How to Test Soil pH

The two primary ways to test soil pH from field samples are slurry testing and direct soil testing. It is important that the soil samples and tests take place in the same spots and the same way every time.

Slurry pH Testing

The slurry method allows you to get a representative sample and measurement of an entire area with just one test. Because soil pH can vary within a small area, be sure to take a representative sample. The soil should be taken from the same depth below the surface each time you test.

When using the slurry method, take soil from next to the plants, as well as some from further away. (Keep these two samples separate.) While this means a little extra work, you will get measurements that are more accurate since the amount of nutrients, types of soil, and moisture content can vary across a planted area.

All these things affect the pH of soil, so it’s important to track your pH at many points.

How to Test Soil Using the Slurry Method

1. Gather some soil from the test area.
2. Take the homogeneous sample and add equal parts of soil and distilled or deionized (DI) water in a 1:1 ratio. So, for 25 grams of soil you would add 25 mL of water.
3. Stir the sample for 5 seconds.
4. Let it sit for 15 minutes.
5. Start stirring the sample again after 15 minutes, and take your measurement.

Direct Soil pH Testing

Direct soil pH testing gives you the benefit of not needing to take soil samples because the pH is tested right in the ground.

How to Test Directly in Soil

1. Using an auger or ruler, first put a hole down into the soil. The hole needs to be the same depth each time you test to avoid pH discrepancies.
2. Add some distilled or deionized water to the hole; the soil should be damp but not saturated with water.
3. Insert your testing instrument into the hole, and allow for the reading to develop or stabilise.

Simple!

Soil pH Testing Options

Now that we have gone over soil and how important pH is, let’s talk about what you can use to test your soil pH.  We’ve narrowed it down to four main groups: test strips, chemical soil test kits, digital pocket testers, and portable soil testing meters.

pH Test Strips

Pros: Easy to use, inexpensive
Cons: Hard to read, loss of precision, hidden costs

pH test strips (aka litmus paper) are paper strips that have been saturated with pH-sensitive dyes. When exposed to a damp substance, the strips will change colour relative to that substance’s pH. This colour change corresponds to a colour chart provided with the test strips. This method for testing is quick, easy, and inexpensive, but it does have some disadvantages.

Soil is very dark in colour, even when mixed with water. The muddy colour could stain the test strips and make them hard to read. Even when a colour change can be seen, it’s subjective since colours can look different depending on the lighting, as well as from person to person. This leads to inconsistent and poor results.

Test strips will not give the most accurate test results either; they only have a resolution of 0.5 pH units. This means the closest your test results can get to your soil’s true pH would be 0.5 pH +/-. Being 0.5 pH units off means a greater cost to treat the soil. If the soil treatment is not accurate you can have low crop yield and dead plants.

Soil pH Chemical Test Kits

Pros: Easy to use, all-inclusive
Cons: Multiple kits needed, hard to read & dispose of, a limited number of tests

pH chemical test kits are like test strips in that they are easy to use, but also have several drawbacks. Using a soil test kit involves adding your soil, distilled or DI water, and some chemicals (these will be included when you purchase a kit) to a tube. The chemicals, like test strips, react with the pH levels in your sample to create a colour change. Also like test strips, the colour change of the test kits will be subjective, and readings will vary between different people.

pH test kits have lower resolution, generally between 1 or 0.5 pH points, and tend to test specific ranges of pH. That means you need to buy many kits to test your different types of soil, or when you are just beginning and don’t know your starting pH.

The number of tests you perform with a chemical test kit limited to the number of reagents included. Regular kits include enough chemicals for anywhere between 1 and 25 tests. Disposal of these chemicals poses another issue; chemical compositions differ from manufacturer to manufacturer, and many cannot just be poured down the drain or into the trash.

Digital Soil pH Pocket Testers

Pros: Pocket size, better accuracy, easy to keep clean
Cons: Need to know how to care for the device

Soil pH pocket testers are digital, portable testing instruments that utilize a pH electrode. The integration of a pH electrode in the durable casing of a tester allows for much greater accuracy than test kits or strips. The pH electrode takes a pH reading in your soil or soil slurry and displays it on an LCD screen.

Testers have fewer things to interfere with taking a reading compared to test kits and strips. You no longer have to worry about the dark soil sample interfering with colour change, or the subjectivity of color change tests in general. Many testers also have a much higher resolution and accuracy than chemical options, generally between 0.1 and 0.01 pH units.

Soil pH testers allow for easy testing out in the field, with simple, single-handed, one- or two-button functions. Some testers have special features like waterproof capabilities or durable bodies which let you test in humid environments without affecting the readings.

Certain pocket testers also feature a cloth junction, which helps prevent clogging of the electrode. To unclog soil particles from the junction, the cloth can be gently tugged with tweezers to remove debris and uncover fresh cloth in the junction. This gives you more accurate readings and longer electrode life.

As the temperature changes, the way the electrode behaves will also change; this can affect your pH readings. To help counter this, some pH soil testers have automatic temperature compensation, a feature that allows the device to correct the error.

These handy instruments do take a little bit more know-how to operate and properly care for. You’ll need solutions to keep the electrode calibrated, hydrated, and cleaned. More information on care for a pH electrode can be found under Care and Maintenance below.

