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Topic: [Help] Measuring soil pH Meter. (Read 2152 times) previous topic - next topic

kidlat14

Good day,

I'm building a digital pH Meter using Arduino and I'm looking for advice. I just want to know whether I can use this sensor PH meter(SKU: SEN0161) to measure soil pH. If no, do you guys have any suggestions.

Thanks.

Delta_G

That probe is for measuring pH in water.   This will only work if you are preparing slurries and measuring the pH of the water afterwards. 
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wvmarle

That's designed to measure the pH of watery solutions. You'll break the fragile glass bulb when trying to stick it into soil.

In fact I still don't know what the pH of soil actually IS. Maybe you can explain?

The normal definition of pH is the negative logarithm of the concentration of H+ ions in a watery solution. How does that even work for soil? Is it the pH of the moisture that's present in the soil? Or what is it, really? Still wondering. Dry soil or wet soil would give a quite different result, too.

You could take a soil sample, mix it in water, take the pH of that. The pH of the resulting solution is of course dependent on the soil/water ratio (more water dilutes the solution bringing the pH closer to 7) and any minerals present in the water.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

kidlat14

That probe is for measuring pH in water.   This will only work if you are preparing slurries and measuring the pH of the water afterwards.  
Thanks for the quick reply. If that's the case, can I dilute my soil sample with distilled water and then measure the mixture? Will my reading still be accurate if that is the case?

Thanks

kidlat14

That's designed to measure the pH of watery solutions. You'll break the fragile glass bulb when trying to stick it into soil.

In fact I still don't know what the pH of soil actually IS. Maybe you can explain?

The normal definition of pH is the negative logarithm of the concentration of H+ ions in a watery solution. How does that even work for soil? Is it the pH of the moisture that's present in the soil? Or what is it, really? Still wondering. Dry soil or wet soil would give a quite different result, too.

You could take a soil sample, mix it in water, take the pH of that. The pH of the resulting solution is of course dependent on the soil/water ratio (more water dilutes the solution bringing the pH closer to 7) and any minerals present in the water.
Thanks for the reply. Soil pH determines the acidity or alkalinity of a soil. I'm building one to help me determine what fertilizer is best applicable to a particular soil.

Based on your point, is it possible if I can just mix the soil with distilled water(i've read that distilled water is more neutral compared to tap) and then measure the pH of that mixture?

Delta_G

Thanks for the quick reply. If that's the case, can I dilute my soil sample with distilled water and then measure the mixture? Will my reading still be accurate if that is the case?

Thanks
I think you should take some time to study up on techniques for measuring soil pH.  There is a method that involves making a slurry, but it is a little more exacting than "just mix with some distilled water". 
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

kidlat14

I think you should take some time to study up on techniques for measuring soil pH.  There is a method that involves making a slurry, but it is a little more exacting than "just mix with some distilled water". 
Thanks for your advice. Got a tutorial on YouTube on how to test soil using the Slurry Method.

https://youtu.be/GB5HLqmJzVs

Delta_G

#7
Jan 11, 2019, 07:12 am Last Edit: Jan 11, 2019, 07:44 am by Delta_G
The normal definition of pH is the negative logarithm of the concentration of H+ ions in a watery solution.
That's the most basic definition.  That's a Bronsted (sp?) acid.  A Bronsted acid is a species that liberates H+ when dissolved and a Bronsted base is a species that liberates OH- when dissolved. 

There's also the Lewis definition which is more complete.  A Lewis acid is a species with an empty outer shell orbital that can accept a pair of electrons.  A Lewis Base is a species with a "lone pair" of electrons that it can donate.  So by that definition, Al3+ is an acid and Cl- is a base.  When you get to upper level chemistry those are the definitions you'll use more. 

Soil pH is a measure of the amount of acid in the soil.  As rain falls, the carbonic acid in the rain tends to leach out the more basic ions by forming bicarbonates (leaving one H+ behind) and also leaves behind more metal ions like Al3+ which don't form soluble carbonate or bicarbonate salts and that would act as stronger Lewis acids. 

There are also many plant based actions that affect soil pH.  A plant's roots generally take up a lot more cations than anions.  They maintain charge balance by releasing H+ into the soil.  Now it is true that it isn't going to just stay as a free proton, it will most likely form H30+ with water.  But that is still an increase in the amount of acid there. 

