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PRIZE ESSAYS AND COMMUNICATIONS.
THE AGRICULTURAL VALUE OF PHOSPHATE OF LIME.
BY JOSEPH HARRIS, ROCHESTER, N. Y. Phosphorus, an element never found in nature in an uncombined state, but usually manufactured from bones; is semi-transparent, of nearly the consistence of wax; internally of a reddish or flesh color, but coated with a white film, arising from its partial decomposition. It so strongly attracts oxygen from the air as to become slowly decomposed at a very low temperature; and hence it is usually kept under water. When ignited it gives off dense white fumes. This is phosphoric acid, and it is in this form united with lime, iron, lead, copper, and other bases, that it is usually found. It enters into the composition of all plants and animals, and is found in all soils not absolutely sterile. The inorganic parts of the bones of all animals are composed principally of this acid, united with lime, called phosphate of lime, and wheat, indian corn, timothy, and other cereal grains, contain from forty to fifty per cent. in their ashes. It will easily be seen that it is a substance of vast importance in an agricultural point of view; and when it is understood that no plant will grow on a soil destitute of it-that no soil, however fertile, contains but a comparatively small amount, and that in the bones and flesh of animals, and in the grains of wheat, indian corn, and timothy, it is annually exported from the soil in large quantities, never to return, its real value and importance will be clearly perceived.
The whole of the phosphate of lime which exists in the bones of man and animals, must have been originally in the soil, and being taken up by plants, was, by them conveyed into the animal organism. Under the system of agriculture commonly adopted, phosphate of lime is continually being abstracted from the soil, and unless an equivalent portion to that which is taken is liberated from the soil by annual decomposition, or is returned by the purchase of manure containing it, a gradual diminution of fertility must follow. Those engaged in the pursuit of Agriculture, cannot but derive some benefit, from a knowledge of the chemical properties and agricultural uses of phosphate of lime.
As is usually the case, practice got the start of science, in the use of bones as a manure, and without knowing why, the farmer found them of great value to his soil. It is said, they were first used on the natural grass dairy meadows of Cheshire, England, where froni the large amount annually taken from the soil, in cheese, and the bones and flesh of animals, the soil had become exhausted of phosphorio acid. Their effect was surprisingly beneficial. They were used with profit, at a cost of from $20 to $60 per acre. Few turnips were then grown, but the value of this crop being acknowledged, great efforts were made to extend its culture, the land owners in many cases compelling the tenant farmer to grow a certain number of acres. Bones were found to be the best manure that could be applied to this crop, their beneficial effect lasting for many years. I have seen a crop of turnips three times as large, on a portion of a field where bones had been applied 12 years previous, as on another part of the same field, where none had been used, though treated similarly in every other respect. It is often said that soil dressed with bones, never forgot it when turnips were sown; but this opinion is formed from the lasting effects of roughly broken bones, as first used, at the rate of one hundred bushels per acre; it was soon found, that if they were finely ground, a much smaller quantity would suffice, though the benefit would not be as lasting. The reason of this, is found in the fact, that plants cannot take anything from the soil, but in a state of solution. Now bones in their | Ag. Trans. 1852.]
natural state, are insoluble in water, and have first to decompose and be incorporated with the soil, before they can nourish plants. But if finely ground they are easily intimately mixed with the soil; where being attacked by its acids, they readily decomposé, and are more speedily rendered in a fit state for assimilation by the plants. Twenty bushels of dust, will in this way be more beneficial, apparently, than 100 bushels of merely crushed bones.
The reason why boiled bones are supposed to be better than unboiled, is the same. They have absorbed considerable water, and are in consequence, more speedily decomposed in the soil, and taken up by the plants. Common experience proved the value of bones as a manure, but science had to explain the cause of their beneficial action. This science not only did, but having done so, discovered a plan to decompose these bones before they were ap plied to the soil, thus concentrating the effect into one, which would have extended over twenty years, or producing a better effect for one year, with a twentieth part of the bones.
There are several phosphoric acid and lime compounds known to chemists, but the only two that concern us at present, are the neutral phosphate of lime, often called the bone' earth phosphate, because it forms the chief earthy ingredient of bones; and the biphosphate, or as it is called in commerce; superphosphate or acid phosphate of lime.
Pure neutral phosphate of lime contains in 100 lbs.:'
The bi-phosphate, or acid phosphate of lime, is composed of, Phosphoric acid,
The neutral phosphate of lime is all but insoluble in water, but the bi-phosphate is readily soluble. Now, what science has discovered is, the method of cheaply converting the insoluble phosphate into the soluble bi-phosphate, and simple as it may seem, this discovery has benefited agriculture more than all others combined.
Phosphate of lime consists of two atoms of phosphoric acid and two of lime; and bi-phosphate has three atoms of phosphoric acid combined with one of lime; now to convert the one into the other, we have either to add phosphoric acid or take away lime; the latter is the only way, practical in a manufactory, though the former can be done in the laboratory. Phosphorie being what chemists term a weak acid, sulphuric acid will take away its lime, setting the phosphoric acid free; if but a small quantity of acid is applied it will unite with a portion of the lime, forming sulphate of lime, and setting free its phosphoric acid which will unite with the remaining phosphate of lime, forming the required soluble bi-phosphate. If sufficient acid is not added to convert the whole into bi-phosphate, a portion of the neutral phosphate will remain untouched. If more than is required to convert the whole into bi-phosphate is applied, a portion of the phosphoric acid will be liberated and remain uncombined, and by adding sufficient acid the whole may be set free, and the result of the mixture would be sulphate of lime and free phosphoric acid.
To exemplify this, let us suppose 100 lbs. of the pure phosphate of lime taken, this would contain 483 lbs. phosphoric acid ; now then if 712 lbs. of phosphoric acid make 100 of bi-phosphate of lime, 484 will make 68. Therefore the 100 lbs. of pure phosphate of lime can be converted into 68 lbs. of bi-phosphate of lime, and this is accomplished by abstracting 32 lbs. of lime. The quantity of pure sulphuric acid required to effect this is easily calculated. Sulphate of lime or plaster is composed of: Lime,
411 Sulph. acid,
So that 32 lbs. of lime would require 45 lbs. sulphuric acid to convert it into sulphate of lime. Therefore to convert 100 lbs. of neutral phosphate of lime into bi-phosplıate, would require 45 lbs. of pure sulphuric acid. If we take away the 32 lbs. of lime, we should have as the result 68lbs. bi-phosphate and 77 lbs. of sulphate of lime.
The composition of bones varies considerably according to the age and kind of animal-less phosphate of lime and more gelatine being found in a young, than in an older animal. But on an average, 100 parts of raw bones may be estimated as containing: Water,....
11 lbs. Phosphate of lime,.
It will be seen that besides the phosphate of lime, hones contain 38 per cent. of organic matter, so that in judging of the results of experiments with fresh bones it is necessary to take this into account and not attribute all the benefit to the phosphate. To avoid any discrepancies of this kind we will present the reader with a few results selected from experiments made at Rothamsted Farm, Herts, England, by Mr. Lawes and Dr. Gilbert, in which calcined bones were used containing about 90 per cent. of phosphate of lime