Portable Soil pH Meters

Pros: Portable laboratory accuracy, customizable, no more guesswork
Cons: Larger investment, more technical

Portable soil pH meters are the next step up from soil pH testers. They are a convenient way to have laboratory accuracy in field testing. A bit larger than the testers, portable soil pH meters offer many functions from multiparameter testing to data logging.

All choice portable soil pH meters have automatic temperature compensation; they will come with either an integrated temperature sensor, or a separate temperature probe. Portable soil pH meter measurements are nearly exact, with resolutions as low as 0.001 pH units. Both of these functions give you readings with much greater accuracy.

If you need to report your pH values, a portable soil pH meter is a great choice. Some of these meters are able to provide Good Laboratory Practices (GLP) data, which includes things such as date, time, calibration data, and logged data. This gives traceability to your readings.

Certain soil pH meters come with an electrode diagnostics feature called CAL Check™. The meter will check the condition of the electrode and pH buffers during calibration. Some of these meters also come with a HELP button which will bring up tutorials right on the screen.

Looking for something smaller than a typical portable soil pH meter, but need the accuracy of one? Some portable soil pH meters do not even need a regular meter! There are electrodes on the market that can wirelessly link to a smartphone or tablet.

Care & Maintenance of Your Soil pH Electrode

Proper care and maintenance of your pH electrode is essential. Appropriate care of electrodes will extend its useful life. Our maintenance motto will help you to remember the three main concepts in electrode maintenance: Clean Regularly, Calibrate Often, and Condition Always.

Clean Regularly

When testing the pH of soil, it’s important to properly clean the pH electrode since soil can clog the junction. If soil gets stuck on the electrode, do not wipe it! Instead, rinse the electrode with distilled water.

Is the soil really sticking to the electrode? Soak it in a cleaning solution specially formulated for soil or humus deposits . Both solutions help to remove residue left behind after rinsing the electrodes with distilled water. The cleaning solution for soil deposits is great for general agricultural samples; the humus solution is the best for highly organic soils (such as compost).

Cleaning the electrode will give you maximum efficiency and accuracy when taking pH readings. After using a cleaning solution, the electrode should be placed in a storage solution for at least one hour before using it again. The cleaning solutions are available in disposable one-use-only packets as well as bottles.

Electrode Cleaning Steps

1. Fill a squeeze bottle or spray bottle with deionized (DI) or distilled water.
2. Using the bottle, rinse down the electrode with the water.
3. Gently shake down the electrode to remove residual water.
4. The electrode is ready for use or storage!
5. Further Cleaning:
   
   1. Rinse the electrode with DI or distilled water.
   2. Let the electrode soak in the cleaning solution for at least 15 minutes. Use a cleaning solution formulated for soil deposits or humus deposits.
   3. Remove the electrode from the cleaning solution.
   4. Rinse the electrode with DI or distilled water.
   5. Place the electrode in a storage solution for at least 1 hour before using it again.

Calibrate Often

Calibrating your electrode will give you the greatest accuracy when testing pH. Calibration will help to correct your electrode as its response changes over time, due to aging and other factors.

Electrode response changes due to several factors. It’s important to calibrate to at least two pH points which brackets your expected pH value. Bracketing simply means calibrating to one pH point below the expected range, and one pH point above the expected range. (For example, if your expected reading is pH 8.6, then pH 7 and pH 10 buffers should be used.)

Electrode Calibration Steps

1. Use a spray or squeeze bottle and rinse down the electrode with DI or distilled water.
2. If using a disposable one-use-only packet of pH buffer, tear or cut open the packet.
   
   1. Enter calibration mode on your soil pH meter.
   2. Insert the electrode until the bulb and junction on the side of the electrode are covered by the buffer.
   3. Let the reading stabilize and accept the buffer.
   4. Remove the electrode from the buffer and rinse it again.
   5. Repeat these steps for the other pH buffers.
   6. When finished, exit calibration mode.
3. If using a bottle of pH buffer, pour the buffer into a clean beaker and place a magnetic stir bar in the beaker.
   
   1. Place the beaker on a stir plate to keep the buffer stirring while taking measurements.
   2. Enter calibration mode on your soil pH meter.
   3. Insert the electrode until the bulb and junction on the side of the electrode are covered by the buffer.
   4. Let the reading stabilize and accept the buffer.
   5. Remove the electrode from the buffer, and rinse it again.
   6. Repeat these steps for the other pH buffers.
   7. When finished, exit calibration mode.

Condition Always

The most important part of the pH electrode is the sensing bulb at the bottom. The bulb is made of glass that is sensitive (responsive) to hydrogen ions. It’s important to maintain equilibrium in the electrode to keep your readings stable by keeping the bulb hydrated.

Proper hydrating means always storing your electrode in a storage solution. Storage in other liquids, like distilled or DI water, can damage the glass bulb and cause slow, inaccurate pH readings.

Electrode Conditioning Steps

1. Rinse the electrode with distilled or deionized water.
2. You can use either a disposable one-use-only packet of conditioning solution, or a beaker with some of the solution poured into it.
3. Place the electrode in the conditioning solution.  (It’s important to make sure the junction on the side of the electrode is covered by the conditioning solution. Both the pH glass bulb and the junction need to be hydrated for the probe to work!)
4. Let the pH electrode sit in the conditioning solution from an hour to overnight to completely rehydrate.