FACTORS OF SOIL FORMATION - Midlands State University

the composition of this parent material are expressed by differences in
soil texture. For surface soils, we may write the following equation:
Nitrogen = f (texture) cl, o, r, t, . . . (10)
The constant soil-forming factors cl, o, r, and t are specified as
follows:
cl = climate of Iowa (approximately identical for entire series),
o = prairie (now cultivated),
r = gently undulating to rolling,
t = unknown, but presumably the same for the entire series.
In the foregoing equation it is assumed that the texture of the
surface soil defines the parent material of the surface soil. Brown's
comparisons are summarized in Table 10. Both nitrogen and carbon
increase as the soils assume a heavier texture. The mean values for N
and C are significant inasmuch as the variability within soil types is
considerably less than between textural groups.
Types of Parent Materials.—In Figs. 28, 29, and 30 are shown
the distribution of various types of parent materials within the United
States. These maps were constructed from Marbut's "Distribution of
TABLE 10.—THE AVERAGE NITROGEN AND CARBON CONTENT OF SOIL
TYPES OF DIFFERENT CLASSES OF THE CARRINGTON SERIES
(Surface soils, 0 to 6 ⅔ in. depth)
Nitrogen, Organic carbon,
Texture of surface soils per cent per cent C/N
Sand 0.028 0.40 14.1
Fine sand 0.043 0.58 14.5
Sandy loam 0.100 1.25 12.5
Fine sandy loam 0.107 1.32 12.5
Loam 0.188 2.21 12.2
Silt loam 0.230 2.68 11.7
parent materials of soils" in his Soils of the United States in the Atlas
of American Agriculture, Part III. According to Marbut,
No attempt has been made to make it accurate in detail. In considerable
areas, there may be some legitimate difference of opinion as to the source and
character of the materials, such, for example, as on the plains of southern
Idaho and parts of central Oregon and Washington. In central Texas, the
western part of the area of residual accumulations from sandstones and shales
contain areas of Great Plains materials and sands. The distribution of loess
has been extended over areas about which there is no universal agreement.
Notwithstanding these and many other areas of detail about which there is no
universal agreement, the maps represent a mass of useful information.
B. SOIL FORMATION ON IGNEOUS ROCKS
Before the advent of the climatic theories of soil formation, parent
material was considered the major soil-forming factor. In spite of a
vast amount of work on the relationships between soil properties and
underlying geological strata, relatively little information is at hand
that can be interpreted in the light of functional analysis. As a rule, the
correlations are suffering from lack of control of soil-forming factors,
particularly climate.