|Abstract or Summary
- The variability of selected physical, chemical, and morphological
soil properties in two landtype mapping units on'the Rogue River National
Forest in southwestern Oregon was studied.
The objectives of the study were (i) to quantify soil variability
in several soil resource inventory mapping units, (ii) to explore methods
of describing soil variability in order to make soil map information more
useful to the map user, and (iii) to attempt to identify sources of
Two mapping units, in the Siskyou and Cascade Mountains, were
selected to represent extremes of internal variability. Eight delineations
of each map unit were sampled with randomly located transects for
a total of 40 sites per map unit.
Soil properties exhibited various types of frequency distributions.
Normal, skewed, and multi-modal distributions were observed. Nearly half
the property-horizon combinations measured in both map units had normal
distributions. Chemical properties, such as extractable bases, were
consistently positively skewed or approximately log-normal. Square root
and logarithmic transformations of the data normalized these distributions
and stabilized the variance. These results suggested that for
multi-modal and badly skewed populations, assumption of a normal
distribution may lead to considerable error if the arithmetic mean and
standard deviation are used for predictive purposes.
Map unit 74 in the Siskyou Mountains was considered to be more
variable, over most properties measured, than map unit 33 from the
Cascades. For most properties coefficients of variation (CV) were higher,
the sample requirements to estimate population means were greater, and
the ranges were wider in map unit 74, as compared to map unit 33.
In both map units, chemical properties were more variable than
physical or morphological properties, which were about equal in their
The number of samples required to estimate the means of properties
varied widely and were often prohibitively large (264 for organic matter
in the surface of map unit 74). This number could be reduced if the
sampling scheme was stratified using estimates of within and between
Between 50 and 75 percent of the total variation in most properties
of both map units occurred within delineations. This result is desireable
from a management and broad planning perspective and tends to support
the validity of the map units as designed and mapped.
When tested by analysis of variance, most properties in both map
units had significantly different delineation means. These differences
could often be traced to one particular delineation and were many times
not of practical significance.
Large or small changes in the values of most properties were found
to be as likely to occur at 660 foot separation distances as at 15 foot
distances. This tends to indicate a random distribution of variation,
when distance alone is considered, which could have important consequences when attempting to characterize soil properties for management interpretations
or site evaluations.
Chemical properties expressed volumetrically were more variable
than the same properties expressed on a weight basis. Volumetric chemical
concentrations were greater in map unit 74 than in map unit 33, the
reverse of the relationship found with those properties on a weight basis.
The proportions of within delineation variance in map unit 74 increased
as a result of conversion to volumetric concentrations.
Use of principal components analysis confirmed the chemical properties
as contributing greater amounts of variation than the physical or
morphological properties. Ordination of sites along axes of selected
factors also confirmed the general uniformity of the map units as well as
the greater variability of map unit 74.