well hardened and
cemented,--in which case they must again undergo the softening processes
of weathering before they become available for use. Where soils become
buried under other rocks and become hardened, they are classed as
sedimentary rocks and form a part of the geologic record. Many residual
and transported soils are to be recognized in the geologic column; in
fact a large number of the sedimentary rocks ordinarily dealt with in
stratigraphic geology are really transported soils.

The development of soils by weathering should not be regarded as a
special process of rock alteration, unrelated to processes producing
other mineral products. Exactly the same processes that produce soils
may yield important deposits of iron ore, bauxite, and clay, and they
cause also secondary enrichment of many metallic mineral deposits. For
instance the weathering of a syenite rock containing no quartz, under
certain conditions, as in Arkansas, results in great bauxite deposits
which are truly soils and are useful as such,--but which happen to be
more valuable because of their content of bauxite. The weathering of a
basic igneous rock, as in Cuba, may produce important residual iron ore
deposits, which are also used as soils. Weathering of ferruginous
limestone may produce residual iron and manganese ores in clay soils.

COMPOSITION OF SOILS AND PLANT GROWTH

The mineral ingredients in soils which are essential for plant growth
include water, potash, lime, magnesia, nitrates, sulphur, and phosphoric
acid--all of which are subordinate in amount to the common products of
weathering (pp. 20-22, 23-24). Of these constituents magnesia is almost
invariably present in sufficient quantity; while potash, nitrates, lime,
sulphur, and phosphoric acid, although often sufficiently abundant in
virgin soil, when extracted from the soils by plant growth are liable to
exhaustion under ordinary methods of cultivation, and may need to be
replenished by fertilizers (Chapter VII). Some soils may be so
excessively high in silica, iron, or other constituents, that the
remaining constituents are in too small amounts for successful plant
growth.

Even where soils originally have enough of all the necessary chemical
elements, one soil may support plant growth and another may not, for the
reason that the necessary constituents are soluble and hence available
to the plant roots in one case and are not soluble in the other. Plainly
the mineral combinatio