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Method to determine pH after combining two soils? Is lime different than other alkaline additives?

 
Posts: 6
Location: Willamette Valley Region, Oregon, Pacific Northwest
Zone 8, Soil: Silty Clay, pH: 5, Flat-ish
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Are there any soil-chemists here? After a lot of online research, I'm still confused and here to ask:
Is there a method of determining the resulting pH after combining two soils of equal mass and density but with different known pH values?

For instance:
If both soilA (with pH of 5) and soilB (with pH of 8) both weigh 1kg and each have a volume of 1liter,  then what would be the pH if you mixed the two?
I tried to apply this situation into the equations given in this video, and I came out with a pH of:  -log((10^-5+10^-8)/2) = 5.3
Did I do that right?
Even if I did, I have a strong feeling that things aren't that simple for soils, due to the multitudes of different chemical compounds that might react with one another in order to create a much different soil chemistry and therefore pH.
Therefore it's my belief that there's no real way to do this without simply testing the resulting soil. Which brings me to my next question:

Is lime different than other alkaline soil additives?

My reasoning for asking is that I'm making seedballs and I'd like the pH to be around 6 or so.  The major component is Red Clay which sometimes has a pH of 4 to 5 and I was thinking of mixing it with azomite (pH of 8), and possibly ag lime (pH of 12.4) to raise the pH.  Another thread mentions that ag lime "has its neutralizing effect on acidic soils by ion exchange".  Does something like azomite have neutralizing effects because of ion exchange too? or is Azomite (Hydrated Sodium Calcium Aluminosilicate) not active in such a way and would therefore only raise pH based on the equations in the previously mentioned video?  Are certain soil compounds more free to exchange ions than others?

Thanks.
 
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Hi Kevin,

Can I suggest though, just buy a pH test kit. They are handy.

I hope that helps!
 
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Location: Cache Valley, zone 4b, Irrigated, 9" rain in badlands.
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Kevin Vernoy wrote:Are there any soil-chemists here? After a lot of online research, I'm still confused and here to ask: Is there a method of determining the resulting pH after combining two soils of equal mass and density but with different known pH values?



I worked as a chemist for 20 years before I returned to my roots as a farmer...

It can't be calculated, it can only be measured after the fact. If you used the same starting ingredients every time, you could come up with a recipe, but the first time the recipe is made, the resulting pH would need to be measured. Something like soil has too many complex chemistries occurring to reduce it to an equation. Soil testing companies are fond of generic recipes. They tend towards average conditions for average soils. The generic recipe might be way off for particular soils.



 
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Kevin Vernoy wrote:Are there any soil-chemists here?



Full disclosure: I am not a soil chemist. I’m a guy that has read books, has a basic understanding of soil chemistry and biology, and I’m totally into it.

I’ll do my best to answer your questions with what I know.

Is there a method of determining the resulting pH after combining two soils of equal mass and density but with different known pH values?



Yes, a pH test of the two combined soils or a lab analysis for the most accurate data.

Is lime different than other alkaline soil additives?  



There are several kinds of lime and it’s the calcium and magnesium that is doing the pH raising. The one you mention with a pH of 12.4, which is extremely caustic, is called hydrated or slaked lime. It’s used in industry to make products such as cement and plaster, and is also used in municipal water treatment facilities. Some people do use it in soil applications, I personally do not recommend it. It’s too easy to over do it, and can be quite a shock to the system.

The ag lime that I use and most farmers use is ground up limestone. It’s one of the most gentle ways to modify a soils pH. There is also calcitic lime and dolomite lime, usually sold in 50lb sacks at the co-op or a farm supply. These kinds, that I’ve seen, appear to be a manufactured lime and consist of little pellets or beads. The big difference between the two is dolomite has magnesium in it, which like calcium, is effective in raising a soils pH. I only recommend using dolomite lime if a soil analysis has been done and the soil magnesium levels are low.

Real quick, here are two side effects of calcium and magnesium in soil besides raising the pH. Calcium loosens soil particles, and helps keep the soil colloids from sticking together. Think clay like soils, and with enough calcium in them, they start to loosen up and become friable. Magnesium makes soil particles stick together. Too much magnesium can make a soil less friable, which in turn affects water and oxygen infiltration.

