At their first introduction, bones were applied in large fragments, and in quantities of from 20 to 30 cwt., or even more, per acre, but as their use became more general they were gradually employed in smaller pieces, until at last they were reduced to dust, and it was found that, in a fine state of division, a few hundredweights produced as great an effect as the larger quantity of the unground bones. Even the most complete grinding which can be attained, however, leaves the bones in a much less minute state of division than guano, and they necessarily act more slowly than it does, the more especially as they contain no ready-formed ammonia. They may be still further reduced by fermentation, which acts by decomposing the organic matter, and causing the production of ammonia; but not as is frequently, though erroneously supposed, by converting the phosphates into a soluble condition, for this does not occur to any extent, and their more rapid action is solely due to the partial decomposition of the organic matter, by which it is brought into a condition capable of undergoing a more rapid change in the soil. The rapidity of action of bones is still more promoted by solution in sulphuric acid, by which they are converted into the form of dissolved bones or superphosphate. At the present moment, however, very little of the superphosphates sold in the market are made exclusively from bones in their natural state, by far the larger portion being manufactured from mineral phosphates, or from bones after destruction of their organic matter, sometimes with the addition of small quantities of unburnt bones, but more frequently of sulphate of ammonia, to yield the requisite quantity of ammonia. These substances may therefore be best considered under the head of mineral manures.
CHAPTER XI.
COMPOSITION AND PROPERTIES OF MINERAL MANURES.
Mineral manure is a term which is now used with great laxity. In its strict sense, it means manures which contain only, and owe their exclusive value to the presence of, those substances which go to make up the inorganic part or ash of plants. It has, however, been usually taken to include all saline matters, and especially the compounds of ammonia and nitric acid, which are indebted for their manurial effects to the nitrogen they contain; and thus is so far incorrect. It would, however, be manifestly impossible to arrange these compounds with any degree of accuracy among either animal or vegetable manures, and hence the necessity of including them amongst those which are strictly mineral. The most important practical distinction between them and the substances discussed in the two preceding chapters is, that the latter generally contain the whole or the greater part of the constituents of plants. Even bones yield a certain quantity of alkalies, magnesia, sulphuric acid, and chlorine, and may in some sense be considered as a general manure. But those to which the term mineral manure is applied for the most part contain only one or two of the essential elements of plants, and hence cannot be applied as substitutes for the substances already discussed, although they are frequently most important additions to them.
Sulphate and Muriate of Ammonia.—These and other salts of ammonia have been tried experimentally as manures, and it has been ascertained that they may all be used with equal success; but as the sulphate is by much cheaper, it is that which probably will always be employed to the exclusion of every other. It contains, when pure, 25·7 per cent ammonia.
It is now manufactured of excellent quality for agricultural use, and when good, contains from 95 to 97 per cent of actual sulphate, the remainder consisting chiefly of moisture and a small quantity of fixed residue; but specimens are occasionally met with containing as much as 10 per cent of impurities, which, as its price is high, makes a material difference in its value. Inferior descriptions are also occasionally sold, among which is a variety distinguished by containing a large quantity of water and fixed salts, although it appears to the eye a good article. Its composition is—
| I. | II. | |
| Water | 9·05 | 5·77 |
| Sulphate of ammonia | 79·63 | 85·21 |
| Fixed salts | 11·17 | 9·02 |
| ——— | ——— | |
| 100·00 | 100·00 | |
| Ammonia | 20·55 | 21·94 |
An article called sulphomuriate of ammonia is also sold for agricultural use. It is obtained as a refuse product in the manufacture of magnesia, and is a mixture of sulphate and muriate of ammonia, with various alkaline salts. It differs somewhat in quality, and is sold by analysis at a price dependent on the ammonia it contains.