Superphosphate. The term superphosphate is applied to the phosphates that have been treated with sulphuric acid to make the phosphoric acid available.  Dissolved bone, dissolved bone black, and the dissolved phosphate rocks are superphosphates.

Fish Scrap, mentioned as a source of nitrogen, is also a valuable source of phosphoric acid, containing 6 to 8 per cent., which is quite readily available owing to the rapid decay of the scrap.

SOURCES OF POTASH

The chief sources of potash used for fertilizers are the potash salts from the potash mines at Stassfurt, Germany, where there is an immense deposit of rock salt and potash salts.

The principal products of these mines used in this country are the crude salts: 

Kainite, containing 12 per cent. of potash.

Sylvinite, containing 16 to 20 per cent. of potash, and the higher grade salts manufactured from the crude salts: 

Muriate of Potash, containing 50 per cent. potash.

High grade Sulphate of Potash, containing 50 per cent. potash.

Low grade Sulphate of Potash, containing 25 per cent. potash.

Wood Ashes, if well kept and not allowed to get wet and leach, contain 4 to 9 per cent. of potash.

Cotton Hull Ashes contain 20 to 30 per cent, of potash and 7 to 9 per cent. of phosphoric acid.

The potash in all these forms is soluble in water and equally available to plants.  The crude salts, kainite and sylvinite, and the muriate contain chlorine and are not considered good for potatoes and tobacco as the chlorine lowers the quality of these products.

In tobacco regions tobacco refuse is a valuable source of potash, the stems are about five per cent. potash.

LIME

Lime is generally supplied to the soil in the form of quicklime made by burning lime stone or shells.  Other forms are gypsum or land plaster, gas lime (a refuse from gas works) and marl.  Most soils contain sufficient lime for the food requirements of most plants.  Some soils, however, are deficient in lime and some crops, particularly the legumes, are benefitted by direct feeding with lime.

Lime is valuable for its effect on the soil properties which constitute fertility.

Physically lime acts on the texture of the soil making clay soils mealy and crumbly, and causing the lighter soils to adhere or stick together more closely.

Chemically, lime decomposes minerals containing potash and other plant foods, thus rendering them available for the use of plants.  It also aids the decay of organic matter and sweetens sour soils.

Biologically lime aids the process of nitrification.

The action of lime is greatest in its caustic or unslacked form.

Too much or too frequent liming may injure the soil.  It should be carefully tried in a small way, and its action noted, before using it extensively.