Reverted Phosphoric On standing, a change sometimes occurs in superphosphates by which a part of the phosphoric acid becomes less easily soluble and to that extent the value of the fertilizer is decreased. This change, known as is much more likely to occur in superphosphates made from rock than in those derived from bone. It also varies in different samples, a well-made article u&ually undergoing little change even after long stand ing. It is supposed to be caused by the presence of undecomposed tricalcium phosphate and of iron and aluminum sulphates.
The decrease in solubility of the superphos phate is due in part at least to the interaction of monocalcium phosphate and tricalcium phosphate leading to the formation of dical cium phosphate and perhaps to the formation of ferric and aluminum phosphates by the action of monocalcium phosphate upon the iron and aluminum sulphates.
Double making super phosphates, a material rich in phosphoric acid must be used, not less than 60 per cent trical cium phosphate being necessary for their profit able production. The poorer materials are sometimes used in making what are known as double superphosphates. For this purpose they are treated with an excess of dilute sulphuric acid; the dissolved phosphoric acid and the ex cess of sulphuric az'd are separated from the mass by filtering and are then used for treating phosphates, rich in calcium phosphate, forming superphosphates. The superphosphates so formed contain more than twice as much avail able phosphoric acid as those made in the ordinary way.
Relative Availability of Phosphate Fer tilizers.— Superphosphates and double super phosphates contain their phosphoric acid in a form in which it can be taken up by the plant at once. They are therefore best applied at a time when the crop is planted or just before, or they may be applied when the crop is grow ing. Well-made superphosphates contain no free acids and therefore are not injurious to vegetation unless used in excess.
Reverted phosphoric acid, although not soluble in water, is readily soluble in dilute acids. Its value as a ready source of food for plants has been the subject of much discussion among agricultural chemists. It is now quite generally believed that it furnishes an avail able supply of phosphoric acid to the plant.
Crude phosphate rock is sometimes used as a fertilizer without being treated with acid. Owing to the slight solubility of this material it acts slowly and should be applied in very large quantities — enough to last several years. It should also be very finely ground. It ap pears to be used more profitably on the rich prairie soils of the Middle West than in the Eastern States. Experiments in Indiana, Ohio
and several States further east have given results unfavorable to the use of ground phos phate rock, or ((floats' as they are sometimes called.
Potash Fertilizers.— The production of potash fertilizers is largely confined to Ger many, where are inexhaustible beds varying from 50 to 150 feet in thickness lying under a region of country extending from the Harz Mountains to the Elbe River and !mown as the Stassfurt deposits. These deposits of potash salts have been mined since 1862, during which time they have constituted the world's princi pal supply of potash. They are operated by a syndicate comprising all the large owners and maintaining a uniform price on the output. The principal salts placed on the market are sylvine (potassum chloride) ; sylvinite, a mix ture of sylvine, rock-salt and kainite; carnallite (magnesium chloride and potassium chloride) ; schoenite (magnesium sulphate and potassium sulphate) • kieserite (magnesium sulphate with carnallite); kainite (magnesium sulphate and potassium chloride, or magnesium and potas sium sulphates and magnesium chloride) ; poiyhalite (potassium, calcium and magnesium sulphates).
The potash fertilizers contain their potassium either as chloride or sulphate. The chloride has the advantage of being more diffusible in the soil but in most respects the sulphate is preferable. Potassium chloride has an injurious effect upon such crops as tobacco, sugar-beets and potatoes. In tobacco the potassium chloride in the ash prevents the proper burning of the tobacco. In the case of clover, corn and grass, however, potassium chloride appears to have no harmful effects.
Kainite.— The potash contained in kainite is in the form of sulphate, but because of the large quantities of other salts it contains, chiefly sodium and magnesium chloride, it has the same effect upon plants as has the chloride. It contains from 12 to 20 per cent of potash and 25 to 45 per cent of sodium chloride, with some chloride and sulphate of magnesium. Kainite should be applied to the soil a con siderable time before the crop which it is in tended to benefit is plguitefl. It 'should not be drilled in with the seed. By this method of application the injurious properties of the chlorides will not affect the crop. Indirectly kainite serves to supply the plant with food other than potash. The salts associated with the potash, particularly the chlorides, aid plant growth by making other substances in the soil, particularly phosphoric acid, more available to the plant, besides improving the physical con dition of the soil.