peruvian guano 30 Basic slag, No. 3[238] 13 Bone-meal 10 Coprolites 9

From these results the value of the commercial article has been attempted to be ascertained. As it contains 80 per cent or thereby of fine meal and 20 per cent of coarse, its activity may be stated to be 50, or half as active as super. Thus 2 cwt. of basic slag is equal to 1 cwt. of super. This only refers to the first year's effect. Professor Wagner has made further experiments as to the after-effects of the different manures, with the result that he has found that the after-effects of the basic slag are even better than those of the "super." This stands to reason, for if twice as much phosphoric acid be added in the form of basic slag as is added in the form of "super," and the effect of the first year is similar—that is, the same quantity of phosphoric acid is assimilated by the plant from the soil in both cases—there is naturally more phosphoric acid left behind in the soil manured with basic slag than in that manured with superphosphate of lime. For example, if 100 lb. of super has the same effect in the first year as 200 lb. of basic slag, and it is found that only 60 lb. of the super and the basic slag have been assimilated the first year by the plant, it is only natural to conclude that the remaining 140 lb. of the basic slag will have a better after-effect than the remaining 40 lb. of super. This has been actually proved to have been the case in Professor's Wagner's experiments. The following are the results of some experiments which Professor Wagner has carried out on the after-effects of different manures:—

Out of 100 parts of phosphoric acid, there was removed by the first year's crop—

Super 63 Peruvian guano 22 Bone-meal 7 Coprolites 6 Thomas-meal— No. 1 fineness 39 No. 2 " 43 No. 3 " 15

Out of 100 parts of phosphoric acid left by the first crop, there was removed by the three succeeding crops—

Super 30 Peruvian guano 9 Bone-meal 13 Coprolites 6 Thomas-meal— No. 1 fineness 14 No. 2 " 29 No. 3 " 24

Numerous other experiments have been carried out by various experimenters in different parts of Germany which it is unnecessary to cite here. None, however, are so complete as those of Professor Wagner.

Results of other Experiments.

In this country experiments have been carried out at Rothamsted, Cirencester, Downton, Bangor, and by Dr Aitken at the Highland and Agricultural Society's stations, as well as elsewhere. The results of these various experiments naturally differ considerably, this being owing to the difference in the nature of the soils upon which the experiments were carried out, as well as the different degrees of fineness of the slag used. They all, however, serve to confirm Professor Wagner's general results. The results obtained in Scotland by Dr Aitken at the Highland Society's stations were especially favourable to basic slag as a phosphatic manure. The experiments were carried out on turnips, and it was found that the Thomas-slag was, weight for weight, superior to superphosphate. It may be added that the slag used in these experiments was rich in phosphoric acid, and was in an unusually fine state of division. Experiments carried out by the author have proved slag to be, on various Scottish soils, one of the most economical phosphatic manures to apply to turnips.[239]

We will sum up, in conclusion, the deductions which may be fairly drawn from the results of all the above-mentioned experiments as to the value of basic cinder as a manure.

Soils most suited for Slag.

Although its action is undoubtedly more favourable on some soils than others, it may be broadly stated that generally its phosphoric acid is half as valuable as that in soluble phosphate. The soils on which it will have the most marked effect will be those of a peaty nature, poor in lime, but rich in organic matter. The beneficial results obtained by an application of lime to peaty soils are well known. As the slag contains a large percentage of free lime, it thus performs on such soils a double function. On meadow-lands, all kinds of pasture-lands (if not of too dry a character), and clay soils poor in lime, its action has been shown to be especially favourable. Of different kinds of crops, those best suited to benefit from the slag as a phosphatic manure are those of the leguminous kind. This arises from the fact that their period of growth is longer than that of most other crops.

Rate of Application.

As to the rate per acre at which the slag ought to be applied, there will naturally be a difference of opinion. Professor Wrightson, of Downton Agricultural College, recommends that it should be applied at the rate of from 6 to 10 cwt. per acre. This, of course, is very liberal manuring. We must remember, however, that phosphatic manures, unlike nitrogenous manures, and to some extent potash manures, may be applied in even excessive quantities without any risk of loss. It is impossible to measure out our phosphate manures in the same accurate manner as we measure out our nitrogen. It is safer, therefore, and on that account more economical in the long-run, to apply our phosphate in excessive quantity than the reverse. The reason of this may be shortly explained. The phosphoric acid which is naturally present in most soils is with difficulty soluble. Only a small quantity is yielded daily to the plant. This quantity may, under favourable climatic conditions, be sufficient; but these favourable influences never last very long at a time.

