- The variability in morphologic, chemical, and physical soil
properties within the Willakenzie silty clay loam, 2 to 12 percent
slopes, map unit in Yamhill County, Oregon was measured.
The objectives of the study included (1) describing the frequency
distributions of individual soil properties, (2) measuring the
taxonomic composition of the map unit, (3) comparing soil property
variability over three separation distances, (4) analyzing the variability
of soil properties within single delineations versus the
variability of properties between delineations, and (5) diagnosing
the major sources of map unit varibility.
One pedon in each of 35 randomly selected delineations was
sampled to meet objectives (1) and (2). Samples were taken at 35 m
intervals along transects placed within four delineations to meet
objectives (3) and (4).
Thirty-seven percent of the morphologic, chemical, and physical
properties measured exhibited positively skewed frequency distributions.
The median and/or the mode, rather than the mean, were more accurate
descriptors of central tendency in two-thirds of these distributions. CV values were higher for the skewed properties than for the normally
Ranges of all of the soil properties measured, except color hue
of the surface horizon, were greater than those described specifically
for the Willakenzie silty clay loam, 2 to 12 percent slopes, map unit.
Slope gradient and Al horizon color were mapped correctly most often,
whereas texture/character of the C or Cr horizon was mapped correctly
None of the 35 pedons classified exactly as the Willakenzie
series, resulting in a 0 percent taxonomic purity. Fifty-seven percent
of the sampled pedons were Mollisols. Thirty-one percent of the pedons
contained contrasting soil characteristics, which resulted in their
designation as dissimilar soils. Sixty-nine percent of the sampled
pedons could be managed similar to or more intensively than the
Soil property variability was compared between three different
separation distances along a transect. Chemical properties tended to
have higher variances within 70 m distances, and physical properties
tended to have higher variances within 105 m distances. Both chemical
and physical properties achieved minimum variances within the 35 m
distance. Morphologic properties showed no trends.
Delineation mean values were significantly different for sixty-one
percent of the properties measured. Management and land use predictions
concerning the Willakenzie map unit as a whole can not be applied to every delineation, because of the significant differences
in morphologic and physical properties between individual delineations.
Three major sources of observed variability within the map unit
were determined. High variability in the underlying geologic strata
resulted in a wide range of C horizon textures and rock types, from
clays to sandy loans to mudstones to sandstones. Some of the pedons
were mapped at elevations low enough to have been affected by lacustrine
silts. These pedons contained uniform silt loam horizons,
which is not typical for the Willakenzie. The fact that soil mapping
in Yamhill County took place in the 1950's under the 1938 Classification
system accounted for much of the variability in soil profile
features such as mollic epipedons and argillic horizons. These features
were not differentiating criteria for the Willakenzie series under the
earlier system as they are under the present system.