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Soil test - Ca/Mg needed?

Posted by mandolls 4 WI (My Page) on
Sun, Mar 20, 11 at 13:23

I am planning on getting soil tests done this year (I have been just fertilizing blind for the past few years). The WI extension charges extra for Calcium and Magnesium testing, yet I see those to minerals being discussed in this forum a lot - should I opt to pay the extra $3 for that additional test?

My plan is to test both the amended soil in my raised beds and the general soil in the yard, where new beds will be put in this year.


Follow-Up Postings:

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RE: Soil test - Ca/Mg needed?

As long as I was having the test done, I would spring the $3 for the Ca/Mg test. These are two very important minerals for plant growth. Both too much or too little can be a big problem.

Are you having two separate tests done? Or are you sending samples from the raised beds and the native soil into the same test?


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RE: Soil test - Ca/Mg needed?

I am definitely planning on two different tests one for the established beds and one for future beds.

It seems odd to me that they dont include the minerals in the standard test.


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RE: Soil test - Ca/Mg needed?

  • Posted by jolj 7b/8a-S.C.,USA (My Page) on
    Sun, Mar 20, 11 at 18:33

I agree, $50.00 today will save you $200.00 in minerals in the future.
If you know what you need & how much. Then you will not waste money on thing you do not need.

Here is a link that might be useful: Minerals for your garden


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RE: Soil test - Ca/Mg needed?

Since Calcium and Magnesium affect what your soils pH is they should be a normal part of what the lab finds out about your soils pH, however logic does not always seem to be a strong concept some places. Your plants need to have Calcium and Magnesium in balance to properly utilize each other, ie. low Mg levels can cause a plant to not be able to use Ca, and vice cersa. Since guessing that your soil needs some lime and adding the wrong type (calcitic when you need dolomitic) could create more problems it would be worth the extra money to me to know the balance between Ca and Mg, whether I needed to add any as well as how much and which type.


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RE: Soil test - Ca/Mg needed?

Since Calcium and Magnesium affect what your soils pH
This statement is patently false. Calcium and magnesium have precisely zero, zilch, zip, nada, to do with pH.
low Mg levels can cause a plant to not be able to use Ca
High magnesium can inhibit plant uptake of calcium. You don't need a minimum amount of one to uptake the other.

Do you need to test for calcium and magnesium? Well, for three bucks...what the heck.


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RE: Soil test - Ca/Mg needed?

Yep - I will definitely pop for the extra test - thanks for the feedback


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RE: Soil test - Ca/Mg needed?

Hmmm, let's think about what gangwarb said. When your soil is tested and your soils pH is low (acid) you are told to apply X amount of lime per 1,000 square feet and lime is Calcium Carbonate. In the soil the Calcium helps bind the free radical hydrogen ions that make your soils pH low and binding those ions helps your soils pH rise. If your soils pH is high (alkaline) the test will show too much Calcium Carbonate and you will be told to use, say sulfur, which helps create free radical Hydrogen ions which cause the soils pH to drop.
So does Calcium have anything to do with a soils pH?


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RE: Soil test - Ca/Mg needed?

No. Calcium does not. But, in regards to your comment concerning specifically calcium carbonate lime, let's start with a couple of basics. The term 'pH' is actually a mathematical statement. 'p' means "find the anti-log of" and 'H' stands for hydrogen. Now, it's not just any hydrogen and it's not a free radical. It's what's called hydronium or basically a water molecule with a hydrogen atom piggy backed to it.
Now, there is a very specific correlation between hydronium and hydroxyl ions (OH-) in the solution. So, the pH is actually a mathematical expression of the relationship between hydronium and hydroxyl. By calculating the pH (anti-log hydronium)you can also calculate the pOH (anti-log hydroxyl). When you have a pH of 7 (neutral) they are present in equal amounts.
So, what lime does is react with and, thereby increase, the amount of hydroxyl ions (OH-) in the soil solution and through other reactions, decreases the amount of hydronium (H+). As you shift the balance of the reaction towards more (OH-) and less (H+), you get a net increase in the pH.

