How to test soil pH at home

Most of the testing we do requires expensive equipment and specialized knowledge, but what about some of the more basic testing? Anyone can buy pH strips at the aquarium store and probes are relatively inexpensive. Knowing this, is sending samples off to a lab really necessary? This article will walk you through each step of the process to help you determine which choice is right for you.

Understanding pH

When we think about pH, most of us understand that we are looking at acidity and alkalinity. We might know the ranges preferred by certain plants or that distilled water is neutral; But when it comes to understanding how these measurements are made and what they really mean, even the most seasoned growers may find themselves scratching their heads in confusion. While you don’t need a PhD in chemistry to have a successful crop, knowing how to read your pH test results will allow you to make the most productive amendments for your plants’ needs.

So what is pH?

Whether using soil or a hydroponic system, pH can only be measured from an aqueous solution. As a measurement of “potential Hydrogen” we want to know how much H+ (hydrogen ion, acid) is available in water with respect to OH- (hydroxyl ion, base). Whether the solution can accept or donate H+ ions determines how it reacts with chemicals in the soil. Because soil chemistry is critical to providing plants with vital nutrients, it is important to make sure any soil amendments react in a way that is available to the plant. The pH values of common liquids are shown in figure 1 as a reference.

How is pH tested (at home or in a lab)?

The American Society for Testing and Materials (ASTM) allows the use of pH sensitive paper or test strips when a rough estimate is all that is needed (ASTM D4972 Method B). However, for more accurate results a pH sensitive electrode potentiometer is preferred (ASTM D4972 Method A) and is necessary when cloudiness of the water skews the color indicator on the test strip. To do this we soak at least 10 g of 2mm or less (number 10 sieve) air-dried soil particles in a 1:1 ratio with water (10 mL) to dissolve the soluble ions in the soil. After at least one hour the solutions is ready for pH testing. For a soilless medium the fertilizer-water solution you use on your plants can be tested directly. You can do a pH test at home by inserting your test strips or probe directly in the 1:1 water-soil slurry, however the sediment may make the test difficult to read or give widely varying results due to the “suspension effect.”

What do my results mean?

If your test comes back higher than 7 you have more alkaline soil, while readings lower than 7 indicate acidic soil. Most soil in high rainfall areas such as the Eastern US and Pacific Northwest are slightly acidic. This is due to leaching of the alkaline elements from the soil and the decay of greater amounts of plant material. Dryer areas such as the American West tend to be slightly basic with higher pH being more common in soils with high clay content. Despite these trends, soils within regions can vary greatly and can also change with time. Once you know the pH of your soil you can check the list below for the preferred pH of common plants and make amendments to your soil as needed. If you are growing a variety of vegetables a pH of between 6.0 and 6.5 is usually ideal for most plants. To raise pH, add lime or wood ash. To lower it, add sulfur, peat, or compost.

FAQs for pH testing

1. What is the “suspension effect” and how can I avoid it?

The suspension effect refers to the difference in pH between the sediment and the water in a soil slurry solution. This can cause problems if you have a sample with many fine clay particles that remain suspended in the aqueous solution. To resolve this issue an electrolyte solution such as CaCL2, KCl, or BaCl2 can be used (question 3), or the sample can be filtered prior to testing (question 4).

2. Do I have to soak my samples in a glass container?

Ideally yes, however soaking it in another non-reactive material such as porcelain or Nalgene labware is permissible for home testing. Do not use a metal container as this can affect ion exchange in the solution.

3. Should I measure pH in a Calcium Chloride Solution?

A solution of 0.01 M Calcium Chloride (CaCl2) is needed to counter the dilution effect on the exchange equilibrium of the soil slurry solution. While it is not necessary in most backyard applications, it can be helpful when there is a high concentration of metals, mineral salts, or significant amendments have been previously applied to the soil.

4. Is there a way to use pH paper with an opaque sample?

Because rinsing sediment off of the paper will change the result, the best way to get a reading is to filter the sample before submerging the pH paper. This can be done with a coffee filter, but is best with a number 42 or higher filter and a vacuum pump. When filtering, be mindful of the water absorbed by the paper to prevent losing too much of your sample. The filter paper may be dampened with DI water to prevent loses, but do not soak as the soil-water ratio may be compromised.

5. What if my clayey soil absorbs all of the water?

In this case you may need a larger sample size to get enough water to insert the probe or paper. Samples of 100-300 g should be sufficient. If not enough sample is available a higher water to soil ratio may be used, but make a note of it and do not exceed 2:1.

6. Can’t I just check the pH of my fertilizer solution?

If you are using a hydroponic, aeroponic, or aquaponic system, you may measure the water directly, however if you are using a soil based medium you will need to measure soil pH as well as water pH. This is because the soluble ions in soil can alter the pH of your fertilizer solution before the plant absorbs it.

7. Do I need to get my probe back to neutral between readings?

Yes. Because pH is a ratio and not a true value your probe will respond to the change in ratio from whatever value it started. If your probe does not yet read between 6.5 and 7.5 it could jump higher or lower than the actual pH based on the change it registers rather than the actual value. While it should eventually level off to the correct number the longer drift time could lead to premature readings. Clean the probe and place it in DI water until it reads neutral between each measurement.

8. Why is my probe drifting?

There are several reasons that a probe might drift, but a few are more common and easier to fix. Your sample could have too much sediment leading to the “suspension effect” (see question 1), or the probe may have sediments on it from a previous reading. This can be cleaned off using a chemwipe with DI water or 10% HCl for mineral buildup. Do not clean with rough bristles or scouring pad as this may cause damage to the sensor. If the probe and sample are clean, it may need to be recalibrated. This can be done using a premeasured buffer solution or pillow powder. Make sure to calibrate on both the high and low end, ideally bracketing your sample. Typical calibration buffers are around 4, 7, and 10 pH. See the user manual on your probe for specific instructions. It is a good idea to recalibrate your probe after a long period of no use or every couple of weeks for frequent use.