ry fertilizer for
that land would necessarily contain a large percentage of phosphorus.
The fertilizer to be used in a soil depends upon the character of the
soil and upon the crops previously grown on it.

[Illustration: FIG. 163.--Water cultures of buckwheat: 1, with all the
food elements; 2, without potash; 3, without nitrates.]

The quantity of fertilizer needed by the farmers of the world is
enormous, and the problem of securing the necessary substances in
quantities sufficient to satisfy the demand bids fair to be serious.
But modern chemistry is at work on the problem, and already it is
possible to make some nitrogen compounds on a commercial scale. When
nitrogen gas is in contact with heated calcium carbide, a reaction
takes place which results in the formation of calcium nitride, a
compound suitable for enriching the soil. There are other commercial
methods for obtaining nitrogen compounds which are suitable for
absorption by plant roots.

Phosphorus is obtained from bone ash and from phosphate rock which is
widely distributed over the surface of the earth. Bone ash and
thousands of tons of phosphate rock are treated with sulphuric acid to
form a phosphorus compound which is soluble in soil water and which,
when added to soil, will be usable by the plants growing there.

The other important ingredient of most fertilizers is potash. Wood
ashes are rich in potash and are a valuable addition to the soil. But
the amount of potash thus obtained is far too limited to supply the
needs of agriculture; and to-day the main sources of potash are the
vast deposits of potassium salts found in Prussia.

Although Germany now furnishes the American farmer with the bulk of
his potash, she may not do so much longer. In 1911 an indirect potash
tax was levied by Germany on her best customer, the United States, to
whom 15 million dollars' worth of potash had been sold the preceding
year. This led Americans to inquire whether potash could not be
obtained at home.

Geologists say that long ages ago Germany was submerged, that the
waters slowly evaporated and that the various substances in the sea
water were deposited in thick layers. The deposits thus left by the
evaporation of the sea water gradually became hidden by sediment and
soil, and lost to sight. From such deposits, potash is obtained.
Geologists tell us that our own Western States were once submerged,
and that the waters evaporated and disappeared from our land very