Lime in the soil causes a whole slew of reactions to accomplish this but here are some of the high lights:

Here's one where we see lime reacting with water to increase (OH-). You'll notice that the lime simply disassociates and it's the carbonate (CO3) that does the reacting, leaving free calcium behind. Calcium does not do the reacting. Something like sodium bicarbonate or potassium carbonate would have a similar on the pH.
CaCO3 +H2O < == > Ca2+ + HCO3- + OH-

In this one, one unit of lime reacts with two hydroniums (2H+) to turn them into water and carbon dioxide, thereby decreasing the (H+) in solutions. You'll notice that the lime simply disassociates and it's the carbonate (CO3) that does the reacting, leaving free calcium behind. It's the carbonate that reacts with (2H+) to form CO2 and H2O.
CaCO3 + 2H+ < == > Ca2+ + CO2 +H2O

Now, we have lime reacting with (H+) to create bicarbnoate (HCO3-) and decrease the hydronium in the solution. You'll notice that the lime simply disassociates and it's the carbonate that does the reacting, leaving free calcium behind.
CaCO3 + H+ < == > Ca2 + HCO3-

After that reaction, you can then have the bicarbonate (HCO3-) going back to react with more (2H+), creating more water and decreasing the (H+) in the solution further. You'll notice that calcium isn't even in the neighborhood for this reaction.
HCO3- + 2H+ < == > CO2 + H2O

And so on....
It's the CO3 part of the CaCO3 that does all of the work. Calcium has absolutely, positively and totally nothing to do with the pH or the reactions that go in to changing it. You can add and remove calcium all the live long day and it won't budge your pH one way or the other unless that calcium is associated with something that will have an effect on the pH of the soil solution.

Seriously Kimm, after all these years, haven't you learned that I don't just make this stuff up?


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OH yeah...

P.S.
If your soils pH is high (alkaline) the test will show too much Calcium Carbonate
Sometimes yes, sometimes no. The presence of lime is not required for an alkaline reaction. I'd explain but I have to get back to work.

P.P.S.
I like when you called me gangwarb instead of gargwarb. Including 'gang' in my name gives me some street cred. Thank you.


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RE: Soil test - Ca/Mg needed?

Of course what garg wrote we all got in our first few weeks of our first semester chem at Uni., so no sense commenting on that.

Props, yo, to G Fresh.

Nonetheless, I like the term pH to read as "potential of Hydrogen" , which allows us to visualize the higher the pH, the lower the number of H+ ions floating around.

This also helps us visualize, for example, why iron is less available in basic soils, because there are fewer H+ to react with.

Just a hint to help with soil chemistry.

Dan


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RE: Soil test - Ca/Mg needed?

I have read that excess magnesium in relation to calcium [don't know just what compounds of each] makes the clay soil tighter and stickier and lumpier. It has to do with + and - charges.


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RE: Soil test - Ca/Mg needed?

You can add and remove calcium all the live long day and it won't budge your pH one way or the other unless that calcium is associated with something that will have an effect on the pH of the soil solution.

Thank you, thank you, thank you. I love it. I'm on my way out to buy a bag of Ca++ 'cause I know my plant need it.


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RE: Soil test - Ca/Mg needed?

Yes: both are basic. Regards, Peter.


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RE: Soil test - Ca/Mg needed?

Peter, I think what you may be referring to is the fact that calcium and magnesium (along with potassium and sodium) are sometimes referred to as the "base cations".
Although it can be a handy phrase when used correctly, it can a bit misleading when it isn't. It can give the impression that these cations will act as a base in solution, which simply isn't the case.
Another phrase that's a little better, but can still be a bit confusing, is "base forming cations".
That is, these cations (Ca, Mg, K and Na) will react readily with hydroxide (the OH- I was talking about up there) to form a strong base:
Ca(OH) - calcium hydroxide
KOH - potassium hydroxide
etc.
The cations are no more a base than a set of tires is a car.
Remember, it's the balance of OH- to H+ that dictates the acidity or alkalinity of the solution.
If you have (and I am really reeeeeally simplifying things here):
10 OH- and 10 H+, your solution will be neutral. If you throw in some calcium, you will still have 10 OH- and 10 H+ in solution and it will still be neutral. Of course, the chemistry is very dynamic and the calcium atoms will be spontaneously hooking up with, and then disassociating from, the OH- constantlty, just like the H+ will be spontaneously hooking up with and disassociating from water to form the hydronium I was talking about up there. In a solution, nothing sits still but that won't change the balance of 10 OH- to 10 H+ in the solution and it will remain neutral.


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