232 QTAXTITATirE AGRICULTURAL AX-\LYXIX
soil analysis in the determination of geological origin has been
considerably underestimated, although few dat^> are at band to
aid in such an interpretation.
Soil Classification Based upon Mode of Formation.—For con-
venience in the study of soils they may be divided according
to manner of formation into eight groups1 as follows: Cumulose
soils are chiefly deposits of vegetation in various stages of decay.
Residual soils—unmoved from the rocks from, which they were
formed. Lo^ss—residue deposited as dust carried by wind.
Glacial soils are deposits which have developed from glacial
action. Cdluvial soils are deposits which ha~ve been moved
down hill by gravity. Alluvial soils consist of residues deposited
from flowing water. Marine soils are formed by deposits carried
Into seas. Lacustrine soils are formed by deposits carried into
lakes.
Classification Based upon Composition.—It is to be expected
that soils whose origin is so different as is noted above would
vary greatly in chemical analysis. Ames has made a study
(unpublished work) of the relation between soil "type and compo-
sition, considering at the same time the geological formation.
The chief differences observed were with respect to the calcium
carbonate, total organic matter and nitrogen content. For
example the soils of limestone origin, as compared with, those
from sandstone and shales, contained larger amounts of calcium,
and magnesium. In many cases these larger amounts of calcium
and magnesium are accompanied by larger amounts of phos-
phorus, organic matter and nitrogen. It is evident that if only
disintegrating forces had been active the soil pairticles would be
of the same general composition as the original rock. However,
many agencies tend to change this original material until only
the most resistant minerals remain.2
The Report.—Determinations of the inorganic constituents of
soil are usually reported in terms of their most stable compounds or
of their oxides although nitrogen is reported astho element. The
determination is usually made on an air-dried sample. Besides
reporting results as per cents on this basis it is sometimes desir-
able to report the amount per acre (2,000,000 Ib. is considered as
ITROWBRIDGE, /. GeoL, 22, 420 (1911).
2 See also A7. C. Exp. Sta. Tech. Bull., 9 (1914).