And just like with water, to fix acidic soil you add base like lime.  To fix basic soil you add sulfur which is converted by bacteria into sulfuric acid in the soil, lowering the effective pH. 
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Delta_G

Thanks for your advice. Got a tutorial on YouTube on how to test soil using the Slurry Method.

https://youtu.be/GB5HLqmJzVs
The only thing I would add about that video is that if you want your probe to last, don't just jam it down in the dirt like they do in that video.  Try to keep it in the watery part and try to get as little dirt on it as you can.  If the junction on the probe clogs up with soil or salts, then the probe is finished. 

Hannah sells pH probes, so they probably think ruining your probe is good for business.  But you should treat it a little better than they treated the one in that video. 

And make sure you clean it really well.  Rinse with DI or distilled water and store in a 3M (or stronger) KCl solution.  The probe should come with some.  It will dry out and leave a salt crust behind.  You can just redissolve it with more water.  The exact concentration of KCl isn't important, but it should be pretty darn strong.  Close to 3M is optimal. 
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

wvmarle

That's the most basic definition.  That's a Bronsted (sp?) acid.  A Bronsted acid is a species that liberates H+ when dissolved and a Bronsted base is a species that liberates OH- when dissolved.   
Most of soil will indeed be the Lewis acid/base type as it's solid minerals. That's the problem, those are not as easy to measure, particularly if done in situ.

I'm a chemical engineer but never worked really with Lewis acids/bases, my knowledge is quite limited on that part. That's also why my question: what IS soil pH? What chemicals give it its pH, and how to measure it? This as indeed the normal measure (free protons) breaks down totally.

Thanks for your advice. Got a tutorial on YouTube on how to test soil using the Slurry Method.
That feels terribly simplified. No mention of the actual amounts of water vs. soil. More water means higher dilution, a pH closer to 7 (so increased when it's <7, decreased when it's >7, and even that is a simplification), and a lower EC and TDS value.
Then there's the problem of water in the soil: the more moist the soil, the less solids in a sample, and the more diluted the final solution will be. This alone makes the measurement a major problem. I'd guess you first have to thoroughly dry the sample, to make sure you know the actual dry solids weight you add to the slurry.
A greater dilution on the other hand may allow for more minerals to dissolve, as some may reach saturation (CaSO4 has low solubility, and both calcium and sulphate are important nutrients and will be present in soil). That means the TDS drops less than you would expect with increased slurry dilution. The solution for this is of course to dilute the slurry enough to have all minerals that can dissolve to dissolve.
The solubility of these minerals in turn will also affect the pH, and the pH in turn may affect the solubility of certain minerals.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Delta_G

Quote
I'm a chemical engineer but never worked really with Lewis acids/bases, my knowledge is quite limited on that part. That's also why my question: what IS soil pH? What chemicals give it its pH, and how to measure it? This as indeed the normal measure (free protons) breaks down totally.
You're still trying to apply the solution definition.  Soil pH is a slightly different definition.  It's about what you get when you mix the soil with water.  But it is still just a measure of the concentration of acidic species in there. 

Say I had a bottle of solid oxalic acid.  I can still call it an acid even though it isn't dissolved yet.  When I dissolve it then it will make the water acidic.  But the oxalic acid is still an acid whether I dissolve it first or not. 

It's like when we talk about color in terms of temperature.  I have some 12K fluorescent lights over my fish tank.  They're not really a black-body.  And they certainly aren't actually at 12,000 kelvin.  But we've agreed on a system that allows us to describe it that way and it works. 

Same here.  In the soil we don't have exactly what you would describe as pH in water, free hydronium floating around.  But we can still describe a property based on what happens when you put it in water and call it that.  In a proper chemistry setting I probably wouldn't use that term.  But for gardeners it works and it is well documented how it is defined.   

Or kind of it does.  The actual definition of pH is the inverse log of the concentration of hydronium.  The soil has a certain concentration of hydronium does it not?  I can talk about the concentration of an ion in a solid. 



Either way, if the soil has more acidic species present, then we call it acidic.  And if it has more basic species present then we call it alkaline.  We measure that by slurry with water and measure pH of resulting solution. 

These are biologists for crying out loud.  Do you really expect them to be proper on their use of chemistry terms?  Have you ever tried to follow a recipe from a paper in some biology journal.  "Add 50ml of EDTA solution"  What concentration?  "The one in the fridge, you know, EDTA solution.  Aren't they all the same?"


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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

Delta_G

That feels terribly simplified. No mention of the actual amounts of water vs. soil.
It said 2 parts water to 1 part soil.  It didn't mention weight or volume, but I would assume volume. 


