According to Payen and Boussingault, raw bones contain 6-1/4 per cent of nitrogen and 8 per cent of water. Pure bones are thus seen to contain about 29 per cent of phosphoric acid and 6-1/4 per cent of nitrogen. The composition of the commercial article, however, differs very widely. This is due to the fact that bones collected from India and America, where they have been long exposed to atmospheric influences, have lost much of their organic matter. The amount of sand and earthy impurities also varies very considerably.
Bones are used for the manufacture of glue and gelatine. These are extracted from them by steaming the bones. The bones after treatment are used as a manure. The improvement noted in the action of the bones thus treated led to the introduction of the use of steamed bones as a manure. Raw bones are now rarely used. The fat present in raw bones retards their decomposition in the soil. Probably, as has been suggested, it forms along with lime an insoluble soap which prevents the mineral matter in the bone being dissolved by the carbonic acid of the soil. In the process of boiling or steaming a certain loss of nitrogen takes place, greater or less, according to the length of time they are boiled or steamed, and in the latter case the pressure applied. A more economical method for extracting the fat has been introduced by using benzine, but this process is not used to any extent. The loss of nitrogen in the former case is more than compensated for by their more speedy action as a manure when applied to the soil. Bone-meal of good quality contains from 45 to 55[217] per cent of phosphate of lime, and 3-1/2 per cent of nitrogen. Our present total consumption of bones is probably little less than 100,000 tons per annum, of which about half is obtained from home collections, over 20,000 tons being annually collected in and around London alone.
It is well known that bones are a slow-acting manure. They may be said to possess both a mechanical and chemical action when applied to the soil. When they putrefy, their nitrogen is slowly converted into ammonia, and carbonic acid as well as various organic acids are formed, which, acting upon the insoluble mineral matter in the bones, renders it available for plant uses. Bones thus, when applied in large quantities, may not merely act directly as suppliers of plant-food, but in the course of their putrefaction may act upon a certain amount of the inert fertilising matter of the soil and render it available. The more readily, then, bones putrefy, the more speedy will be their effect. As we have already pointed out, bones, in order to increase their efficiency, are often fermented before application. The removal of the fat is another means of increasing the rate of their action, but the fineness to which they are ground determines this more than anything else. Much ingenuity has been expended in perfecting machinery for grinding bones. At one time in Germany they were pounded in stamps similar to those used for ore. In America what has been called "floated bone" has been prepared. This bone is so fine that it actually floats in the air like flour-dust, and is made by whirling the bones against one another. The action of bones prepared in this way is of course very speedy, but the difficulty of applying a manure in such a fine state of division to the soil is great. The expense of the process also is considerable.
The ease with which bones when ground into a fine state of division putrefy, is evidenced by the fact that bone-flour has to be salted in order to enable it to keep. Another condition which determines the rate at which the fertilising matters in bones become available is the nature of the soil. Fermentation, as we have already seen, requires a plentiful supply of air, and a certain amount, but not too much, of moisture. Consequently bones act best in medium soils—soils which are "neither too light and dry, nor too close and wet." There can be no doubt that what gives to bones a peculiar value in the eyes of the farmer is the fact that they form a manure of a lasting character. They give what has been termed backbone to a soil. But the tendency of modern agricultural practice is to use quick-acting manures rather than slow. This has been admirably put by Professor Storer in the following words: "The old notion, that those manures are best which make themselves felt through a long series of years, is now recognised to be an error. The adage, that 'one cannot eat the cake and have the cake' is conspicuously true in agriculture; and just as it is the part of prudence in household or maritime economy to abstain from laying in at any one time more provisions than can be properly disposed of in a year or during a voyage, so should the farmer refrain from bringing to the land an unnecessary excess of plant-food. Such food is liable to spoil withal in the soil, as well as other kinds of provisions that are kept too long in store. A just proportion of food, properly prepared, is the point to be aimed at always."
In view, therefore, of what has just been said, it might seem best to use bones in the form in which they are most speedily available—viz., as dissolved bones. This would be so if bones were the only source we possessed for the manufacture of superphosphate of lime; but we now have, in the various mineral phosphates, abundant and cheaper sources of this valuable manure. The opinion of leading agriculturists and agricultural chemists is rather in favour of applying bones in the undissolved condition. For one thing, it seems far from economical to utilise an expensive material such as bones for manufacturing an article which can be equally well manufactured from cheaper materials; for once the phosphate of lime is dissolved, it is equally valuable from whatever source it may be derived. Of course this is not tantamount to saying that dissolved bones as a manure are no more valuable than superphosphate. In dissolved bones we have, in addition to soluble phosphate, a considerable proportion of undissolved bone-tissue, containing a certain quantity of nitrogen and organic matter; but so far as the soluble phosphate is concerned, it seems only rational to conclude that its efficacy is equally great, whether it be derived from bone or mineral phosphate. Another reason is, that much of the characteristic action of bones is lost by treating them with sulphuric acid. As Dr Aitken has pointed out, the germ life in the soil and in the bones gradually converts them into a form available for the nourishment of plants; but to dissolve bones with sulphuric acid is to kill out the germ life and retard the decay of any nucleus of bone in the dissolved manure.
Dissolved Bones.
Dissolved bones, however, are still manufactured. Formerly the manure called dissolved bones was often a mixture of mineral superphosphate along with undissolved bone-meal, but recent legislation has stopped the continuance of this practice. The composition of dissolved bones varies somewhat, the percentage of soluble phosphate being about 20 to 23 per cent, the insoluble amounting to from 9 to 10 per cent, and the nitrogen from 2-1/2 to 3-1/2 per cent.[218] Another reason against dissolving bones is to be found in the difficulty experienced in dissolving their phosphate. Bones, especially when raw, are not easily acted upon by acids.
Crops suited for Bones.