; 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:

1. An investigator found in his laboratory that a plant growing in a
water solution was injured when magnesium was added, and that the
injury was checked when calcium in equal amount was added to the water.
The theory was worked out that a soil should not contain a greater total
amount of magnesium than of calcium, and as the soil's supply of calcium
tends to leach out more readily than the supply of magnesium, it was
best to use a high-calcium lime. If this discovery of the laboratory had
been carried into the field, its signifi