Often not considered, soil pH has as much effect on plant health as disease, insects, drought and drainage.
Keeping pH in the right range for certain plants is essential, according to Weston Miller, Oregon State University Extension Service horticulturist. Plants with specific pH requirements include vegetables, blueberries and rhododendrons.
Soil pH is a measurement of acidity or alkalinity and measured on a scale from 0 to 14, with 7 being neutral. Lower numbers equal a more acidic or sour soil and higher numbers more alkaline or sweet.
“The key to pH is that it is going to influence the key nutrients available to plants – nitrogen, phosphorus and potassium – and other nutrients like calcium and boron,” Miller said. “When pH is in the right range, it makes it easier for the plants to uptake the nutrients they need.”
Both the inherent minerals present in native soil and annual precipitation (rain and snow) affect soil pH. In western Oregon, rain tends to leach out minerals and results in soils that become more acidic. In eastern Oregon soils are more alkaline.
On the west side of the Cascades, pH greater than 7.0 is uncommon, so gardeners typically only acidify their soil for growing acid-loving plants like rhododendrons and blueberries. Soils on the east side of the Cascades are alkaline, and may need to be acidified for all pH-sensitive plants, Miller said.
Plants differ in their preferences for soil pH, Miller said. In general, most plants grow best in a neutral soil pH, although there are important exceptions. For example, blueberries, azaleas and rhododendrons do well in an acidic soil between 4.5 and 5.5. Lawns favor a pH of 5.5 to 6. Roses do best in soils with a neutral pH of 6.5 to 7. Similarly, vegetables prefer a slightly acid to neutral pH of 6 to 7.
When pH levels are too high or too low for pH-sensitive plants, adding more fertilizer won’t do any good, Miller explained.
For pH-sensitive crops, get your soil tested before you plant. Collect soil from your garden from the areas where you intend to grow pH-sensitive plants. Learn how in the Extension publication A Guide to Taking Soil Samples for Farms and Gardens.
For a list of labs, see the Extension publication Analytical Laboratories Serving Oregon. Ask if they provide interpretation for the kinds of pH sensitive plants you intend to grow. If you get test results without the interpretation, you can use this guide or go to an Extension office and talk to an OSU Extension Master Gardner.
Often gardeners aren’t aware of the importance of pH, but it can make quite a difference when growing blueberries.
“The biggest mistake people make with blueberries is that people don’t adjust the pH before planting,” Miller said. “It’s important that the pH is in the correct range. That’s the range where nutrient uptake, especially nitrogen is highest for this acid-loving plant.”
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, refer to the Extension publication Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden, which includes a chart of the pH needed by various plants.
For new vegetable gardens, add lime (to increase pH) and sulfur (to lower pH) in the fall to give the material time to change the soil chemistry. Follow the recommendations from your soil test. In lieu of a soil test, follow the instructions on lime or sulfur product packages.
For existing vegetable gardens, take note of your yields and the health of your crops. If you’re wondering why your crops are not healthier or more bountiful, your soil pH might be off. Perform a soil test as needed.
Miller recommends changing soil pH gradually over several years for vegetable gardens.
“Two small applications of lime a year apart are better than a single large application,” he said. “Soil pH reactions may take a year or more to complete, so check soil pH annually to monitor change and check pH at the same time each year, as soil pH varies seasonally.”
For more guidance on pH and a deep dive into soil check out these Extension resources:
Soil: The Dirty Secrets of a Living Landscape, Living on the Land: Managing Soil pH, Testing Soil pH, and Improving Garden Soils with Compost.