Are soil pH Meters Accurate?
What is soil pH Meter
A variety of garden soil 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.
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 value is of limited use to gardeners.
Important: I am not including lab grade instruments here, which can be very accurate – we are talking about the electronic soil pH meters being sold to gardeners.
It turns out that none of the garden grade soil meters will give you a result that is accurate enough.
Soil Science for Gardeners book by Robert Pavlis
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. 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 soil pH meters 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.