This is a scanned version of the text of the original Soil Survey report of Washington County, Oregon, issued July 1982. Original tables and maps were deleted. There may be references in the text that refer to a table that is not in this document. Updated tables were generated from the NRCS National Soil Information System (NASIS). The soil map data has been digitized and may include some updated information. These are available from http://soildatamart.nrcs.usda.gov. Please contact the State Soil Scientist, Natural Resources Conservation Service (formerly Soil Conservation Service) for additional information. SOIL SURVEY OF WASHINGTON COUNTY, OREGON By George L. Green, Soil Conservation Service Fieldwork by George E. Otte, Duane K. Setness, Richard T. Smythe, and Calvin T. High, Soil Conservation Service United States Department of Agriculture, Soil Conservation Service, in cooperation with the Oregon Agricultural Experiment Station WASHINGTON COUNTY is in the northwestern part of the shape of slopes, the size and speed of streams, the kinds of Willamette Valley (fig. 1). The total land area is native plants or crops, the kinds of rock, and many facts about approximately 458,200 acres. the soils. They dug many holes to expose soil profiles. A The southern and eastern parts of the survey area consist of profile is the sequence of natural layers, or horizons, in a soil; a smooth valley area that is used for farming and urban it extends from the surface down into the parent material that communities, and the western and northern parts consist of a has not been changed much by leaching or by the action of hilly or mountainous area that is used for timber. plant roots. With the exception of the headwaters of the Trask, Wilson, The soil scientists made comparisons among the profiles and Nehalem Rivers, which originate near the summit of the they studied, and they compared these profiles with those in Coast Range and flow westward to the Pacific Ocean, the counties nearby and in places more distant. They classified drainage of more than 90 percent of the county is eastward and named the soils according to nationwide, uniform through forks and tributaries of the Tualatin River into the procedures. The soil series and the soil phase are the Willamette River, which flows into the Columbia River at categories of soil classification most used in a local survey. Portland. Soils that have profiles almost alike make up a soil series. Except for different texture in the surface layer, all the soils of How this survey was made one series have major horizons that are similar in thickness, arrangement, and other important characteristics. Each soil series is named for a town or other geographic feature near Soil scientists made this survey to learn what kinds of soil the place where a soil of that series was first observed and are in Washington County, where they are located, and how mapped. Aloha and Hillsboro, for example, are the names of they can be used. The soil scientists went into the county two soil series. All the soils in the United States having the knowing they likely would find many soils they had already same series name are essentially alike in those characteristics seen and perhaps some they had not. They observed the that affect their behavior in the undisturbed landscape. steepness, length, and Soils of one series can differ in texture of the surface layer and in slope, stoniness, or some other characteristic that affects use of the soils by man. On the basis of such differences, a soil series is divided into phases. The name of a soil phase indicates a feature that affects management. For example, Hillsboro loam, 0 to 3 percent slopes, is one of several phases within the Hillsboro series. After a guide for classifying and naming the soils has been worked out, the soil scientists drew the boundaries of the individual soils on aerial photographs. These photographs show woodlands, buildings, field borders, trees, and other details that help in drawing boundaries accurately. The soil map at the back of this publication was prepared from aerial photographs. The areas shown on a soil map that are identified by the same symbol are called mapping units. On most maps detailed enough to be useful in planning the management of farms and fields, a mapping unit is nearly equivalent to a soil phase. It is not exactly Figure 1.-Location of Washington County in Oregon. equivalent, because it is not practical to show on such a map then adjusted the groups according to the results of their all the small, scattered bits of soil of some other kind that studies and consultation. Thus, the groups that finally have been seen within an area that is dominantly of a evolved reflect up-to-date knowledge of the soils and their recognized soil phase. behavior under current methods of use and management. Some mapping units are made up of soils of different In mapping the survey area, two intensities of soil mapping series, or of different phases within one series. Two such were used. In the forested, hilly or mountainous part of the kinds of mapping units are shown on the soil map of county, soils were examined at moderate to wide intervals Washington County: soil complexes and associations. and were mapped at medium intensity. On the smooth valley A soil complex consists of areas of two or more soils, so floor, the soils were examined at close intervals and were intermingled or so small in size that they cannot be shown mapped at high intensity; that is, in considerably more detail separately on the soil map. Each area of a complex contains than in the forested, hilly or mountainous part. Most of the some of each of the two or more dominant soils, and the soils mapped at high intensity have a narrow range of slope. pattern and relative proportions are about the same in all They are in the southern and eastern parts of the survey area, areas. Generally, the name of a soil complex consists of the and the soils mapped at medium intensity are in the western names of the dominant soils, joined by a hyphen. and northern parts. Kilchis-Klickitat complex, 60 to 90 percent slopes, is an A list of all the soils in the survey area, their map symbol, example. and the capability unit, woodland group, and wildlife group An undifferentiated group is made up of two or more soils to which each has been assigned can be found in the "Guide that could be delineated individually but are shown as one to Mapping Units" at the back of this publication. unit because, for the purpose of the soil survey, there is little value in separating them. The pattern and proportion of soils are not uniform. An area shown on the map may be made up of only one of the dominant soils, or of two or more. If there are two or more dominant series represented in the group, the name of the group ordinarily consists of the names of the General soil map dominant soils, joined by "and." Cornelius and Kinton silt loams, 2 to 7 percent slopes, is an example. The general soil map at the back of this survey shows, in In most areas surveyed there are places where the soil color, the soil associations in the survey area. A soil material is so stony so shallow, so severely eroded, or so association is a landscape that has a distinctive pattern of variable that it has not been classified by soil series. These soils in defined proportions. It typically consists of one or places are shown on the soil map and are described in the more major soils and at least one minor soil, and it is named survey, but they are given descriptive names, such as Urban for the major soils. The soils in an association can occur in Land, or are named for a higher category in the soil other associations, but in different patterns. classification system, such as Udifluvents, nearly level: A map showing soil associations is useful to people who While a soil survey is in progress, soil scientists take soil want to have a general idea of the soils in a survey area, who samples needed for laboratory measurements and for want to compare different parts of that area, or who want to engineering tests. Laboratory data from the same kind of soil locate large tracts that are suitable for a certain kind of land in other places are also assembled. Data on yields of crops use. Such a map is a useful general guide for broad planning under defined practices are assembled from farm records and on a watershed, a wooded tract, or a wildlife area or for broad from field or plot experiments on the same kind of soil. planning of recreational facilities, community developments, Yields under defined management are estimated for all the and such engineering works as transportation corridors. It is soils. not a suitable map for detailed planning for management of a Soil scientists observe how soils behave when used as a farm or field or for selecting the exact location of a road or growing place for native and cultivated plants and as material building or other structure, because the soils within an for structures, foundations for structures, or covering for association ordinarily vary in slope, depth, stoniness, structures. They relate this behavior to properties of the soils. drainage, and other characteristics that affect their For example, they observe that filter fields for onsite disposal management.