For three weeks, perhaps, the plant may experience drought, and during this period it takes up no phosphoric acid, and its growth practically comes to a standstill; but this period of drought is followed by rain and warm weather, and the plant, if it is to be ripe by harvest-time, must make up for lost time. It must grow as much the next few days under these favourable climatic conditions as it would have grown under normal conditions in double or treble the time. In order to do so, however, it must be able to obtain plenty of phosphoric acid, and this is only possible where there is a decided excess of phosphoric acid present in the soil.

The richness of a soil, therefore, in phosphoric acid, must be such that it is not only able to supply the ordinary wants of the plant, but to provide an excess when such an excess will be needed; for one must remember that the amount of plant-substance formed in the course of a few days under favourable conditions is very great, and that the amount consequently of phosphoric acid which plants assimilate during that period must also be very considerable.

Method of Application.

In conclusion, as to the method of application of the slag, agriculturists must be warned against mixing it with sulphate of ammonia; for if this is done, a considerable loss of ammonia will ensue, set free from the sulphate by the action of the free lime which the Thomas-slag contains. With nitrate of soda and potash salts it may be freely mixed. Such mixtures, however, are apt to form themselves into little balls, which soon become very hard. They should therefore only be mixed shortly before use. To overcome this difficulty, Professor Wagner recommends the mixture of a little peat or sawdust with the slag.


[233] See Appendix, p. 417.

[234] Vide paper on "Basic Slag: Its Formation." By Stead and Ribsdale. 'Journal of the Iron and Steel Institute,' 1887, p. 230.

[235] Vide Professor Wagner's pamphlet, 'Der Duengewerth und die rationelle Verwendung der Thomas Schlacke,' Darmstadt, 1888.

[236] No. 1 fineness was such as passed entirely through a fine gauze sieve of 250 wires to the linear inch.

[237] No. 2 fineness was such as passed entirely through the regular standard sieve—i.e., containing 120 wires to the linear inch.

[238] No. 3 was what would not pass through the standard sieve.

[239] 'Transactions of the Highland and Agricultural Society,' 1891; 'Chemical News,' 1893.


NOTE (p. 404).

For those more particularly interested, we append a full analysis of the slag, taken from Messrs Stead and Ribsdale's paper in the 'Journal of the Iron and Steel Institute,' 1887, vol. i. p. 222:—

Lime 41.58 Magnesia 6.14 Alumina 2.57 Peroxide of iron 8.54 Protoxide of iron 13.62 Protoxide of manganese 3.79 Protoxide of vanadium 1.29 Silica 7.38 Sulphur } .23 Calcium } .31 Sulphuric anhydride .12 Phosphoric acid 14.36 ——- 99.93



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.



In addition to the manures which have been discussed in previous chapters, there are a number of minor manures which are used to a very much smaller extent—dried blood, hoofs, horns, &c.

Among these one of the most valuable is dried blood. Fresh blood, containing 80 per cent of water, has from 2.5 to 3 per cent of nitrogen, about .25 per cent of phosphoric acid, and about .5 per cent of alkalies. When dried it forms a very concentrated and valuable nitrogenous manure, which has long been used in France. The commercial article contains, on an average, about 12 per cent of nitrogen, and slightly over 1 per cent of phosphoric acid. When mixed with the soil it ferments, and the nitrogen it contains is converted into ammonia. Although not so quick-acting a manure as nitrate of soda or sulphate of ammonia, it can by no means be described, as is done in ordinary agricultural text-books, as a slow-acting manure. Its nitrogen may be regarded as of equal value to that in Peruvian guano. It is peculiarly suited for horticulture, and is chiefly used in this country as a manure for hops. It has also been used with beneficial results for wheat, grass, and turnips. As a manure it is best suited for sandy or loamy soils. Considerable quantities are exported to the sugar-growing colonies as a manure for sugar-cane. Manures are made from other animal refuse. It may be mentioned that lean flesh (containing 75 per cent of water) has about 3 to 4 per cent of nitrogen,.5 per cent of alkalies, and .5 per cent of phosphoric acid; that is to say, a ton of lean flesh would contain about 70 lb. of nitrogen and 10 lb. of phosphoric acid. In air-dried flesh, according to Payen and Boussingault (containing 8-1/2 per cent of moisture), there is 13 per cent of nitrogen. Flesh, therefore, is, when properly composted, a valuable nitrogenous manure. Dried flesh is generally made into a manure called meat-meal guano, the composition of which we have already referred to in the chapter on Guano.[240]

