CHAPTER XII
SOUS
A soil analysis is made to find out the extent and distribution
of plant food elements aad thus to determine which elements are
the limiting factors in crop production. The term " soil fertility "
is often used to express this relation and this is understood to
mean the crop producing power of any soil under specified
climatic conditions. "Fertility'7 is really an Indefinite term as
the property indicated is the resultant of many forces which
are frequently opposed to each other in their action.
Total and Acid-soluble Material.—The analysis of soils with
a view to measuring their fertility and studying their geological
origin has received much attention in recent years. It -was
formerly thought, and it is still held to some extent, that the
fertility can best be measured by extracting the soils with strong
acids, thus obtaining an invoice of the total plant food con-
tained. But by this method there is left some available potas-
sium and a mass of substances of unknown composition, which
yet need to be determined in order to have complete information
concerning the geological origin of the soil. On this account
the value of many early analyses is being called into question
and at present it is regarded as desirable that the total constitu-
ents of the soil should be determined. The complete examina-
tion of a soil involves its study, from chemical, physical and
biological points of view. The chemical phases of this subject
will be given most attention here.
Soil Constituents.—Soil has been defined as that portion of
the earth's surface, climatic conditions being favorable, which
makes possible complete growth and development of plants.
Ordinarily soils are made up of mixtures of organic matter, rock
at various stages of disintegration, water, gases and bacteria.
The great mass of this material is not directly essential to the
growth of plants but aids in holding moisture and making a
medium in which the roots may anchor themselves.
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