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Soil pH Testing

taking a soil sample
Combine soil from similarly marked areas into one composite sample.

Soil pH governs availability of nutrients to plants and also affects activity of soil microorganisms. Parts of the Northeast, Midwest, Rocky Mountains, Texas, Great Plains, Southwest, coastal Florida and many urban areas throughout the country have alkaline (high pH) soil.

Trees such as pin oak, sweet gum, red maple, queen palm, many pines and some others develop chlorosis and grow poorly in these soils. Some, like junipers and goldenchaintree, prefer alkaline soil. Choose trees that are adapted to alkaline soils if the pH is above 7.

Do not guess the pH. Have it tested! It is a simple, quick and inexpensive test. Consider using two different labs to test the soil. If results do not agree, use a third lab.

A pH test should be conducted in several areas of the site (see figure to right), wherever soil color or texture appears distinctly different. Site pH may vary too much to plant the same species or cultivar across the entire job site. To collect samples for testing from an open area such as a lawn where soil may be fairly uniform, dig about 10 small holes five to ten feet apart with a trowel or shovel. Remove a slice of soil from the side of each hole from the surface down to 12 inches deep.

You might choose to use a soil coring device to collect the samples if one is available. Mix soil together in a clean plastic bag or clean bucket or jar and take a subsample (about a pint) to a lab to be tested. The test report you obtain from the lab will include corrective measures for adjusting pH if needed.

If soil at the site is not uniform because of natural soil variability or significant soil disturbance, several samples are needed. For example, urban sites can be so diverse and variable that one soil test may be required for each island in a parking lot in order to select the right tree for the project. One tree may not be suited for the entire project due to soil variability. To plant the same tree species over an entire site with variable soil pH, choose one that tolerates the highest and lowest pH at the site.

Most trees can grow in soils with a pH between 4.8 and 7.2. If soil pH is less than 4.8, check with local specialists regarding proper species selection. Trees planted in a soil with a pH of between 7.2 and 7.8 should be tolerant of slightly alkaline conditions. Only trees tolerant of alkaline soil should be planted in soil with a pH above 7.8. Few trees grow well in soils with a pH above 9.0.

As you dig soil cores, take note of its color and texture. Indicate on your landscape sketch which areas contain loose soil and which contain dense clay or other types of soil. If you are unable to do this, hire someone who can because this procedure provides important information to the manager of the completed landscape. For example, trees planted in loose, well drained areas may need more frequent irrigation until they are well established. Those in poorly drained, clayey soil may need less water and could even drown before they become established.