Hoofs, horns, hair, bristles, and wool, wool-waste and the intestines of animals, have been used as manures. Hoofs and horns form a regular source of artificial nitrogenous manure; the latter being obtained as a bye-product in the manufacture of combs and other articles. They are in the form of a fine powder; and in order to increase their rate of action, which is very slow, they are often composted in America with horse-manure before use. They have also been composted with slaked lime. There can be no doubt that such treatment increases very considerably their value. Their percentage of nitrogen seems to vary very much according to the kind of animal from which they are derived. In nine samples of horn the nitrogen was found to vary from 7-1/2 to 14-1/4 per cent; giving an average of 11-1/3 per cent. The nitrogen seems rarely to exceed 15 per cent. The amount of phosphoric acid they contain has been found by various investigators to range from 6 to 10 per cent. S. W. Johnson found only from .08 to .15 per cent in buffalo-horn shavings. In France what is known as "torrefied" horn has been used. This is horn which has been subjected to the action of steam. The nitrogen in this material is considered to be more active than in ordinary horn. According to Way, horns have been used for the hop crop with good results. Ground hoof is very similar in composition to horn, and contains about 14 to 15 per cent of nitrogen. Considerable quantities are now used. It must be remembered, however, that horns, hoofs, hair, bristles, &c., although rich in nitrogen, possess a comparatively low manurial value. The home production of these articles may be estimated at 6000 to 7000 tons.


Scutch is the name given to a manure made from the waste products incidental to the manufacture of glue and the dressing of skins. It contains about 7 per cent of nitrogen, and is manufactured in London to the extent of several thousand tons annually.

Shoddy and Wool-waste.

Shoddy, which is a manure made from waste-wool products, is a material largely manufactured in this country, and which was formerly (it is now used to a considerably less extent) used to a large extent as a manure. Its annual production amounts to about 12,000 tons. There are three qualities,—the first containing 8 to 12 per cent of nitrogen; the second, 6 to 8 per cent; and the third, 5 to 8 per cent. Shoddy is by no means a very valuable manure. Woollen-waste products were formerly much richer in nitrogen than is now the case. This is due to the fact of the adulteration with cotton, now so prevalent in the manufacture of woollen goods. Pure woollen rags should contain 17 to 18 per cent of nitrogen. It has been strongly recommended to treat woollen waste with caustic alkali before being used as a manure, in order to render their nitrogen more quickly available; and there is a good deal to recommend this treatment. When wool-waste is applied as a manure, it should in every case be in autumn, so as to allow as long a period as possible to elapse before it is required for the plant's growth.

Leather has also been used as a manure. Its nitrogen may be stated at from 4 to 6 per cent; and it may safely be described as of all materials used as nitrogenous manures the least valuable. Leather is, from its very nature, admirably adapted to resist decomposition when applied to the soil, and unless it is reduced to a very fine condition, might be trusted to remain undecomposed for a long period. Torrefied leather, however, is probably of greater value. It is obtained in the same way as torrefied horn, already referred to—namely, by treatment with steam. The grease and fatty matters which so largely aid it in resisting decomposition being extracted, it is much better suited for manurial purposes than ordinary leather. Torrefied leather contains from 5 to 8 per cent of nitrogen.


A manure which has long been used and highly esteemed is soot. Obtained in the usual way, it generally contains some 3 per cent of nitrogen, chiefly in the form of sulphate of ammonia, and small quantities of potash and phosphates. A varying proportion of the nitrogen is present in the form of ammonia salts; and this undoubtedly confers upon soot its manurial value. It has long been used as a top-dressing for young grain and grass, and has been applied at the rate of from 40 to 60 bushels per acre. It has an indirect value as a slug-destroyer.

Many of the above-mentioned manures, of comparatively low value, will probably be less used in the future than they have been in the past, owing to the more abundant supplies of nitrate of soda and ammonia salts which are now available. Many of these substances have probably been used in mixed manures.


