Measuring pH

[JPB]

Which equipment do you use? How is the measuring protocol? I had one for measuring in situ (ground-meter), but it didn't worrk properly...

[Rodger Whitlock]

For measuring soil pH, the simplest method is to mix a sample of the soil with water (ideally, *distilled* water), shake it a bit, let the crud settle to the bottom, and dip a pH test strip into the supernatant.

pH test strips aren't an everyday item in the world at large, but if there is a working chemist near you or a university chemistry department, they may be able to help you.

Or look online: *everything* is sold online these days.

You want strips that cover the range pH4 to pH 8, approximately. In horticultural terms, that runs from very acid to moderately alkaline.

[David Nicholson]

Hans, little kits, of the kind Rodger describes, are available in the UK at most garden centres. I may even have a spare one in my garage I'll look later.

[Darren]

Hans,

Also remember that soil pH even within a garden can be very variable. If you test a specific area for suitability for a particular plant then make sure you take a sample that is as typical as possible and at the depth where the roots would be - for example my soil is generally fairly alkaline but I would get a more acidic reading if I took soil from immediately beneath a layer of organic mulch. The root zone for many plants would be further down and thus more alkaline than my test would indicate. Also - if I tested during a drought when net movement of dissolved minerals is upward I may get a more alkaline reading than if i tested during a rainy period!

If you want a general reading for your garden then take a few samples in different places and then you can average the readings and also get an idea of variation.

[David Pilling]

It's common to see meters that claim to measure ph and moisture for the gardener, typically with a couple of big metal prongs, I had two of these and they didn't work. Has anyone ever got one of them to work, or is it some sort of 'emperors new clothes' mass delusion.

You can get proper digital meters, type "checker hanna ph" into Google. Around 30 quid, the catch is that the sensor is supposed to be stored in a special solution when not in use, so you have the expense and trouble of doing that.

The above google, leads to a range of ph measuring solutions on amazon, from the simple variety ("don't work" say the reviews) to the digital meters.

The only suggestion I head for why the simple meters don't work is that the chemistry involved requires a certain temperature.

[Diane Clement]

It's common to see meters that claim to measure ph and moisture for the gardener, typically with a couple of big metal prongs, I had two of these and they didn't work.
The only suggestion I head for why the simple meters don't work is that the chemistry involved requires a certain temperature. 

I think some of the problem is that they require good conductivity between the probe and the soil, which needs a level of moisture to fill the "gaps" in the soil.  The probes also corrode or oxidise on the surface, limiting the conductivity, hence the need to sandpaper them before use (sometimes suggested on the cheap ones) or storing them in the special solution you mention.

[David Pilling]

The special storage solution I mentioned is for the expensive accurate meters - which I assume are used in chemistry labs. But sand papering the probes is an idea.

I gave up with the cheap model when after inserting the probes in vinegar and in salty water, I could see no difference in reading.

It is interesting, with an accurate meter, to compare the ph of your tap water and rain water.

[JPB]

Today my search for pH test strips was without result. I finally bought a reasonally priced pH meter (Adwa AD-100). Dissolving some soil into destilled water after callibrating at pH 4 and 7 (with disposable sachets) and it works indeed much better than the in situ/soil measurements I did! Accuracy is specified at +/- 0.1 (rather 0.2-0.3 I found) but that's enough as replicate measurements, even on different samples, were comparable (within +/- 0.1-0.2).

BTW, I only have measured the soil in my pots, not in my garden.

Thanks for your suggestions!

Hans 

[iann]

The meter may be specified to 0.1, but in practice you shouldn't consider your readings to be this accurate.  The most important reason is that soil isn't soluble A pH measurement applies only to a solution, so what comes out of your soil when you mix it with water is rather poorly defined.  Soil testing laboratories dissolve the soil in special solutions that most definitely aren't distilled water in order to determine the acidic or alkaline reaction of the soil.  There are several standard methodologies best suited to different soil types and they give slightly different results.  A second reason for "inaccuracies" is simply the amount of soil and water you use.  Doubling the amount of water relative to the soil amounts to a pH change of about 0.3, so unless you very carefully use exactly the same amounts of soil and water each time you test, it will vary by at least a few tenths.  You should also make sure that your soil is ground rather finely to get the best results.  A chunk of limestone will not test strongly alkaline because it isn't very soluble, but limestone grit will certainly make your soil alkaline because it will react (albeit quite slowly) with any acidic components.

Who knew it could be so hard!

[JPB]

Ian, I think you have a point here. I did some reaserch and I found that air-dried and homogenized soil should be mixed with 5 parts (v/v)  of 1mol/l KCl. After shaking and incubating for two hours, the suspension can be sieved and measured. That should give a fairly good estimate.

I will try this method once I've received the KCl. Maybe I have some samples measured by a lab to see what the differences are

BTW, I did measure the pH in differend amounts of dest. water and found no or little difference. Not surpising since dest. water has no buffering capacities. 

Also, the samples which had been standing for a few days gave approx. the same values.

[iann]

I did measure the pH in differend amounts of dest. water and found no or little difference. Not surpising since dest. water has no buffering capacities.

I don't know what value of pH you read, but it should certainly change if you use more or less water unless it is the same pH as the water.  This is not an issue of the buffer capacity of the water, but simply of dilution.  pH is a (reverse logarithmic) measure of the concentration of hydrogen ions in a solution.  Add more water and the concentration goes down, and just to confuse the pH goes up.

Also, you should remember that even distilled water does not have a pH of 7 (shock horror!).  Pure water does have a pH of 7, but any water in contact with the atmosphere dissolves carbon dioxide which forms a weak acid with a pH typically around 5.5.  The effect on testing soil should be small, just a few tenths.  This is also the pH of clean rainwater although in many cases it is lower (more acidic) due to pollution, or very occasionally higher from alkaline dusts in the atmosphere.  Boiling rainwater or distilled water raises the pH closer to 7 as carbon dioxide is temporarily driven off.

[Ragged Robin]

This thread is full of information that is fascinating about soil PH - thanks to all contributing....what PH is the optimum for Camellias?

[iann]

Around pH 6 is good for Camellias.  Perhaps different species vary slightly?  They are related to the ericaceous plant family Ericaceae but not so strict about needing acid soil.  I've even seen them grown on alkaline soil, but not such great plants.

[JPB]

Ian,  I don't think the CO2 dissolving into water will have a major effect unless the solution is badly buffered (like in pure water). And doubling the amount of water will only change the pH by +0,3 due to the -LOG function. But theoretically it is impossible to measure the pH in moist soil as it can only be measured in a solution. I do not know how this problem is tackled , but in the protocols of labs it is standardized to 1 part dried soil into 5 parts water (v/v) or 1 mol/l KCl

I'll try to find out if this is correct an do some measurements later.

Interesting topic!