Phosphates of Lime
Chemical substances which contain the elements calcium and phosphorus, both of which are found to be present in all plants. The ash which remains when the plants are burned invariably contains them in greater or less amount. Experiments on plants which are supplied with food of definite and strictly-ascertained composition, have proved that when either of these elements is with-held, the plants remain small and stunted, though the other elements required in the food are all supplied. On adding the missing element to the food, growth begins, and goes on in a healthy way. But, though this has been ascertained, there is still much uncertainty as to the way in which each of these elements exerts its beneficial action on plants. It is believed that calcium is of use both in enabling plants to obtain the sulphur and phosphorus that they require more readily in combination with it (in sulphate of lime and Phosphates of Lime) than these elements could be otherwise supplied to them, and in combining with oxalic acid, which is largely formed in growing tissues of plants, but is hurtful to them in excess. The calcium unites with this to form oxalate of lime, in the shape of small crystals, called "raphides," which are very common in the cells of many plants--in some resembling bundles of minute needles; in others, in the form of cubes; in others, like balls, studded over with sharp points. Oxalate of lime is harmless to plants. Phosphorus is believed to be of value chiefly in helping to form protoplasm, and to transfer it from place to place, as needed for growth in the plants. Phosphates are employed chiefly to supply phosphorus, and they are drawn from the soil by the plants as they require it. There are several Phosphates that contain calcium. They differ from one another in the percentage of calcium that they contain, and also in their solubility; but, for a discussion of their composition and preparation, the reader is referred to works on chemistry, such as Miller's "Elements of Chemistry," or Johnston and Cameron's "Elements of Agricultural Chemistry and Geology." Soils generally contain a small quantity of the Phosphates of Lime, but not enough for the requirements of cultivated plants, as each crop removes some of the small supply, which, in time, becomes exhausted. Hence, it is necessary to add to the soil new supplies of Phosphates; and this is largely done by means of artificial manures. The chief sources of Phosphates are bones and coprolites, which latter are hard, grey or brown, rounded pebbles, abundant in certain localities in England. They were formerly believed to be the fossilised dung of animals (from the Greek word kopros, dung). They are probably the fragments of bones of great reptiles that lived in the period to which the rocks and strata (Greensand) in which they are found belong. There are also other less important sources of the mineral. The Phosphate contained in bones and in coprolites is not soluble in pure water, but is so in acetic acid and in the stronger acids. Experiments show that various substances insoluble in water are rendered soluble by contact with the roots of plants, probably by means of the excretion of an acid from them. Bone Phosphate is acted on in this way, and the effect is largely increased when the bones or minerals are in a finely-powdered state. Bone-dust, prepared by grinding bones, after they have been steamed, to remove the animal matter, and thus render them more easy to reduce to powder, and ground coprolites and mineral Phosphates, are now largely used as manures. In this form, the Phosphates have the advantage of remaining in the soil for a considerable time, until absorbed by the roots of plants; hence, their action is spread over more than one year. It is, however, frequently desired to allow plants to absorb them more rapidly than can be done where the roots have to render the Phosphates soluble. To permit of this, the Soluble Phosphate, or Superphosphate of Lime is made use of. It is prepared by subjecting bones, or a mixture of bone-ash and mineral Phosphates, to the action of sulphuric acid (oil of vitriol), in the proportion of about three parts of the Phosphates to two of acid. The commercial Superphosphate is a somewhat indefinite substance, composed of the soluble Phosphate of Lime in large proportion, along with small quantities of the insoluble Phosphates and a good deal of sulphate of lime. The soluble Phosphate readily dissolves in water, and plants obtain it in larger proportion the first season after it is put on the soil; but it is liable to be washed out of the soil by rain: hence, its effects are not lasting. It should be mixed with two or three times its bulk of ashes or mould before sprinkling it on the soil. It is asserted by Mr. Jamieson, as the result of his experiments, that Superphosphate of Lime favours the development of "Finger-and-Toe" (the work of Plasmodiophora Brassicae) in Turnips; but this is denied by others, who advocate its use.