Agriculture Reference
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between genetic horizons and diagnostic horizons. Genetic horizons, for example,
A, B, and C horizons, reflect a qualitative judgment about soil forming processes.
Diagnostic horizons such as mollic epipedon, cambic horizon, and argillic horizons
have defined quantitative properties that may include two or more genetic horizons.
It seems that diagnostic horizons provide a more quantitative method of evaluating
soil properties that contribute to sustainability. However, the majority of data in the
literature are reported based on genetic horizons.
The time factor is important in understanding formation rates of soil. The aging/
development of a soil in the simplest case can be thought of as starting at time zero
with the exposure of unweathered parent material to physical, chemical, and biologi-
cal processes. Most cases are not this simple. Methods of determining the absolute
age related to time zero include radioisotope dating and historical records. Chemical
composition data have also been used on a watershed basis. Alexander (1988) used
outputs of silica and major cations to estimate rates of formation. Wakatuski and
Rasyidin (1992) used geochemical mass balance equations to estimate rates of
weathering and soil formation. Geomorphic surfaces can also aid in refining the time
factor in a relative way. A geomorphic surface is a portion of the land surface that
can be defined in space and time and is mappable (Ruhe 1969). It must be defined
in relation to other geomorphic surfaces to place it in the proper spatial and time
sequence. Parsons et al. (1970) and Ruhe et al. (1975) used this approach to develop
time lines for formation of surface and subsurface horizons.
Another method of evaluating the time factor utilizes the degree of development
or horizon differentiation of soil profiles. However, this approach has many prob-
lems. For example, combining data from Hutton (1947) and Ruhe (1969) demon-
strates that on stable geomorphic surfaces dated at about 14,000 years before present
(YBP), soils developed in Wisconsinan loess and classified as Mollisols range from
those with minimal cambic horizons to those with argillic horizons having clay con-
tents in excess of 55%. Differences in profile differentiation are related to distance
from loess source and sorting, loess thickness, depth to restrictive layer, and internal
drainage and are not necessarily controlled exclusively, or even dominantly, by the
time factor.
Today's soils result from a myriad of processes occurring over hundreds to thou-
sands (and even millions) of years. Some components of soil development such as
soil organic matter additions can occur in a relatively short time—a period of years.
Chemical weathering of parent materials into clay minerals may take centuries or
even longer. Leaching of materials, such as clays, forming different soil horizons
will take even longer as this process will occur subsequent to conversion of parent
materials to clay minerals. Thus, clearly articulating soil renewal rates must involve
integration of multiple soil development processes that occur at different rates and
give different results based on other supporting soil formation factors. Identifying
soil development rates is not an exact science; however, it is one that is critical in
evaluation of soil erosion impacts on soil sustainability.
Many studies report both soil loss and soil formation in units of soil depth,
while arguably the world's most recognized soil conservation agency, the Natural
Resources Conservation Service of the United States Department of Agriculture
(NRCS), bases soil erosion and conservation programs on mass of soil eroding or
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