May 3, 1912] 
eral rock structure will largely determine 
the relation of the soil to any particular 
rock formation. 
V. The Series—The soil series is the 
most complex of the separations made in 
practise. Unlike the others, it does not 
rest upon a single character. Its separa- 
tion requires the employment of a group 
of correlated characters. These are more 
intimate in their nature, more concerned 
with the material itself than the bases of 
the larger separations. Having applied 
those, the final grouping of types or units 
is determined first of all by 
(a) Color.—Color as a physical property 
is not of great importance, but when con- 
sidered in connection with the properties 
which are correlated with color it is of the 
greatest significance. Color is usually in- 
dicative of the proportion of organic mat- 
ter, of the drainage, of the state of oxida- 
tion, of the proportion of lime carbonate 
and to a degree of the mineral composition 
of the soil. 
(0) Organic Matter—The proportion of 
organic matter is suggestive of the nitrogen 
content of the soil. It has a large influ- 
ence on the availability of the mineral par- 
ticles. It is indicative of the natural 
drainage, of the proportion of lime carbon- 
ate and of the activity and type of micro- 
organisms. 
(c) Lime Carbonate.—The presence of a 
fair amount of this constituent has been 
pretty generally recognized as essential to 
a fertile soil. 
(d) The Total Plant Food Content of 
the Soil.—While the average soil contains 
a relatively large quantity of the mineral 
plant food constituents, soils which are 
decidedly deficient in some one or more of 
these are sufficiently abundant to warrant 
the special consideration of this property. 
It does not necessarily involve general 
chemical analysis and may often be in- 
SCIENCE 
685 
ferred from other properties. The solu- 
bility of the soil constituents, while seldom 
a distinetive series basis of separation, is 
often indicative of other characteristics of 
the soil and as alkali, especially, of the 
type of climate. These several elements 
will seldom, if ever, have equal value in 
deciding a separation. They can not well 
be applied independently in general field 
work. In some cases one, as color, may be 
dominant; in another drainage, and in still 
other separations it may be the lime con- 
tent which will determine the grouping. 
If one consider all soil series together 
without reference to their grouping accord- 
ing to the broader bases of classification, 
such as formation, ete., it will generally be 
observed that soil series are chiefly distin- 
guished by chemical differences, that is, dif- 
ferences in composition and chemical form. 
Organic matter is suggestive of the nitro- 
gen content, solubility, ete. The presence 
of lime is a matter of composition. Differ- 
ences in rock material mean certain differ- 
ences in composition. On the other hand, 
the last or unit separation of soils is based 
chiefly on physical properties—texture and 
structure. The series includes all material 
having the same characteristics, but vary- 
ing in texture from the coarsest to the 
finest. 
VI. The Type. Texture and Structure. 
—The fineness of the material of which a 
soil is composed is the most broadly influ- 
ential of the physical properties of a soil. 
Through its influence on porosity, moisture 
relations, aeration, temperature, tillage 
properties, etc., it is probably the most 
dominant in deciding crop adaptation and 
agricultural value. The more distinct tex- 
tures of clay, loam, silt and sand are gen- 
erally recognized. These rest upon the 
proportion of particles of different sizes 
and a great variety of proportions may be 
recognized and given names. These are 
