Superphosphates as manufactured may be divided, generally speaking, into three classes--viz., low class, medium, and high class. The ordinary or medium class contains from 25 to 27 per cent of soluble phosphate; and here it may be pointed out that by soluble phosphate is meant the percentage of tribasic phosphate which has been dissolved--not, as might at first sight be supposed, the percentage of monocalcic phosphate. The lower-class superphosphates are those containing less than 25 per cent, generally 23 to 25 per cent, of soluble phosphate; while the high-class superphosphate may contain from 30 to 45 per cent. For the manufacture of high-class superphosphate only a certain number of raw phosphates are available, such as Curacao and Somme phosphates, phosphatic guanos, bone-char, &c. Certain processes have been patented for the manufacture of even more concentrated superphosphates, and by them phosphates containing as much as 40 per cent of soluble phosphoric acid--_i.e._, equal to 87 per cent of soluble phosphate--have been prepared. To this class belongs the so-called double superphosphate, manufactured at Wetzlar in Germany. Such a concentrated form of manure is naturally very expensive to manufacture, and is hardly to be recommended for home consumption. Where, however, manures have to be conveyed long distances, and the freight is consequently very high, such a concentrated article may be found most economical. _Action of Superphosphates._ When superphosphate is applied to the soil it is converted into an insoluble state. In short, the process of reversion is carried on on a wholesale scale. This is due to the lime, iron, and alumina salts which the soil contains. In all probability the phosphate is finally converted into a hydrated ferric or aluminic phosphate, in which form it is gradually acted upon by the sap of the plant-roots as required. This being the case, it may be asked, Why is superphosphate so much more rapid in its action than insoluble phosphate; or why should we be at the trouble and expense of dissolving the phosphate if it has to become insoluble again in the soil? This question is one of very great importance, for the answer to it furnishes, in our opinion, the key to the whole phosphate question. When superphosphate is added to the soil, being soluble in water, it is soon dissolved and carried down by the rain into its pores, and becomes thoroughly mixed with the soil-particles. It is thus soon fixed in the soil, beyond the risk of being washed away. The result is, that the phosphate is obtained in a state of division infinitely more minute than could ever be obtained by mechanical grinding, and is, further, most intimately mixed with the particles of the soil. It is this intimate mixture of the phosphate with the particles of the soil, and its minute state of division, that constitute the only reason for rendering superphosphate superior in its action to even the most finely ground insoluble phosphates.