Commercial and recreational gardeners are showing a growing interest in taking accurate pH measurement of soil samples. The pH of soil indicates more than its alkalinity or acidity strength; it affects the relative availability of nutrients, the soil life, and the type of plants that will thrive.
The common range of soil pH varies from 4.0 to 8.0; the range of soil pH for optimal availability of plant nutrients is 6.0 to 7.0. The ability of soil to provide adequate nutrition to the plant depends upon the following factors:
• Essential elements in the soil—The nutrients present in soil depend upon the elemental nature of the soil and the organic material content. Soil nutrients exist both as complex insoluble compounds (organic materials) and as simple soluble forms.
• Release of nutrients to plants—Simple elements in the soil are readily available for plant uptake. The complex forms (organic materials) must be broken down through decomposition to simpler, more available forms to benefit the plants.
• pH of the soil solution—pH directly affects the availability of essential nutrients. For example, though iron, manganese, and zinc become less available as the pH rises above 6.5, molybdenum and phosphorus become more available. When the soil is acidic, minerals such as zinc, aluminum, manganese, copper, and cobalt become more soluble for plants’ uptake. However, an excess of these ions can be toxic to plants. Alkaline soil contains a higher quantity of bicarbonate ions, which interferes with the normal uptake of other ions, harming plant growth.
Soil life refers to living organisms that live in the soil and break down the organic materials. Soil bacteria that assist in the decomposition of organic material thrive at a pH of 6.3 to 6.8. Fungi and mold prefer a more acidic soil, making soil more prone to souring and putrefication.
Plants also have different soil pH preferences—several gardeners’ web sites offer charts of preferred pH levels for different plants. Knowing the pH of soil can help you choose the correct plants and the required treatment for your soil.
Testing the pH of your soil sample
Method A. LAB: To get started you will need a standard pH meter, a pH electrode, an ATC probe, a stirrer with stir bar, beaker, deionized water, and buffers. We recommend a Sure-Flow™ electrode because the free-flowing, easy-to-clean junction never clogs.
Method B. If the testing is being performed for recreational gardening, you can use a pH tester with ATC (35634-10) and buffers for calibration (35653-04).
Scoop up soil into a clean, dry plastic jar or plastic bag. Remove stones and crush any clumps of soil for better results. Gather two to three representative samples of each soil sample to confirm results. No sample preparation or preservative required.
Laboratory Testing—Method A
1. Weigh 20 g of soil sample into a 100 mL beaker.
2. Add 20 mL of deionized (DI) water and place on a stirrer to mix for 30 minutes.
3. Cover and let stand for an hour.
4. For the most accurate measurements, allow the buffers and the soil sample both to come to room temperature. (A difference in temperature will add error to your measurement.)
5. We recommend a 2-point calibration with a pH 7 and a pH 10 buffer solution. The electrode slope should be between 92 and 102%.
6. Rinse electrode and ATC with DI water and blot dry. Place probes in soil sample and measure pH and record measurement.
Alternative Testing—Method B
1. Place soil sample about ¾ full in sample jar and add distilled water to cover soil.
2. Cap jar and shake the soil vigorously a few times.
3. Let mixture stand 10 minutes to dissolve the salts in the soil.
4. Calibrate the pH tester with a pH 7 and a pH 10 buffer solution.
5. Remove the cap and place the pH tester into the wet soil slurry.
6. Measure pH and record measurement.
A minor (< ±0.5 pH) difference between results of the same soil sample indicates good technique and high confidence in results.
One of the easiest ways to correct the pH of your soil (both acidic and alkaline) is by adding compost. The alternative is to add an alkaline source (such as ground limestone) to acidic soil or an acidic source (such as pine needles or peat moss) to alkaline soil. Consult with authorities from a local agricultural extension office, local growers associations, or university before you apply chemicals to correct soil pH.