is
amount. The United States has thus far been using up the original
materials stored in the soil by nature, but these have not been
sufficient to yield anything like the crop output per acre of the more
highly fertilized soils of Europe.

In addition to the nitrates, phosphates, and potassium salts, important
amounts of lime and sulphuric acid, and some gypsum, are used in
connection with soils. Lime is derived from crushed limestone (pp.
82-83), and is used primarily to counteract acidity or sourness of the
soil; it is, therefore, only indirectly related to fertilizers.
Sulphuric acid is used to treat rock phosphates to make them more
soluble and available to plant life. It requires the mining of pyrite
and sulphur. Gypsum, under the name of "land-plaster," is applied to
soils which are deficient in the sulphur required for plant life;
increase in its use in the future seems probable. There are also
considerable amounts of inert mineral substances which are used as
fillers in fertilizers to give bulk to the product, but which have no
agricultural value. The proportions of the fertilizer substances used in
the United States are roughly summarized in Figure 4.

The United States possesses abundant supplies of two of the chief
mineral substances entering into commercial fertilizers,--phosphate rock
and the sulphur-bearing materials necessary to treat it. For potash the
United States is dependent on Europe, unless the domestic industry is
very greatly fostered under protective tariff. For the mineral nitrates
the United States has been dependent on Chile, and because of the
cheapness of the supply will doubtless continue to draw heavily from
this source. However, because of the domestic development of plants for
the fixation of nitrogen from the air, the recovery of nitrogen from
coal in the by-product processes, and the use of nitrogenous plants, the
United States is likely to require progressively less of the mineral
nitrates from Chile.

The fertilizer industry of the United States is yet in its infancy and
is likely to have a large growth. Furthermore much remains to be learned
about the mixing of fertilizers and the amounts and kinds of materials
to be used. The importance of sulphur as a plant food has been realized
comparatively recently. The use of fertilizers in the United States has
come partly through education and the activity of agricultural schools
and partly through advertising by fertilizer companies. T