[240] See p. 324.



The value of sewage as a manure has been in the past enormously overrated, and much misunderstanding has existed on the part of the public on the question of the profitableness of the disposal of town sewage as an agricultural manure. Not a few of the erroneous opinions prevalent in the past regarding sewage have been due to statements made by scientific and other writers as to the enormous wealth lost to the world by many of the present methods of sewage disposal. Fortunately, however, the sewage question is now increasingly regarded as a question, in the first instance, of sanitary interest. As much has been written on the subject, and many schemes have been devised, at the expense of much ingenuity, for utilising its manurial properties, it may be desirable here to say a few words on the purely agricultural side of the question.

The two most important points about sewage are its enormous abundance and its extremely poor quality. If the most important consideration were not the sanitary one, but its manurial value, then indeed our water system, so universally used in towns, must be regarded as a most wasteful one; for by its means the value of the excrementitious matter from which it derives its manurial ingredients is tremendously lessened. When we reflect that a ton of sewage, such as is produced in many European cities, contains only 2 or 3 lb. of dry matter, and that the total amount of nitrogen in this is only an ounce or two, while the phosphoric acid is considerably less, and that it is on those two ingredients that its value as a manure entirely depends, we see very strikingly how poor a manurial substance sewage is. Various methods have been devised and experimented with for extracting these manurial ingredients, and many methods are in operation in different parts of the world. The methods of utilising sewage for agricultural purposes may be broadly divided into two classes.


One of these, which may be classed under the heading of irrigation, consists in pouring the sewage on to certain kinds of coarse green crops. Sometimes the land is made to filter large quantities of sewage by special arrangements of drains and ditches. The land is first carefully and evenly graded down a gentle incline. At the top of the field the sewage is conducted along an open ditch from which it is permitted to escape, by the force of gravity, by several smaller ditches running at right angles from the main ditch. By means of stops which may be shifted at will, the sewage can be directed to flow over different parts of the field. Modifications in this plan may be made so as to suit the nature of the ground. In the case, for example, of a steep incline, the field may be sewaged by means of what are known as "catch-work" trenches running horizontally along the hill. In this way the sewage is allowed to pass over the whole of the field, and is caught at the bottom in a deep ditch, whence it is allowed to flow into the nearest river or stream. This is the system which has been employed at the famous Beddington Meadows, near Croydon.

Another method of distributing the sewage is by means of underground pipes, which are laid in a sort of network over the ground to be manured. At certain intervals pipes with couplings for hose are fitted on, and by keeping a certain amount of pressure on the main pipes the sewage may be distributed over the different parts of the field as it is required.

A third modification is subsoil irrigation. This resembles the last-named system, with this difference, that the pipes used are either porous or perforated with small holes.

Total submersion can only be applied in the case of absolutely level lands, and is practised to an enormous extent in Piedmont and Lombardy.

There has been little dispute as to the thorough efficiency of irrigation—when conducted under favourable conditions—as a method of purifying sewage and utilising to the full its constituents of manurial value. It is the only method which has been conclusively shown to extract from sewage that to which it owes most largely its value as a manure—viz., ammonia; and from this fact it deserves a first place in the consideration of agriculturists. For however admirable other methods may be from a sanitary point of view, it is obvious that a method which would allow the ammonia in sewage wholly, or at least to over 90 per cent, to be lost, cannot claim the same place in the judgment of agriculturists as a method which can extract for the soil not only the whole of this valuable constituent, but all else in the sewage which in any way is of value to plant-life.

Effects of continued Application of Sewage.

When sewage is continuously applied to the same land, what generally takes place is this: At first the sewage is purified, and the soil derives corresponding benefit from the valuable fertilising ingredients it thus extracts. After a time, however, the land becomes what has been termed "sewage-sick." The pores in the soil become choked up by the slimy matter the sewage contains in suspension; the aeration of the soil, which, as we have already mentioned, is so necessary, is consequently to a large extent stopped; and the result is, that the land rapidly deteriorates, and the sewage is no longer purified.

Intermittent Irrigation.

This is obviated to some extent by intermittent irrigation. The land, instead of receiving sewage continuously, only receives it at intervals, and is allowed some time to recover between each dose. It is, however, the opinion of those who have given the subject much attention, that land, even although intermittently sewaged, never recovers its original efficacy.