Does something like azomite have neutralizing effects because of ion exchange too?



I've used azmomite as a micronutrient source only, not to change a soils pH. All soil pH adjusting is a result of ion exchange. pH stands for potential Hydrogen. Acidic soils have lots of hydrogen ions bound to exchange sites on soil colloids (the soil particles). A soil colloid can have tens of thousands of ion exchange sites. When lime is added, the calcium and magnesium cations displace those hydrogen ions and take their seat on the soil colloid. At any given time, there is never an empty seat, or exchange site, on a soil particle. They are always occupied by a cation or hydrogen ions. Always.

Are certain soil compounds more free to exchange ions than others?  



Well, not compounds but elements for this discussion*, and staying in the context of soil particles, specifically cations only, but yes some are more free to exchange than others. Cations have a positive charge (+ or ++). Hydrogen ions on soil particles are the easiest to be knocked off their exchange site. From what I remember, Sodium has the weakest bond to a soil particle and is the easiest cation to be removed. Here’s a list of soil cations. Some have a double charge and some a single. The doubles will thus occupy two exchange sites on a soil colloid.

Calcium - Ca++
Copper - Cu++
Iron - Fe++
Magnesium - Mg+
Manganese - Mn++
Potassium - K+
Sodium - Na+
Zinc - Zn++

*Ammonium (NH4+) is a compound that can bond to an exchange site
 
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Kevin Vernoy wrote:Are there any soil-chemists here?  Is there a method of determining the resulting pH after combining two soils of equal mass and density but with different known pH values?

For instance:
If both soilA (with pH of 5) and soilB (with pH of 8) both weigh 1kg and each have a volume of 1liter,  then what would be the pH if you mixed the two?
I tried to apply this situation into the equations given in this video, and I came out with a pH of:  -log((10^-5+10^-8)/2) = 5.3
Did I do that right?
Even if I did, I have a strong feeling that things aren't that simple for soils, due to the multitudes of different chemical compounds that might react with one another in order to create a much different soil chemistry and therefore pH.
Therefore it's my belief that there's no real way to do this without simply testing the resulting soil. Which brings me to my next question:

Is lime different than other alkaline soil additives?

My reasoning for asking is that I'm making seedballs and I'd like the pH to be around 6 or so.  The major component is Red Clay which sometimes has a pH of 4 to 5 and I was thinking of mixing it with azomite (pH of 8), and possibly ag lime (pH of 12.4) to raise the pH.  Another thread mentions that ag lime "has its neutralizing effect on acidic soils by ion exchange".  Does something like azomite have neutralizing effects because of ion exchange too? or is Azomite (Hydrated Sodium Calcium Aluminosilicate) not active in such a way and would therefore only raise pH based on the equations in the previously mentioned video?  Are certain soil compounds more free to exchange ions than others?

Thanks.



Yes there is at least one soil scientist here.
As Kola Lofthouse brought up the only way to determine pH of any soil, mixed or not, is to perform a pH test, the more accurate the meter the more accurate the results.
There is no magic pH formula for determining soil pH, it has far to many influencing factors for any formula to be able to be developed for the specific purpose of determining pH of any soil or soil mix.

As was pointed out by James Freyr, there are many different "limes" that can be used for soil amendment purposes, the one which is not useable for this purpose is slaked lime (burnt limestone) it is far to caustic and using this product will kill off any microorganisms in the soil treated, turning it into dirt.
James also gave a lot of really good information on what lime does in soils.

Anions and Cations are the transferable portions of a molecule, they have electrical charge which is what allows for the exchanges and formation of compounds, most all compounds are more stable since they have become neutral through the passing off of electrons or the gathering in of electrons.
Neutral charge is the goal of any atom.  In soil, the acidifying molecules will be those that have free-able hydrogen atoms. Again, James gave you good information about this.

I would point you to my soil series Soil Series for more information on soils and how we can make lasting improvements.

Redhawk
 
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