APPENDIX TO CHAPTER XIII.
NOTE I. (p. 388).
The formulæ, and molecular and percentage composition, of the different phosphates, are given in the following table:—
| Composition in terms of— | ||||||||
| Molecular weight. | Per cent. | |||||||
| Name. | Symbol. | Lime. | Water. | Phosphoric acid. | Total. | Lime. | Water. | Phosphoric acid. |
| Tri- or bone-phosphate. | 3CaO, P2O5 | 168 | 0 | 142 | 310 | 54.19 | 0.00 | 45.81 |
| Bi- or di-phosphate. | 2CaO, H2O5 | 112 | 18 | 142 | 272 | 41.18 | 6.61 | 52.21 |
| Mono- or super-phosphate. | CaO, 2H2O, P2O5 | 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, P2O5 | + | 2(H2O, SO3) |
| (Tricalcic phosphate), | (Sulphuric acid), | ||
| = | 2(CaO, SO3) | + | CaO, 2H2O, P2O5 |
| (Gypsum) | (Monocalcic phosphate). | ||
(2.) 3CaO, P2O5 + 3(H2O, SO3) = 3CaO, SO3 + 3H2O, P2O5, or 2H3PO4.
This equation gives the chemical reaction taking place when soluble phosphate is reverted, owing to the presence of undissolved phosphate:——
| 3CaO, P2O5 | + | CaO,2H2O, P2O5 | |
| (Tricalcic phosphate), | Monocalcic phosphate, | ||
| = | 2CaO, H2O, P2O5 | + | 2CaO, H2O, P2O5 |
| (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:——
Fe2O3, 3SO3 + CaO, 2H2O, P2O5 = Fe2O3, P2O5 + CaO, SO3 + 2(H2O, SO3),
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 Fe2O3, P2O5, and diminishing in the same proportion the soluble P2O5. Again, we may have simply——
2Fe2O3 + 3(CaO, 2H2O, P2O5) = 2(Fe2O3, P2O5) + 3CaO, P2O5;
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, SO3) + 2O + CaO, 2H2O, P2O5 + 3CaO, P2O5 = 2(Fe2O3, P2O5) + 4(CaO, SO3).
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 |
CHAPTER XIV.
THOMAS-PHOSPHATE OR BASIC SLAG.
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."
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:—
"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 fineness | 43 |
| No. 3. fineness | 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 fineness | 29 |
| No. 3 fineness | 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.