Irrigation, therefore, under favourable conditions, is a most successful method of utilising the manurial value of sewage; but the great difficulty in practice is to obtain those favourable conditions. It has long been known that if soil is properly to discharge its function as a purifier of sewage water, it must be properly aerated; and we now know that in every fertile soil the process of nitrification must be permitted free development. Now the application of large quantities of sewage to a soil is apt to prevent this free development. As we have already seen, absence of air and the lowering of the temperature of the soil distinctly tend to retard nitrification; and these two conditions accompany the application of large quantities of sewage.

Crops suited for Sewage.

Another objection to irrigation has been found in the alleged limited number of crops sewaged land is suited to yield. It has been repeatedly stated that rye-grass is about the only crop it is profitable to grow on it. In opposition to this statement, however, is the opinion expressed in the conclusions arrived at by the committee appointed by the British Association for the consideration of the sewage question. A vast number of experiments were carried out by them between the years 1868-72, and the result they arrived at was as follows: "It is certain that all kinds of crops may be grown with sewage, so that the farmer can grow such as he can best sell; nevertheless, the staple crops must be cattle food, such as grass, roots, &c., with occasional crops of kitchen vegetables and of corn." While, therefore, it is probably a mistake to say that rye-grass is the only crop sewaged land is capable of growing profitably, the bulk of experience goes to show that such a crop is best suited for such land. This being so, the question naturally arises, What is the farmer who uses sewage as a manure to do with the large green crops he obtains from his land? He is, in most cases, unable to use them himself or dispose of them at the time. And while this has hitherto proved to be a most important drawback, now that we have in ensilage a means of preserving our green crops in a condition suitable as fodder for as long a time as is necessary, the grounds on which this objection rests are almost entirely removed.

It will be obvious, of course, that some soils are naturally much better fitted to perform purification of sewage than others; but it must be frankly admitted that even the best of soils can only deal with a certain quantity of sewage. Various calculations have been indulged in as to the amount of sewage an acre of land can successfully deal with. According to one of these, an acre can purify some 2000 gallons per day, or that produced by 100 persons; while other calculations estimate it at 60 persons; and others, again, at 150. The capacity of a sandy soil in this respect will be much greater than that of a heavier soil; and at Dantzic an acre of the sand-dunes is regarded as being capable of purifying the sewage of 600 persons. The late Dr Wallace has calculated that, in order to treat the sewage of Glasgow, over twelve square miles of land would be required. Of course, if the sewage is subjected to previous treatment, which is often the case, by the method immediately about to be described—namely, precipitation—the amount of sewage the soil is capable of purifying will be correspondingly increased. A difficulty which may also be pointed out in connection with irrigation as a means of disposing of sewage, is the impossibility of carrying it on during frosty weather, when the land is frost-bound. In warm climates irrigation has much to recommend it as a means of sewage disposal. In damp and cold climates, on the other hand, there are many objections.

Treatment of Sewage by Precipitation, &c.

We now come to consider the methods grouped under this second heading. Mechanical filtration, of course, only aims at purifying sewage to the extent of removing all insoluble suspended matter which it contains. Different substances have been used as filters, the most generally used being charcoal. Charcoal mixed with burnt clay, gravel, sand, &c., has also been used.

In chemical precipitation, however, we have a method which claims to do more. Beyond the extracting of all solid matters in suspension, it removes (at any rate most chemical precipitants do) nearly all the phosphoric acid, which, next to the ammonia, is the most valuable constituent the sewage contains. Of all precipitants, lime has been the most universally used; and on the whole, it is perhaps the best, for it is both cheap and obtainable almost anywhere. According to an analysis by the late Professor Way, the difference in the percentages of phosphoric acid, potash, and ammonia, before and after treatment with lime, in a sample of sewage, was as follows:—

Grains per Gallon.

Before. After. Phosphoric acid 2.63 .45 Potash 3.66 3.80 Ammonia 7.48 7.50

From the above we see that while sludge caused by lime as a precipitant contains nearly all the phosphoric acid, there is not a trace of the potash or ammonia removed. Sulphate of alumina has also been used, both alone and in conjunction with lime. The advantage claimed by it over lime is, that the resulting precipitate is much less bulky. In other respects, however, it does not seem to be any more efficient as a precipitant. In the well-known A, B, C process, a mixture of alum, clay, lime, charcoal, blood, and alkaline salts, in different proportions, has been used. This mixture is said to extract, in addition to the phosphoric acid, a certain proportion of the ammonia; but the amount is so small as scarcely to be worth considering.

