Composition and variability of a Willakenzie map unit in Yamhill County, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rj4308060

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  • 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 distributed properties. 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 least often. 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 Willakenzie. 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.
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