More water means higher dilution, a pH closer to 7 (so increased when it's <7, decreased when it's >7, and even that is a simplification),
They're actually mostly going to be buffers.  Combinations of acidic and basic forms.  Dilute them and the pH stays the same.


and a lower EC and TDS value.
Yeah but we aren't measuring that.  If you want EC you'll have to be a bit more exacting.  But even then there is a definition for how to make a measurement in a certain amount of water to determine what they call an EC value for the soil even though the soil itself doesn't conduct anything.  The number still tells you about the amount of salt in the soil. 

Then there's the problem of water in the soil: the more moist the soil, the less solids in a sample, and the more diluted the final solution will be.
That's why I'm sure they mean 2:1 by volume.  But remember, we're not worried about small dilutions because these salts will form buffer solutions. 


This alone makes the measurement a major problem. I'd guess you first have to thoroughly dry the sample, to make sure you know the actual dry solids weight you add to the slurry.
A greater dilution on the other hand may allow for more minerals to dissolve, as some may reach saturation (CaSO4 has low solubility, and both calcium and sulphate are important nutrients and will be present in soil). That means the TDS drops less than you would expect with increased slurry dilution. The solution for this is of course to dilute the slurry enough to have all minerals that can dissolve to dissolve.
The solubility of these minerals in turn will also affect the pH, and the pH in turn may affect the solubility of certain minerals.

Now you have a point here.  But it is simple to define a point at which we will measure.  The 2:1 ratio seems to be very common.  That sort of standardizes it here.  Remember, we're growing plants here, so we're really only worried about soluble species anyway.  We aren't trying to describe some chemical property.  We aren't running a saponification reaction and need to know about the rate.  We're growing plants.  It's a pretty rough measure, but it certainly tells you whether you need to add lime or sulfur to your soil. 

Each measurement has a purpose.  If you're creating an ion routing multipole for a mass spectrometer then you might be measuring to the micron.  (actually it's not that ticky).  But if you're building a shed you are probably OK with just centimeters or maybe millimeters. 
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

wvmarle

Say I had a bottle of solid oxalic acid.  I can still call it an acid even though it isn't dissolved yet.  When I dissolve it then it will make the water acidic.  But the oxalic acid is still an acid whether I dissolve it first or not.  
Of course it's an acid.

But the question remains: what then would the pH be of that oxalic acid? I've never seen anyone defining a pH for a solid acid. If you can define a pH for soil, you must be able to do the same for a compound such as oxalic acid.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

wvmarle

It said 2 parts water to 1 part soil.  It didn't mention weight or volume, but I would assume volume. 
Volume of loosened soil (after you started digging it up to make it loose enough to actually put it in a cup) or soil that's been pressed into that cup? Such loosening of soil can vastly increase its volume. Many of the plant matter that's in the soil will also change drastically in volume depending on the wetness.

If you can indeed assume the soil acts as a buffer (you'd have to test this) then indeed it doesn't matter much how much soil and water you mix together, to an extent of course.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

Delta_G

#14
Jan 11, 2019, 08:39 am Last Edit: Jan 11, 2019, 08:42 am by Delta_G
Volume of loosened soil (after you started digging it up to make it loose enough to actually put it in a cup) or soil that's been pressed into that cup? Such loosening of soil can vastly increase its volume. Many of the plant matter that's in the soil will also change drastically in volume depending on the wetness.

If you can indeed assume the soil acts as a buffer (you'd have to test this) then indeed it doesn't matter much how much soil and water you mix together, to an extent of course.
Why do I need to test it?  We know what is mostly in our soil.   Again, we're not doing an elemental analysis.  This isn't absolutely quantitative.  It's a measure of how much acidic stuff is in the soil and how much basic stuff is in the soil.  It's for gardening, not standardizing a soil sample.  You're WAY overthinking this.  It's just to determine whether the soil is acidic or alkaline and by (relatively) how much. 




I've never seen anyone defining a pH for a solid acid.
BS.  I just gave you one.  Here's another.  Not hard to find.  If you want to argue this then you've got a couple hundred years worth of whenever they came up with this concept to argue it.  Hey, I kind of agree that calling it pH is kind of confusing but that's what they call it.  I'd rather they call it soil alkalinity but even then we run into a similarly named solution property.  And there is a long standing definition of it as soil pH.   

There are two guys at my office named Bob.  They aren't the same guy, but both are short for Robert.  You could handle that couldn't you?  Same here.  Soil pH isn't the same as solution pH, but they both relate to relative amounts of hydronium. 


When you're cooking with a friend and they say set the oven to 350, do you calibrate a thermometer and calculate the specific heat capacity of the oven and the relative entropy change on heating?  Or does that just mean set the dial to 350 and whatever happens is probably close enough?  Some things aren't so absolutely scientific.  The corn in the garden probably can't tell if you were off by a few tenths of a pH unit.
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

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