Numerous other chemical substances have been used, alone and also in conjunction with one another, such as perchloride of iron, copperas, manganese, &c. All alike, however, have failed to do more than effect partial purification,—the best results, it may be added, being obtained when the sewage thus treated was fresh. With regard to the manurial value of the resulting sludges, much difference of opinion has existed. The small percentage of phosphoric acid and nitrogen they contain has prevented them from being used to any extent as a manure, as their value did not admit of carriage beyond the distance of a few miles. By the introduction a few years ago of the filter-press, their value has been considerably enhanced. The old method of dealing with the sludge at precipitation-works was to allow it to dry gradually by exposure to the atmosphere. The result, however, of leaving sewage-sludge with over 90 per cent of water in it to dry in the air, was to encourage the rapid decomposition and putrefaction of its organic matter, so that in many cases the decomposing sludge proved to be as great a nuisance as the unpurified sewage itself would have been. By the use of Johnson's filter-press, however, a sludge containing 90 per cent of water was at once reduced to 50 per cent or even less. By this means the percentage of its valuable constituents was very much increased, and the sludge-cake, besides being much more portable, was neither so objectionable nor so liable to decomposition as before.

Value of Sewage-sludge.

As to the value of this sludge-cake as a manure, we are happily in possession of some very interesting and valuable experiments by Professor Munro of Downton Agricultural College. The sludge experimented upon was that produced by sulphate of alumina, lime, and sulphate of iron, and contained, after being subjected to Johnson's filter-press, from .6 to .9 per cent of nitrogen, and over 1 per cent of phosphoric acid. It was found that the benefit resulting from the application of the sludge was far from what in theory might have been expected. The experiments were made with turnips; and the results obtained with superphosphate and farmyard manure respectively, in the same field and under exactly the same conditions, were contrasted with those obtained with sludge. Thus it was found that 53 lb. of phosphoric acid as superphosphate, or 60 lb. as farmyard manure, produced a considerably larger crop than 240 lb. of phosphoric acid in the sludge. That is to say, that the phosphoric acid in the sludge did not exert more than one-fifth of its theoretical effect. The explanation of this somewhat strange result Dr Munro finds in the unsuitable physical character of the sludge-cakes. In farmyard manure we have a loose texture and a large amount of soluble constituents when well rotted. It thus quickly distributes its fertilising elements throughout the soil. In the case of the sludge, on the other hand, its composing particles are closely compacted together, and thus offer the greatest resistance to mechanical and chemical disintegration. "As a matter of fact," says Dr Munro, "the sludge-plots in my experimental series were all readily identified, when the roots were pulled, by the presence of unbroken and undecomposed clods of cake, which had evidently given up, at most, a small portion of their valuable ingredients to the soil."

Briefly stated, therefore, the objections to chemical precipitation as a means of dealing with sewage are these—viz., that while it relieves sewage of all its organic matter, and to a large extent of its phosphoric acid, it fails to extract any ammonia, which is thus lost; that the resulting sludge is consequently so poor in fertilising matters as scarcely to make it worth while to remove it any distance for manuring purposes; and that, further, owing to its unfavourable physical character, as at present made, even the small percentage of plant-food it contains is not realisable, within, at any rate, anything like a reasonable time, to its full theoretical extent.

The most profitable method of treating sewage must be determined by various local conditions; and it must be clearly understood that the question of sewage disposal is primarily a sanitary one, and that it must be dealt with from the sanitary aspect. The most profitable way of applying sewage as a manure, however, will doubtless be found by combining chemical precipitation and land irrigation.



