ns in which the various elements occur are
important factors in making them available for plant use. Similarly a
soil of a certain chemical and mineralogical composition may be fruitful
under one set of climatic conditions and a soil of like composition may
be barren at another locality--indicating that availability of
constituents is also determined by climatic and other conditions of
weathering. Even with the same chemical composition and the same
climatic conditions, there may be such differences in texture between
various soils as to make them widely different in yield.

The unit of soil classification is the _soil type_, which is a soil
having agricultural unity, as determined by texture, chemical character,
topography, and climate. The types commonly named are clay, clay loam,
silt loam, loam, fine sandy loam, sandy loam, fine sand, and sand. In
general the soil materials are so heterogeneous and so remote from
specific rock origin, that in such classification the geologic factor of
origin is not taken into account. More broadly, soils may be classified
into provinces on the basis of geography, similar physiographic
conditions, and similarity of parent rocks; for instance, the soils of
the Piedmont plateau province, of the arid southwest region, of the
glacial and loessal province, etc. In such classification the geologic
factors are more important. Soils within a province may be subdivided
into "soil series" on the basis of common types of sub-soils, relief,
drainage, and origin.

USE OF GEOLOGY IN SOIL STUDY

While the desirability of particular soils is related in a broad way to
the character of the parent rocks, and while by geologic knowledge
certain territories can be predicated in advance as being more favorable
than others to the development of good soils, so many other factors
enter into the question that the geologic factor may be a subordinate
one. A soil expert finds a knowledge of geology useful as a basis for a
broad study of his subject; but in following up its intricacies he gives
attention mainly to other factors, such as the availability of common
constituents for plant use, the existence and availability of minute
quantities of materials not ordinarily regarded as important by the
geologist, the climatic conditions, and the texture. As the geologic
factors are many of them comparatively simple, much of the expert work
on soils requires only elementary and empirical knowledge of geology.
The