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 |