In respect of the processes of analysis there are no especial directions to be given. The phosphoric acid, as given below, and the potash are to be determined by the usual methods, the total phosphoric acid and potash after the destruction of the organic matter.
In old cave deposits the processes of decay and nitrification seem to have long been completed and we have found very little power of inducing nitrification in culture solutions seeded from these samples.
292. French Official Method for Total Phosphoric Acid in Guanos.—To determine the phosphoric acid in guanos, the method officially adopted by the French agricultural chemists may be used.[247]
Two grams of the sample are rubbed up in a porcelain crucible with a decigram of slaked lime to prevent the possible reduction of the phosphoric acid by the organic matter. The mixture is slightly moistened with a few drops of water, dried on a sand-bath, and afterwards heated to redness, best in a muffle, until organic matter is destroyed. The contents of the crucible are detached and placed in a flask of 200 cubic centimeters capacity. The crucible is well digested twice with some hydrochloric acid to dissolve any adhering fragments, and finally washed with hot water, the acid and water being added to the flask. The contents of the flask are boiled for fifteen minutes and then poured into a flat-bottomed dish, the flask well rinsed three or four times with small quantities of water, and the liquor and washings are evaporated to dryness to render the silica insoluble. The residue is taken up by a mixture of ten cubic centimeters each of hydrochloric acid and water, heated for a few minutes and filtered, and the dish well washed with successive small portions of water, but the total volume of the filtrate and washings should not exceed eighty cubic centimeters. In this filtrate the phosphoric acid may be determined by any one of the approved methods.
293. Waste Leather.—This material belongs probably to that class of nitrogenous substances which has already been considered in [paragraph 149]. The chief manurial value of the waste is found in its nitrogenous content. The value of this for available plant food has been investigated by Lindsey.[248] A complete resumé of the literature of the subject is also given by him.
The best way of identifying leather waste is by the process proposed by Dabney.[249] It depends on the color produced in a solution of iron phosphate by the tannin compounds derived from the leather. The reagent is prepared by dissolving a freshly made precipitate of iron phosphate from ten grams of ferric chlorid in 400 cubic centimeters of an aqueous solution of forty grams of glacial phosphoric acid. A gentle heat promotes the solution of the phosphate.
In the case of a fertilizer supposed to contain leather, about one gram of the material is treated with thirty cubic centimeters of water and a few drops of sulfuric acid. The mixture is boiled and poured on a filter. To a portion of the filtrate some of the solution of iron phosphate is added, and the mixture made alkaline with ammonia. If leather be present in the sample, a purple or wine color will be developed. Lindsey could easily detect the leather when it was added in ten per cent quantities by the above method, and he regards this method as superior to the microscope which is unreliable in the case of finely ground material.
While leather, as such, decays slowly, and therefore is not at once available for the nourishment of plants it acquires greater utility after digestion in sulfuric acid. Artificial digestion experiments with leather previously treated with sulfuric acid show that, approximately, seventy per cent of the nitrogen pass into solution. Such a prepared leather has, therefore, a digestive coefficient in respect of nitrogen not much inferior to most organic bodies.
In comparative trials with sodium nitrate it was demonstrated that nitrogen in leather, previously dissolved in sulfuric acid, has a rank of about sixty when it is rated at one hundred in the soda salt.
For the estimation of the nitrogen in leather the moist combustion process is to be preferred.