_Wood Ashes._ A large supply of lime in excellent form was afforded by hardwood ashes, but this product has ceased to have any important value to our agriculture. The chief supply on the market is low in quality, containing moisture and dirt in considerable amount, the form of lime being changed from an oxide to the hydrate and carbonate. _Gas Lime._ Prof. E. B. Voorhees, in "First Principles of Agriculture," says: "Gas lime is also frequently used as manure; in gas works, quicklime is used for removing the impurities from the gas. Gas lime, therefore, varies considerably in composition, and consists really of a mixture of slaked lime, or calcium hydrate, and carbonate of lime, together with sulfites and sulfides of lime. These last are injurious to young plant life, and gas lime should be applied long before the crop is planted, or at least exposed to the air some time before its application. The action of air converts the poisonous substances in it into non-injurious products. Gas lime contains on an average 40% of calcium oxide, and usually a small percentage of nitrogen." _Lime After Magnesium Removal._ A by-product in the removal of magnesium from a magnesian limestone is an excellent material for correction of soil acidity, on account of its physical condition. Its exposure to the air causes much of the hydrate to change to an air-slaked form, and its value per ton lies somewhere between that of very finely pulverized limestone and hydrated lime. CHAPTER XIV MAGNESIAN LIME _Magnesium._ As an element of plant food, magnesium is as essential as calcium. It leaches out of the soil less readily, and there may be even less need of its application as a plant food, though the need of calcium applications for this purpose is assumed to be small. In the correction of soil acidity magnesium is more effective than calcium, 84 pounds of the carbonate being equal to 100 pounds of calcium carbonate. It is a curious fact, however, that there is widespread fear of magnesium as a soil amendment. This is not traceable to any considerable experience by practical farmers that inspires caution in its use, although immense quantities of magnesian limestone and lime have been used. Neither is it due to any weight of evidence against it in the experience or teachings of soil chemists and experiments. The facts of the case appear to be as follows: