Bone Manures are applied to crops on account of the nitrogen—equivalent to 4 or 5 per cent. of ammonia—and the phosphate of lime which they contain—the latter amounting to nearly half their weight. Their value has only recently been discovered. The first crude bone-crushing machines were used in England in 1814. In Scotland, when bones began to be employed as manure, about 1825, they were roughly broken into large splinters with hammers by the farm-servants during wet weather, and 30 bushels applied per acre, at a cost of half-a-crown a bushel. Five hundred weight per acre is now considered a good dressing. Bones are not applied in good practice in a larger size than that of meal or flour—in other words, ground into a fine state of division.—Rough Bones, as quarter-inch size and larger, are rarely used, as these lock up capital in the soil by taking too much time to decay, and do not give a remunerative return in comparison with manures that act quickly. Bones are sometimes fermented to induce decay. They are piled in heaps, and water or urine poured over them. When the heat resulting from the fermentation goes down, they should be turned and watered again, and this kept up for a few months. Bones in an altered form are also used in making up concentrated manure, and are either ground or acted upon by sulphuric acid—for example:
(1) Steamed Bone, or degelatinised bone, results when bone is made friable and more easy of assimilation by plants through the action of superheated steam (at 270°—280° F.) removing the fatty and gelatinous matter; but there is a loss of nitrogen in the process, as there is also in the preparation of ash and char.
(2) Bone-ash is the residue from bones burnt as fuel used in the 'rendering' of the fat of cattle in South America.
(3) Bone-char is made by heating in a close retort, and used largely for refining sugar before it goes for manure.
(4) Vitriolised Bone is a material only partially acted upon by acid—the result of a most wasteful practice deserving of condemnation, as destroying microbes which bring about nitrification in the soil.
(5) Dissolved Bone is bone broken into half-inch size, and then acted upon by sulphuric acid, so as to change a portion (about half) of the insoluble tribasic phosphate of lime into the soluble monobasic phosphate and the sulphate of lime; thus, . Sulphuric acid is peculiarly suitable for this purpose as compared with say hydrochloric acid, as by the formation of sulphate of lime—or in common language, plaster of Paris—a part of the moisture is absorbed, and the resulting material is left in a dry condition, which is essential for its proper distribution on the land. The object—extraordinary minuteness of division—is obtained, although the soluble condition disappears soon after its application to the soil. The dissolving of bones is now held to be wasteful because soluble phosphate of lime is procurable from many other cheaper sources. It is also unsatisfactory on account of the acid destroying the microbes of nitrification, so useful to the soil, which exist in large quantities in bones and all sorts of decaying animal matter. Liebig was the originator of the method in 1839, and during the next few years it was taken up by leading farmers in England and carried out as a farm operation. Now the work has fallen into the hands of manufacturers who can make it cheaper and, if they choose, better. Much adulteration has been practised in this trade at the expense of farmers. This was made possible owing to their ignorance and prejudices against learning anything scientific. Much manure sold as 'dissolved bone' is merely a mixture of soluble phosphate and some such nitrogenous material as nitrate of soda and sulphate of ammonia—the valuable substances being in the proportions present in bone. Soluble phosphate of lime, from whatever source, has a more beneficial influence on the turnip-crop than upon any other crop, because the turnip roots are not so able as the roots of other plants, such as wheat, to make use of the natural supply of phosphates existing in the soil in the insoluble form, even when this is abundant. Should the stock of phosphoric acid in the soil be reduced too far, the crop produce is reduced accordingly; it is necessary, therefore, to apply at least sufficient of this ingredient to make up for the loss resulting from the removal of the sale produce of a farm. The famous example of the deterioration of the Cheshire pastures on the rich soils of the new red sandstone by the yearly drain of phosphates through the sale of dairy produce, is now a matter of history. It is recorded that by the application of bones to these soils the value was doubled. Bone meal or flour is the most valuable form for application to thin, light, or hungry soils, as it decays in time to feed the growing crop, and is not liable to be washed out. In heavy land, ground bones are of little or no value—they get imbedded in the clay, and are preserved so that they cannot yield their substance to act as manure. On such soils the phosphate of lime should be used in the soluble form. Many thousands of tons of bones are annually imported from South America, whence we draw our largest supply. India contributes the next largest amount, the Hindus being prejudiced against the use of bone as manure.