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Soil pH Testers-Are They Accurate?

Every gardening book and web site recommends that you get your soil tested and one of the main tests is for pH. You can get a professional lab to do the test, or you could use one of the convenient test kits made for gardeners.

soil pH tester

soil pH tester – colored dyes

 How useful and accurate is the information about your soil pH? It is more complicated than you think. Let’s have a close look.

Soil pH Testers

Soil testing can be done with 3 different types of soil pH testers; electronic meters, indicator test strips and chemical colored dyes. In each case you take some of your soil and mix it with water or a buffer solution provided by the pH tester. The water is then tested.

Chemical Colored Dyes

Colored dyes are mixed with the soil water and the resulting color is compared to a supplied chart to determine the pH level. The above image shows a pH tester of this type.

soil pH test strip

pH test strip

pH Test Strips

pH test strips are advanced versions of litmus paper and many people still call them this. True litmus paper is extremely inaccurate and completely useless for measuring the pH level of soil. pH test strips are more accurate since they have several color spots on each strip. The ones shown in the picture are lab grade and are much better than those sold for garden use, but they are also more expensive. I have some lab grade pH test strips that use a 3 color strip for a pH range of 5 to 10 (much better than the 0-14 range in the picture). The color differences between 6.5 and 7.5 are so minor that I could not tell them apart with any kind of confidence.

 

 

electronic soil pH meter

electronic soil pH meter

Electronic pH Meter

A variety of garden pH meters are available. The probe that comes with them is inserted into the soil water, and the pH can be read directly from a display. The really cheap models come with a metallic probe and the instructions suggest that you insert this directly in the soil. Yes this is more convenient, but you will never get a useful reading without first making the water solution as described above.

 

 

 

 

 

 What is pH?

pH is a measure of acidity. pH is reported as a number between 0 and 14. Anything below 7 is acidic and anything above 7 is alkaline. A value of 7 is neutral, ie it is neither acidic or alkaline. This is fairly common knowledge, but what most people don’t know is that pH is measured on a logarithmic scale (do you remember high school math?). What this means is that a pH of 5 is 10 times more acidic that a pH of 6. Worded slightly differently, a pH change of 1 unit is actually an acidity change of 10. A change of 2 numbers, example 5 to 7, is a change of 100, which is a huge change.

 So You are Thinking ‘Big Deal’ – Why do I Care it is Logarithmic?

The reason is that a small change in pH numbers is actually a large change in acidity. Unless pH is measured to at least one decimal place, the accurately is of limited use to a gardener.

 If you have a look at the pictures above you can easily see that they don’t measure pH to 1 decimal place. In fact in many cases the color change is so small that you’d be hard pressed getting the whole number correct. You might expect that the electronic pH meters are more accurate, but they aren’t. The probes and electronics they use are just not good enough.

Important: I am not including lab grade instruments here, which can be very accurate – we are talking about the electronic pH testers being sold to gardeners.

It turns out that none of the garden grade pH testers will give you a result that is accurate enough. A good comparison of several options is detailed at this site http://www.waynesthisandthat.com/soiltest.htm and a Consumer Report video saying the same thing can be found here:

Note: I do not agree with the fact that most people need to add lime or sulfur to adjust pH for a lawn, as promoted in this video. Without a soil test from a lab–don’t monkey with the soil pH.

What are Your Options?

You can get an accurate soil pH test done by a professional lab. Their results are reliable. You can also talk to local gardeners. Are they successful with acid loving plants like rhododendrons, and blueberry bushes? If they are, the local soil is acidic. If they are not, it is probably neutral or alkaline.

 Is it Important to Know the pH of Your Soil Accurately?

The answer to that question depends very much on what you will do with the information. If you use the information to select plants for your garden, then you don’t need to know the pH very accurately. Knowing it is very acidic, slightly acidic, slightly alkaline or very alkaline it good enough. The reason for this is that most plants grow quite well in a wide pH range. Most plants will grow in slightly acidic, neutral and slightly alkaline pH. That is a range of approximately 6 to 7.5. Fortunately for us gardeners, most of us are in this range.

I have a pH of 7.4 (measured with a lab grade instrument) and I can grow some ‘acid loving’ plants, but not most of them – they just don’t do well. However, I can grow many plants that ‘prefer’ an acidic soil because these plants can also grow in slightly alkaline conditions, namely a pH of 7.4. Most plants are quite adaptable.

If you only want to know the approximate pH range you have in your garden, the pH testers mentioned above may be accurate enough.

pH measurement accuracy is important if you plan to change the pH of your soil. Remember a pH change of 1 is actually an acidity change of 10, so it is easy to ‘over treat’ your soil if you don’t start with accurate values. Besides being wasteful it can seriously damage your plants.

Accuracy is also critical when adding Lime to make your soil less acidic as discussed in Adding Lime to Acidic Soil.

Don’t Adjust pH.

My own philosophy is simple. There are so many plants that will grow in the soil pH I have – why should I change it and create all kinds of headaches for myself? I’d love to grow Rodos (rhododendrons), but I can’t and I have come to accept it – on most days anyways.