The adoption of irrigation as a means of utilising sewage, suggests a short consideration of the value of liquid manures. It has been a custom on many farms to apply the liquid manure got from the oozings of manure-heaps, the drainings of the farmyard, byres, stables, piggeries, &c., directly to the soil. Indeed, so strongly has the belief in the superiority of liquid manure over other manure been held by certain farmers, that they have washed the solid animal excreta with water, in order to extract from it its soluble fertilising constituents. The late Mr Mechi was one of the foremost exponents of the value of liquid manure. His farm of Tiptree Hall was fitted up with iron pipes for the distribution of the manure over the different fields. Superphosphate, it may also be added, as first made from bones by Baron Liebig, was applied in a liquid form. As to the general merits of liquid manure, there can be no doubt that it is the most valuable form in which to apply manure. It secures for the manurial ingredients it contains a speedy and uniform diffusion in the soil; but, on the other hand, the expense of distributing it makes its application far from economical. The chief ingredient in liquid manure is urine. Now the removal of urine from the farmyard manure-heap entails a severe loss of the ingredient which is most potent in promoting fermentation. Separation of the urine from the solid excreta is on this very account not to be recommended. Urine, when applied alone, is lacking in phosphoric acid, of which it contains mere traces. It is not, therefore, suitable as a general manure. It has to be pointed out, however, that the drainings from a manure-heap in this respect are superior to pure urine, since they contain the soluble phosphates washed out of the solid excreta. The objections against using liquid manure may be summed up as follows:—

First, it is too bulky a form in which to apply the manure, and hence too expensive; secondly, it is not advisable to deprive the solid excreta of the liquid excreta, as the one supplements the other; thirdly, fermentation is largely fostered in the solid excreta by the presence of the liquid excreta—hence fermentation will not take place properly in the solid excreta when deprived of the liquid excreta.

If, however, the production of liquid manure on the farm is in excess of what can be used for the proper fermentation of farmyard manure, it will be best to utilise it for composts. No better addition to a compost can be made than liquid manure, as it induces speedy fermentation in nearly all kinds of organic matter.



The use of composts is an old one. Before artificial manures were so plentiful as they are at present, much attention was paid by farmers to their preparation. A compost is generally made by mixing some substance of animal origin which is rich in manurial ingredients with peat or loam, and often along with lime, alkali salts, common salt, and indeed any sort of refuse which may be regarded as possessing a manurial value. Composting, in short, may be looked upon as a useful method of turning to profitable use refuse of various kinds which accumulate on the farm. The object of composting is to promote fermentation of the materials forming the compost, and to convert the manurial ingredients they contain into an available condition for plant needs. Composts often serve a useful purpose in retaining valuable volatile manurial ingredients, such as ammonia, formed in easily fermentable substances like urine. In fact, we may say that farmyard manure is the typical compost, and its manufacture serves to illustrate the principles of composting.

Farmyard Manure a typical Compost.

Farmyard manure as ordinarily made is not generally regarded as a compost, but in the past it has been widely used for the purpose of making composts. Thus the practice of mixing farmyard manure with large quantities of peat has been in some parts of the world a common one. Peat, as has already been pointed out in a previous chapter, is comparatively rich in nitrogen. When it is mixed with urine or some other putrescible substance, the peat undergoes fermentation, with the result that its nitrogen is to a greater or less extent converted into ammonia. The effect, therefore, of mixing peat with farmyard manure is beneficial to both substances mixed: the escape of ammonia is rendered impossible by the fixing properties of the peat, while the inert nitrogen of the peat is largely converted by fermentation into an available form. The proportion of peat which it is advisable to add in composting farmyard manure will depend on the richness of the quality of the manure: the richer the quality of the manure, the greater the amount of peat it will be able to ferment. Composts of this kind are generally made by piling up the manure in heaps, consisting of alternate layers of peat and farmyard manure. From one to five parts of peat to every one part of farmyard manure is a common proportion. The use of such a manure, containing so much organic matter, will exercise its best effect on light sandy soils.

Other Composts.

But instead of farmyard manure, or in addition to farmyard manure, various other substances may be added, as bones, flesh, fish-scrap, and the offal of slaughter-houses. Sometimes leaves and the dried bracken-fern are used for the manufacture of composts. Some of these substances contain much nitrogen or phosphoric acid, but in their natural condition ferment when applied to the soil at a slow rate. If mixed together before application in pits with peat, leaves, bracken-fern, or some other absorbent material, fermentation proceeds evenly and rapidly. The addition of lime, potash, and soda salts has been found to have a most beneficial effect in promoting fermentation. These substances, as is well known, hasten putrefaction of organic matter. Lime seems especially to be valuable in composting. This is no doubt due to the fact that lime plays a valuable part in promoting the action of various ferments, as has already been illustrated in the case of nitrification. The effect of large quantities of sour organic acids (humic and ulmic), which are the invariable products of the decomposition of organic matter like peat, leaves, &c., is inimical to micro-organic life. The action of lime is to neutralise these acids. There can be no doubt that composting is a useful process for increasing the fertilising properties of different more or less inert manurial substances. But in view of the abundant supply of concentrated fertilisers, the use of composts may considerably decrease in future.




