measuring soil pH – not that easy

The pH (acidity/alkilinity) is an important must know for growing vegetables, fruits and berries.  Blueberries notoriously love acid conditions.  Tomatoes like a pH range between 5.5 to 7.5.  Since pH can range from 1 (very acid) to 14 (very alkaline) it follows that 7 is neutral.  I have tried over the years to independently determine my soil’s pH, with little success.  I have sent samples to the extension service for general soil analysis but have always wanted to determine pH myself.

The pH strips I purchased were little help to me since I had difficulty confidently matching the color on the strip after it had been dipped into the liquid I was testing, with the pH graded color chart.  An inexpensive pH meter was also little help to me.

I decided to go upmarket and purchased a laboratory grade meter and 7.0 buffer, electrolyte cleaning and storage solutions and I again got unusual results.  This may be because I purchased the meter for a discounted price from an alternative supplier and the electrode bulb may have dried before it reached me (I subsequently learned that a dried out bulb doesn’t work) or after it reached me due to my inexperience.

I probably needed a new replacement electrode but this cost >$60 and I was not motivated to go this route.  Then it occurred to me that a) I do not need laboratory grade equipment; and b) a whole lot of decent electronic instruments have been flooding the market at much lower prices than the well known brands and should work decently.

So I studied the options and purchasers’ comments on Amazon and purchased the pH tester below for $11.10 total which included 2 packets of buffer powder.

my new ph meter
my new ph meter

I purchased a gallon of distilled water and made up the 2 buffer solutions by dissolving each packet into a separate container holding 250ml of distilled water.  The instructions said to use deionized water for preparing the buffer solutions and to use distilled water for rinsing the electrode.  I figured nothing could beat distilled water and I had no deionized water available so I made up the solutions with distilled water (more about this later).

my 2 buffer solutions - ph 6.86 and ph
my 2 buffer solutions – pH 4.01 and pH 6.86

Now the moment of truth – as instructed I rinsed the electrode with distilled water and then immersed it into the 6.86 pH solution.  The meter read 6.7 and with the small screwdriver provided, I adjusted the reading to 6.9.  I rinsed the electrode again and put it into the 4.0 pH solution and it read 4.0.  I was seriously impressed – this $11 instrument really worked.  I rinsed it again and measured the pH of distilled water expecting 7.0 (neutral) and it read 5.5 what the !!!!.  Surely distilled water is neutral!  Back into the 6.86 solution and it read 6.9, rinsed and back into the 4.0 solution and it read 4.0 and back into the distilled water and again it read 5.5.  The meter appeared consistent.

So I pressed ahead.  My rainwater measured 4.9 (acid rain?) and my well water 6.0.  Time to do the tomatoes – I took 3 soil samples from 3 different tomato beds – 2 samples from beds prepared this year and 1 from a bed I have used for several years.  Each sample I mixed with distilled water and then filtered the solution to remove large abrasive particles.

some of my utensils - a glass stirring rod, a measuring beaker and a jar to hold the solution being tested
some of my utensils – a glass stirring rod, a measuring beaker and a jar to hold the solution being tested

Samples 1 & 2 both read 6.7 and sample 3 read 7.8.  Wow! – a big difference.  Then I remembered that I had copiously applied lime and wood ash to the existing bed in previous years so the high alkaline reading made sense.  My conclusion was that none of the beds required liming.

The next day I began over again.  The 6.86 buffer solution read 6.8, and the 4.0 solution now read 4.3 so I adjusted it to 4.0. and then rinsed and retested the meter on both buffer solutions and they both read true.  I mixed coffee grounds with distilled water and the pH was 6.0.  My comfrey and coffee compost tea was 5.5 – made using rainwater which had read 4.9 so this made sense.  And I took 2 more samples from the bed which read 7.8 the previous day and they read 7.0 and 7.3 so the bed was too alkaline.

It all seemed to make sense except for one big and growing concern – why was the pH of the distilled water 5.5 and was this low pH affecting all the readings of the samples which had been mixed with distilled water.  For example if with a soil sample and distilled water solution I get a pH of 7.3 and the distilled water by itself has a pH of 5.5 does this mean the soil sample if mixed with water with a neutral pH (7.0) will have a much higher pH reading?

And why does distilled water have such a low pH?  Apparently distilled water when produced has a 7 pH but it reacts with CO2 (carbon dioxide) to produce a mild carbonic acid which lowers the pH of the water.  It appears that the pH of distilled water is easily influenced by what it mixes with.  My analogy is a shopping cart – if empty and you push with a finger it will move some distance.  If heavily loaded then a 1 finger push will barely budge it.  If the distilled water is the empty cart and the CO2 moves it one direction then the pH of a soil sample (I would like to think as a one handed push) will overcome the 1 finger push and get it to where it should be.  Very simplistic and unproven.

I bought another gallon of distilled water from another large chain and its pH was 5.0.  Now I could resolve matters by buying deionized water but it cost >$25 a gallon vs. $1.72 a gallon for distilled water.  There is another solution – how about using bottled drinking water.  I tested one sample and its pH was 7.4 – much closer to the neutral 7.0.  If I mix the soil sample with drinking water with a 7 pH will it give a more accurate result than if mixed with 5.5 pH distilled water?

So a simple test.  I filled a container with 200ml of distilled water and another with 200ml of bottled drinking water.  And added a small quantity of lime to both and stirred and measured their pH’s.  Lime is alkaline so you would think the lime mixed with the 7.4 pH drinking water will have a higher pH than the lime mixed with the 5.5 pH distilled water.  And the result?

The drinking water had a pH of 8.4 and the distilled water a pH of 9.7!  (Repeat tests with 2 drinking water solutions and 2 distilled water solutions gave pH’s of 8.6 for the former and 9.4 for the latter – a similar trend).  So distilled water with a lower starting pH gave a higher pH result when mixed, compared with the drinking water which has a higher starting pH.   Using my analogy, the distilled water is a very light cart with well greased wheels and the drinking water is heavily laden and moves sluggishly.  I  am not scientifically trained so don’t rely on my findings – if this is important to you do your own tests.

For my purposes I will continue using distilled water for my soil pH testing and will not fork out for deionized water.

FYI – a friend who is qualified in this area, says this is a long boring post; not using deionized water for the buffer solutions invalidates my readings; the shopping cart analogy is stupid; and he/she is sceptical of the results but acknowledges that for my purposes distilled water is probably ok for mixing with soil samples.

Additional note:  I noticed that distilled water when first poured has a lower pH than when subsequently poured and I hypothesize that the carbonic reaction at the surface of the water, which produces the acid, does not diffuse through the whole solution (unless stirred) but I did not test this by withdrawing a sample from the bottom of the container.

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