Soil pH Testers-Are They Accurate?

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Robert Pavlis

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 soil pH test kits made for gardeners.  How useful and accurate are the results of such tests? It is more complicated than you think. Let’s have a close look.

soil pH tester
soil pH test kit – colored dyes, source: BBC Gardening Blog

Soil pH Testers

Soil testing can be done with 3 different types of soil pH testers; electronic meters, indicator test strips and test kits (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.

pH Test Kits

These kits us colored dyes that 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 such a common test kit. These kits are very common, easy to use, but they do come with a limited supply of capsules containing the dye. Unlike the test strips, this type of kit allows you to compare the color change to the color of the soil, allowing some correction for the soils natural color.

pH test kits will only give you whole pH values with no decimal places. One advantage of this kit is that it also allows you to test the NPK of the soil.

Compost Science for Gardeners by Robert Pavlis

YouTube video

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.

Package of soil pH test strips showing 4 color spots
pH test strip, source: Michael Krahe

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.

These devices are not very accurate. You would be lucky to get a value that is +/- 1 pH unit – that is useless for adjusting soil pH.

electronic soil pH meter
electronic soil pH meter, source: London Permaculture

Buying a pH Tester

If you want to get a rough idea of pH, one of the above pH testers will give you that. Here are my choices (these are affiliate links).

YouTube video

What is pH?

pH is a measure of acidity and 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 value is of limited use to gardeners.

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 this Consumer Report video says the same thing.

YouTube video

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 try to change 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.

YouTube video

 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 is good enough. The reason for this is that most plants grow quite well in a wide pH range and 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 more 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.

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Robert Pavlis

I have been gardening my whole life and have a science background. 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!

45 thoughts on “Soil pH Testers-Are They Accurate?”

  1. Holy smokes I’m so glad I found you. I just read your post about inaccurate, (& imo) irresponsible articles all over the internet (& this has been driving me nuts for years- ever since I read multiple articles that either contradict information I concretely KNOW to be true, or articles that don’t even make sense within themselves). It’s beautiful to read work that at least makes sense & presents WHY it makes sense. I’ve struggled with pH for a couple years, now- & now I know why.

    Reply
  2. Thanks for the great information! I was wondering what this means for container gardening, especially for acid-loving plants (I’ve got a couple of blueberries in containers). I’ve heard various sources recommend test the soil pH, add some amendment if necessary, then test again, etc., until the pH is right. But if the only reliable test is to mail to a lab (or buying a lab-grade meter), it’s not practical to test each pot several times. Or are the other, less-accurate tests good enough for getting blueberry soil in the right range?

    Related: I’ve read in one of your other posts that pH amendments in the ground don’t work long term, because the mineral content of the soil causes it to revert to its previous pH. Does the same apply to potting mix in containers? If I buy a potting mix designed for acidic-loving plants, will it lose its acidity with time, and if so how quickly?

    Reply
    • A good garden grade pH test can tell the difference between a pH of 7 and 6. Use that.

      The reason amendments in the ground don’t work well is because the soil has a big buffer capacity, which means it tends to revert the pH to the normal pH of the soil. There is no soil in potting mixes, so they don’t behave the same way. The pH depends more the pH of the water and the fertilizer you use, as well as the original potting mix.

      Reply
  3. I am a green keeper (in training) and have had the green lab tested. But i was interested about ph for other places. This information was great, i realy like the way you answered questions. Nice job.

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  4. Read unverified info on net that acid soil deepens blue colour of Meconopsis betonicifolia, so TEMPORARILY acidify at a stage in growth (I don’t know when or how to tell yet!) with water-diluted sulfur ? . I’ll read more RHS.org articles and this site.I

    Yes,different species have preferred nutrient ratios that are affected by Ph. Acidifying is temporary till after- or a short while before – flowering’s finished so’s not to cause a nutrient deficiency.
    I hate it when photos are tinkered with to make the plant look better.maybe photos of Meconopsis flowers vary because grown in different Ph conditions ?

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  5. Am confused about the pH testing and if one is unsure of its accuracy then I cannot now purchase one. I intended to do so since I am in London where I have the opportunity to purchase one. I have a garden where I have been growing vegetables and root plants like yams but I have not been able to get good yield over the years. Vegetables are spotty, yellow or brown leaves and the root crops are small like potatoes. However cassavas do well on the soil. I use compost and animal manure. I do not have the facility of a local lab as I live in Nigeria

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  6. Rhodis have dense roots that are not too extensive. I would think a thick pine needle mulch ought to do it for you to grow them.

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    • If your believing that “pine needles” add acidity to the soil then I believe that you would be incorrect UNLESS they are fresh / green needles. The naturally fallen needles to the ground I believe have been proven time and time again to be at a neutral Ph of 7. It’s just another myth that using pine needles for a mulch will increase the acidity of the soil.

      Reply
  7. What if my tap water is 8.5?. I do raided beds. Soil is basic Midwest farm dirt. Using cheap meters and distilled water to test. Still in high side. 7.9+-. I’m looking for a quality soil ph tester. I don’t have time to get a pro test done.
    To point. Since tap water very High ph and hard. Any suggestions. Crops mainly tomato’s and strawberries. And sweet potatoes. < these do well since I sulfur the hell out of them. Willy nilly. So to speak. I do green beans too but I Sudbury them to. Trying to find a balance. But tap water is hell on house plants. 8.5+ ph. I’ll random but that’s me.

    Reply
  8. Pingback: Cannabis pH: Getting This Wrong Can Ruin Your Plants
  9. how do i get moisture and Ph value at the same time from this sensor by connecting it with arduino not using its toggle switch?

    Reply
      • Arduino is a hobby micro processor that can be programmed for many useful functions. It can be interfaced with many things such as a moisture sensor, temp sensor. But the fact that this guy is asking this question shows he only enough to be dangerous. Why would you need to monitor PH constantly?

        Reply

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