geologist, although he may understand fully the origin of soils and
may indicate certain broad features, must acquire a vast technique not
closely related to geology before he becomes effective in soil survey
work and diagnosis.

For these reasons the mapping and classification of soils, while often
started by geologists of state or federal surveys, have in their
technical development and application now passed largely into the hands
of soil experts in the special soil surveys affiliated with the U. S.
Department of Agriculture and with agricultural colleges.

FOOTNOTES:

[13] A good summary of this subject may be found in _Engineering
Geology_, by H. Ries and T. L. Watson, Wiley and Sons, 2d ed., 1915.

[14] Marbut, Curtis F., Soils of the United States: _Bull. 96, Bureau of
Soils_, 1913, p. 10.

CHAPTER VII

THE FERTILIZER GROUP OF MINERALS

GENERAL COMMENTS

Soils are weathered rock more or less mixed with organic material. The
weathering processes forming soils are in the field of geologic
investigation, but the study of soils in relation to agriculture
requires attention to texture and to several of their very minor
constituents which have little geologic significance. Soil study has
therefore become a highly specialized and technicalized subject,--for
which a geological background is essential, but which is usually beyond
the range of the geologist. To supply substances which are deficient in
soils, however, requires the mining, quarrying, or extraction of
important mineral resources, and in this part of the soil problem the
geologist is especially interested.

Soils may be originally deficient in nitrates, phosphates, or potash; or
the continued cropping of soils may take out these materials faster than
the natural processes of nature supply them. In some soils there are
sufficient phosphates and potash to supply all plant needs indefinitely;
but the weathering and alteration processes, through which these
materials are rendered soluble and available for plant life, in most
cases are unable to keep up with the depletion caused by cropping. A ton
of wheat takes out of the soil on an average 47 pounds of nitrogen, 18
pounds of phosphoric acid, 12 pounds of potash. On older soils in Europe
it has been found necessary to use on an average 200 pounds of mixed
mineral fertilizers annually per acre. On the newer soils of the United
States the average thus far used has been less than one-seventh of th