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.
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.
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.
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
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.
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:
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.
Delta_G:
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.
kidlat14:
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.
Delta_G:
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.
Delta_G:
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.
I'm thinking about disassembling one of these (or similar) and adding a hi-z amplifier to read the analog output and send it amplified/buffered to the Arduino...
You seem to be assuming that the gizmo actually does something useful.