We now come to discuss those manures which we may class under the term Indirect, because their value is due, not to their direct action as suppliers of plant-food—like those manures we have hitherto been engaged in discussing—but to their indirect action. Of these by far the most important is lime.

Antiquity of Lime as a Manure.

Lime is one of the oldest and one of the most popular of all manures. It is mentioned, and its wonderful action commented on, in the works of several ancient writers, more especially Pliny. Of late years, perhaps, its use has become restricted; and, as we shall point out by-and-by, it is well that it is so.

Action of Lime not thoroughly understood.

Despite the fact of the long-established and almost universal use of lime, it can scarcely be said that we as yet clearly understand the exact nature of its action. Much light, however, has been thrown of late years on the subject by the great advance which has been made in our knowledge of agricultural chemistry. Nevertheless, there are many points connected with the action of lime on the soil which are still obscure. Perhaps one reason for the conflicting ideas prevalent with regard to the value of this substance in agriculture is to be found in the fact that it acts in such a number of different ways, and that the nature of the changes it gives rise to in the soil is most complicated. The experience of agriculturists with lime in one part of the country often seems contradictory to the experience of those in other parts of the country. Its action on different soils is very dissimilar. For these reasons, therefore, the discussion of the value of lime as a manure is by no means an easy one.

Lime a necessary Plant-food.

Lime, as we have already pointed out in a former chapter, is a necessary plant-food, and were it present in the soil to a less extent than is actually the case, would be just as valuable a manure as the different nitrogenous and phosphatic manures; and in certain circumstances this is the case. There are soils, though they are by no means of common occurrence, which actually lack sufficient lime for supporting plant-growth, and to which its addition directly promotes the growth of the crop. Poor sandy soils are often of this nature. Another class of soils are also apt to be lacking in lime—at any rate their surface-soil is. These are permanent pasture-soils. Originally there may have been an abundance of lime in the surface portion of the soil; but, as is well known to every practical farmer, lime has a tendency to sink down in the soil. This tendency in ordinary arable soils is largely counteracted by ordinary tillage operations, such as ploughing, &c., by means of which the lime is again brought to the surface. In permanent pasture-soils, however, no such counteracting action takes place, hence impoverishment of the surface-soil in lime eventually results. It is for this reason—partly at any rate—that permanent pasture benefits in an especial degree by the application of lime. We say partly, for there are other important reasons. One is, that lime seems to have a striking effect in improving the quality of pastures by inducing the finer grasses to predominate. It has also a very favourable action in promoting the growth of white clover. Another reason for the favourable effect of lime on pasture-soils is doubtless on account of the action it has in setting potash free from its compounds. Soils, however, which directly benefit from the application of lime in the same way as they benefit from the application of nitrogenous manures, may be safely said to be rare. In the great majority of soils lime exists, so far as the demands of plant-life are concerned, in superabundance.

Lime of abundant Occurrence.

Indeed limestone is one of the most abundant of all rock substances, and it has been calculated that it forms not less than one-sixth of the rock-mass of the earth's crust. Nearly all the commonly occurring minerals contain it, and in the course of their disintegration furnish it to the soil. Vast tracts of country are composed of nothing but limestone; and we have examples, even in this country, of so-called chalk-soils, where it is the most abundant constituent. Nor can it be classed amongst the insoluble mineral constituents of the soil; for although insoluble in pure water, it is soluble in water—such as the soil-water—which contains carbonic acid. This is proved by the fact that it is the chief dissolved mineral ingredient in all natural waters.

Lime returned to the Soil in ordinary Agricultural Practice.

It may be further pointed out, as bearing upon the true function of lime when applied as a manure, that in ordinary agricultural practice nearly all the lime removed from the soil in crops finds its way back again to the farm in the straw of the farmyard manure. For these reasons, then, it is clear that the true function of lime is as an indirect manure.

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