Have you ever wondered why you and your neighbors have the same hydrangea plant, but your blooms are pink while theirs are blue? The answer lies in the pH level of the soil. Soil provides the raw material from which gardens grow, but the pH level of the soil determines which nutrients are available to the plants. Improper pH can result in insufficient or excessive nutrient availability (toxicity) and can create conditions that promote disease.
What is a pH level?
The pH level measures the amount of hydrogen ion concentration in the soil. The more hydrogen the soil contains, the lower the pH level and the more acidic the soil. Levels are measured on a scale of 0-14. A perfectly neutral soil measures 7, the same as pure water. Soil with a pH level of 0-6.9 is acidic, while soil with a pH level of 7.1-14 is alkaline.
The acceptable pH range for all-around gardening is 6.2-7.2, slightly acidic to barely alkaline. This range provides the best mix of nutrients for most plants. A pH of less than 5.5 renders many plant minerals (nitrogen, calcium, potassium) required for healthy growth unavailable. Other nutrients (the same minerals that pose toxicity problems in humans zinc, iron, copper, aluminum) become toxic when the pH approaches 5.0.
Some plants prefer acid soil: blueberries, camellias, tomatoes, and azaleas. A few plants prefer alkaline soil: forsythia, onions, cabbage, and cauliflower. Hydrangeas react uniquely to soil pH by producing blooms ranging in color from blue in acidic soil to pink in alkaline soil.
In general, the soil is more acidic in heavy rainfall areas such as the Pacific Northwest and the East coast. Arid climates produce soil that is more alkaline. New subdivisions everywhere tend to have soil toward the alkaline end of the pH scale since the top layer of soil was bulldozed during construction.
Testing the Soil
The only way to know with certainty whether the soil is acidic or alkaline is to test the soil. Soil tests measure the exact chemical composition of the soil. At a minimum, test pH levels when a garden underperforms and before preparing a new garden bed. Professionals recommend routine testing every two to three years, in either the fall or the early spring.
Testing in the fall allows plenty of time to adjust the pH before the blooming season. Garden and home improvement centers carry a variety of inexpensive do-it-yourself tests, but these tests may prove less accurate than lab tests. Some garden centers will test your soil for free hoping that you will buy materials to alter the soil from them.
Gathering Soil Samples
The first step in soil testing is to gather a good soil sample. Gathering the soil properly is the most important factor in receiving accurate test results. Professionals recommend at least 10 samples from each garden or area you intend to turn into a bed, because pH levels vary from one area to the next.
The test is performed on a combination of the soil from each of the smaller samples. Work with slightly damp soil, and always wear gloves when touching the soil. To gather a good sample, follow the instructions below:
- Remove any debris and loose matter from the top of the ground around the sample area.
- Using a bulb auger, pull a dozen plugs of soil from a depth of approximately six inches.
- Remove a one-inch section from the middle of each plug and place each smaller sample in the same plastic bag.
- Sift through the soil in the plastic bag, and remove foreign objects such as rocks, twigs, leaves, and insects.
- Crumble the soil into small bits and mix the soil well.
- Place the remaining soil on newspaper or plastic, and air-dry at room temperature. Do not apply any external heat to the soil.
- Soil tests usually require one cup or less of soil. Place the required amount of soil in an airtight plastic bag for delivery to the lab, or reserve the soil for your own testing.
Do-It-Yourself Soil Testing
Local, Internet, and mail-order garden centers carry three main types of do-it-yourself soil testing devices. Do-it-yourself testing provides quick results, but what you gain in time, you may lose in accuracy.
- Hand-held electronic pH meters Meters resemble a meat probe–a box with a dial or digital meter and a probe. Unlike the other testing methods, meters provide immediate results and are reusable. Of all the home-testing methods, pH meters provide the greatest degree of accuracy. Insert the probe into the ground, and wait for the meter to register the pH reading. The soil needs to be moist in order to register a reading. Test after a rain shower or light watering for the best results.
- Litmus paper Litmus paper provides limited useful data. It only tests for acid or alkaline soil, not for a specific pH level. Mix the soil with distilled water, and dip the litmus paper into the solution. If the litmus paper turns red, the soil is acid; if it turns blue, the soil is alkaline.
- Testing kits Do-it-yourself testing kits include test tubes and caps, pH tablets, and a color chart. Mix a small sample with distilled water, add a pH tablet, shake the capped tube, and compare it to the color chart after the recommended waiting period. Results vary in terms of accuracy.
Laboratory Soil Testing
Send soil samples to a lab for professional analysis and proper recommendations to neutralize the soil. Labs fall into two categories: state university Cooperative Extension Services and commercial labs.
Most Cooperative Extensions offer inexpensive soil testing and provide recommendations as well as personalized help. Soil types vary according to the locality, and Cooperative Extension scientists are the theoretical and practical experts on local soil properties.
They offer a wide variety of free and inexpensive community services–one of the best-kept secrets of the universe. Their primary function aside from research is to educate farmers and gardeners in their area. Get to know the experts at your local office, and you will have the resources of the entire Agriculture department of your state university at your disposal.
Commercial lab tests are sometimes more expensive than those offered by Cooperative Extensions, but they also offer a comprehensive range of testing. They may present one-size-fits-all recommendations, and the level of customer service and individualized follow-up varies from lab to lab. Ask the lab to help you determine which test you need and what level of support they provide before purchasing a test.
Turnaround times for both types of labs take up to several weeks, and most provide test results via email upon request. Tests show consistency in results among various labs thanks to high quality control standards, but recommendations may vary from lab to lab. If you are unsure about the recommendations or just want a second opinion, call your local Cooperative Extension for help.
Adding Amendments to the Soil
An amendment is an organic material that you add to the soil to alter its composition–in this case, to bring the soil’s pH level closer to neutral. Amendments include compost, peat moss, bark, lime, etc. In addition to altering the pH level of the soil, some organic amendments significantly improve drainage.
It is easier to neutralize an overly acid soil than it is to neutralize an alkaline soil. Highly alkaline soil may require several amendments applied over several growing seasons. If you want to start a new garden bed in an area with excessively alkaline soil, consider one of the following strategies:
- Create raised beds using neutral top soil.
- Bury containers in the alkaline soil to give the appearance of a garden bed while you alter the pH.
If you ordered a lab test, follow the recommendations included with the analysis. General recommendations follow. Be sure to follow the manufacturers’ instructions, and mix well with existing soil before planting. When working with chemicals, wear garden gloves and a dust mask. Wash your hands thoroughly after application.
Neutralizing Acid Soil
Add hydrated lime to acidic soil to increase the pH level. The amount of lime to add is dependent upon the size of the garden and the properties of the soil. For example, clay soil requires more lime than sandy soil, but sandy soil requires more frequent amendment.
Bone meal, crushed marble, ground limestone, and chalk all contain heavy concentrations of lime. Clay soils respond well to gypsum. In addition to lowering the acidity, gypsum creates a lighter soil that improves drainage.
Neutralizing Alkaline Soil
Add sulfur to alkaline soil. Use a combination of rock sulfur, peat moss, and wood chips for a longer-term solution. Each of these materials decomposes at a different rate. It is harder to restore alkaline soil, but alkaline soil levels provide the least hospitable environment for most plants. The rewards are well worth the trouble.
Adjusting soil pH is an ongoing process, not a one-time event. Many factors conspire to leach nutrients from the soil, altering the pH levels over time. Add soil testing and pH adjustment to your permanent list of garden tasks to produce healthy plants, prolific blooms, and bountiful harvests.