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.
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.
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 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.
1) Photo Source for ‘soil pH tester – colored dyes’: BBC Gardening Blog
3) Photo Source for ‘electronic soil pH meter’: London Permaculture