References:

1) Photo Source for ‘soil pH tester – colored dyes’: BBC Gardening Blog

2) Photo Source for ‘pH test strip’: Michael Krahe

3) Photo Source for ‘electronic soil pH meter’: London Permaculture

Robert Pavlis
Editor of GardenMyths.com ,
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

24 Responses to 'Soil pH Testers-Are They Accurate?'

  1. To grow blueberries, you need an acid soil. This is accomplished by using almost pure peat moss and the addition of sulfur in a large pot. It works. Now I have to figure out how to stop the jays from eating all the blueberries. They are very smart and wait until they are ripe.

  2. Michael Dizon says:

    An informative post! As for me, to ensure accuracy I have bought a digital pH meter from a http://www.gainexpress.com. I used the code 10SPECIAL to avail discounts on all of their items. The quality is good and I have never been this contented on my purchase!!

  3. Camilla keeling says:

    So, this is all fascinating, but could you just tell us which is the cheapest, simplest way to test our soil – logarhrits notwithstanding to just try to prevent feeding tomatoes too much of the wrong nutrient? I had a little kit – perhaps inaccurate for your extreme requirements but fine for adjusting my ten tomatoes plants food and its empty … Can’t see anything similar on the uk market and am looking for simple advice? Of the three options you started with … Which is best? Thanknyou

  4. janet says:

    Don’t give up on those Rhododendrons just yet. The Dallas Planting guide recommends the following raised bed mix for our clay-based soil that do not support Azaleas: “mix planting medium with 60% finely-milled pine bark or pecan shells, 40% coarse Canadian peat moss, and one pound micro-nutrient fertilizer per cubic yard….do not skimp on any mixing or moisture. If peat is not completely wet, your plants will not live.” I live west of Dallas in an even more alkaline area and have been successful with this method except that I added acidified compost as the fertilizer and I don’t think my pine bark was finely milled.

  5. Howard McPherson says:

    The pH of an aqueous solution is the negative log of the hydrogen ion concentration in moles of hydrogen ions per liter of solution. A solution below a pH of 7 is acidic, while a pH above 7 is alkaline (basic). pH 7 is neutral. Normally when you measure the pH of a solution you do not dilute the solution with water. Mixing water with soil would not give an accurate pH simply because the sample has been diluted from its original value. There are various types of narrow range pH in which you dip the paper into the test solution and compare the set of colors (not just one color) to estimate the best value. Wetting the pH paper with water before testing the unknown sample is useless. The wide range pH 1-14 paper gives a very approximate value which is probably useless for soil testing.

    • That is all true, except for the soil statement. I don’t see how you can measure the pH of soil without adding water, since pH requires water to be present in order to dissociate the ions. The standard lab procedure for measuring soils is to add either water or a buffer, mix, and then measure the pH.

  6. hotwired64 says:

    I grow strawberries in raised beds. I have a 5.0 pH target. I feed fast acting sulfur through my soaker lines and monitor my pH weekly. I bought a $2K Thermo Scientific pH meter at a bankruptcy sale, so I get pretty accurate readings. I have 3500 plants, and I can tell you from experience that as pH reaches in excess of 6.5 Everbearing suddenly become Neverbearing. I am pretty successful at correcting pH. I love your blog, but don’t always agree. If I had to only grow only what my natural soil pH would support, I’d have to learn to eat rocks.

    • My posts are generally targeted to general gardeners and not someone like you who is willing and able to operate in unusual situations. Most others would not spend the money to buy the pH meter, and they definitely would not measure pH weekly. A lot can be done if one puts their mind to it.

      I have grown everbearing strawberries at pH 7.2, but I am not saying I was doing a great job. They did less well in my current soil at a similar pH, so maybe I need to acidify??

    • Sam says:

      Sounds interesting, what’s your general procedure for measuring the pH? Was it something you developed yourself?

  7. Don Lauer says:

    Mr.Pavlis; Hello from Va. Thank-you for the pH tutorial refresher.You might tell your followers that the local water and wastewater plants in every region has professional pH meters.I work at a wastewater facility and have been ask several times to perform soil pH tests.We are always happy to help. Don L.

  8. Mathilde says:

    A really interesting article & one that has helped me. Thank you. I now know of your website &will keep on reading.

  9. GreenEye says:

    The only option then is to buy ericaceous compost and grow in containers.
    Growing in 100% peat will be acidic and peat will acidify a soil mix for a couple of years, but it shouldn’t be used anyway – to conserve habitats.

  10. GreenEye says:

    The options are:
    Get soil tested by laboratory
    Buy laboratory grade testing equipment
    Guess, use acidifying fertilizers & chemicals (sulphur) and/or matter (peat, spahgnum moss, pine needles) and hope for the best
    Buy premixed acid compost (ericaceous)

    • Robert Pavlis says:

      The first two options make sense, although #2 is expensive. Your third option, guess, is not a good idea. A much better option is to stop trying to change pH, and learn to live with the soil you have. Find out what grows and what does not. Don’t grow things that do not grow.

      By the way, peat, sphagnum and pine needles do not acidify soil. Sulphur is the only reliable way to acidify soil.

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