Soil pH tests determine the acidity or alkalinity of the planting soil. Soil with a pH of 7 is classified as neutral. Soil with a pH of less than 7 is acidic, while soil with a pH greater than 7 is alkaline or basic. Most garden plants grow adequately in soil with a pH of 6.0 to 7.5. However, some plants such as blueberries and azaleas prefer an acidic soil with a pH of about 5. Most plants will not tolerate soil with a pH greater than 8.3 or lower than 4.6. There are several methods to determine the pH of planting soil.
Collect a sample of the soil to test. If garden soil is tested, dig up 10 soil samples from the garden area. Dig samples 6 to 8 inches deep and avoid roots, rocks and plant debris. Place the 10 samples in a clean plastic bag and mix thoroughly. Pour the soil into a pan or plate and dry at room temperature. When the soil is air dry, place 1 to 2 cups in a clean plastic bag. Label the bag with the date and the location of the garden area.
Ship the soil sample to a soil-testing laboratory to determine the soil pH. Often universities with agriculture programs have testing labs. Commercial labs provide the most accurate pH test results. Soil test kits purchased at garden stores are often designed primarily to test acidic soils. For the best results commercially test alkaline soil.
At a garden centre, buy a container of soil pH test strips, which are easy to use and give a general indication of the soil pH. To use most test strips, place 1 to 3 tbsp of soil into a plastic container. Add the same volume of distilled water and mix well for 1 minute. Let the soil mixture stand for at least 30 minutes. Pour the soil mixture through a coffee filter and collect the fluid. Put the test strip into the fluid and compare the colour with the colour chart.
Purchase a pH meter at a garden supply centre. The meters differ is size and shape and give an estimate of the soil pH. Follow the package directions. Many pH meters must be calibrated with a buffer solution. To test soil, mix 1/8 cup of distilled water with 1/8 cup of soil. Shake the solution and let it settle for 10 to 15 minutes. Insert the pH meter into the solution and read the pH. After the test, rinse the electrode with distilled water.
Purchase a soil test kit to determine the pH. Following the package directions, pour pH indicator solution into a test tube and add the soil sample. Mix and note the colour change. Compare the colour of the solution with the colour chart.
- University of Rhode Island Landscape Horticulture Program; GreenShare fact Sheets; Soil Testing
- Lincoln County Master Gardeners; Soil pH Testing Procedures
- Colorado State University Extension: Soil pH; David Whiting, et al; November 2010
- USDA; Soil Quality Test Kit Guide; August 1999
- University of Texas: Describing Soils; William E. Doolittle; July 2004