Soil pH

Asked April 22, 2017, 11:41 AM EDT

What is the soil pH for Clackamas County? Is there a map reference showing the entire state?

Clackamas County Oregon

1 Response

Soils in Western Oregon tend to be in the range of 4.8 to 6.2. The pH in agricultural fields have been changed using inputs, the pH will vary from field to field.

If you're interested in testing your own soil please refer to this pulblication from the Master Gardeners.

Soil pH greater than 7.0 is uncommon west of the Cascades, so gardeners in this area typically acidify their soil only for growing acid-loving plants. Soils on the east side of the Cascades are alkaline, and may need to be acidified for all crops. To lower soil pH, add elemental sulfur. If using powdered form, wear a mask. Like lime, sulfur is best added in the fall. It is more effective to add it to an area before planting. For established trees and shrubs, add a small amount to the soil along the drip line, or broadcast in a band along a row of blueberries. For more information about lowering soil pH, see EC 1560-E. There is no test to determine sulfur amendment amounts. A soil pH test in 3 to 6 months will let you know if more is neede

The excerpt below comes from the Soil Acidity in Oregon: Understanding and Using Concepts for Crop Production Extension publication. For the full publication click here.

The material from which present-day Willamette Valley soil is formed was deposited during the iceage Missoula floods that receded approximately 10,000 years ago. Centuries of leaching by winter rainfall have developed western Oregon’s naturally acidic soils (4.8 to 6.2). Assuming the initial pH of the deposits was neutral, and the current pH of these soils is approximately 5.5, soil pH declined at a rate of 0.00015 unit/year. These naturally acidic soils limited yields for the first Euro-American farmers who settled in the Willamette Valley. Columbia Basin soils have a similar age as those in the Willamette Valley, as both are the result of the Missoula floods. However, before agricultural cultivation, Columbia Basin soils have a pH of 7.8 to 8.2. The lower amount of rainfall in the Columbia Basin has resulted in slower natural soil acidification.