lime, cal/mag ratio, and base saturation rate

StudentOfSoilAugust 17, 2012

I'm trying to get all this information straight in my head so I was hoping for some help. For containers I prefer to grow organically in a peat based mix and so adding something to raise the pH is necessary. I've learned that in lime it is actually the carbonate that raises the pH. So two of the common choices are either calcium carbonate (calcitic) or calcium magnesium carbonate (dolomite).

I understand that for a garden outdoors adding dolomite should only be done when you need to raise the pH, calcium level, and magnesium level. Calcium carbonate is used when the pH and calcium needs raised but the magnesium levels are ok. And gypsum can be used when the pH and magnesium are ok but the calcium needs raised.

So in container growing I am responsible for adding everything that the plant needs. I understand there is one school of thought that says a specific cal/mag ratio needs to be obtained. But i have also seen a number of studies that show various ratios can be used as long as there is adequate levels of both minerals.

The next concept is the base saturation percentage. This is simply the percentage of exchange sites occupied by cations. So say you want a cal/mag ratio of 7:1, is that the same as calcium having a base saturation percentage of 70% and magnesium having 10%? And if the importance of a cal/mag ratio is disproved then does that do the same for trying to get a specific base saturation level?

Now say in my containers I use dolomite as my only source of lime (as many seem to do). This is a cal/mag ratio of around 2:1. Is there any advantage to using a mix of half calcium carbonate and half dolomite? Or 1/4 calcium carbonate and 3/4 dolomite? This would raise the cal/mag percentage higher.

It is also my understanding that the magnesium in dolomite is much more soluble than the calcium, is this correct? That is why i was wondering about using something in addition to dolomite. I understand that things like crab meal also contain calcium carbonate. Is there any difference in the time of availability between different calcium carbonate sources or are they pretty much the same?

Basically what it comes down to is that i want to make sure that the plant gets adequate amounts of calcium and magnesium. I have always used dolomite but am now wondering if there is a better way? Or as long as i have enough dolomite to cover the calcium and magnesium needs is that enough?

Thanks for any help and taking the time to read this

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With a target of 7:1, your base saturation could theoretically be any two percents that meet that ratio. 70:10, 7:1, 14:2, etc.

Base saturation values are rarely used to modify container media. Container media tends to be quite acidic and have a high CEC - and the resultant buffering capacity. Just providing enough dolomitic lime to raise the pH to an acceptable level for your plant should provide plenty of calcium and magnesium for the growing season.

Magnesium is not more soluble than calcium from dolomitic lime. In fact, calcite (calcium carbonate) is more soluble than dolomite (calcium magnesium carbonate).

    Bookmark   August 18, 2012 at 3:17PM
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greentiger: thanks for replying. I wasn't sure if the magnesium became available more quickly than the calcium in dolomite, that was one of the reasons I wondered if using a different source of calcium (calcite lime, crab meal, etc) would allow the calcium and magnesium to be available at a similar time and thus not potentially causing a lockout.

As far as base saturation I forgot to take the high CEC of peat into account and the buffering capacity that follows.

Thanks again

    Bookmark   August 19, 2012 at 7:07PM
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So as I've continued my learning I have discovered that the magnesium component of dolomite does seem to become available more quickly than the calcium component. So mixing calcitic and dolomitic lime would in fact be a good idea. I also saw a recipe that called for 2 parts calcitic, one part dolomitic, and one part gypsum. calcite has a higher dissolution rate than dolomite (approximately 100x) due to a smaller particle size. That would help to even out the more available magnesium from dolomite.

I don't know if others find this as interesting as me but please feel free to add any thoughts you may have.

    Bookmark   September 4, 2012 at 11:57PM
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To be clear, please clarify what you mean by "become available". Is this availability in the soil solution, availability in the root system, in the xylem, at the growing shoot tip, etc.? I'm just trying to understand what you're getting at. Sounds like you're hinting that your reserach leads you to believe your onto something better than the tried and true.

BTW, there are plenty of Cal-Mag fertilizer solutions available that provide Ca and Mg in soluble form. The advantage of dolomite (or calcite for that matter) is that it will slowly dissolve over time and act as an extended release formulation (and as you mentioned, raise the pH).

Since it sounds like you're willing to go to the trouble, I'll suggest you consider using a better medium than peat-based mix. In case you haven't come across it already, I attach the link below.


Here is a link that might be useful: Container Soils - Water Movement & Retention XV

    Bookmark   September 6, 2012 at 11:58AM
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So I typed up a very long reply to you but of course my computer crashed before I could send it! I will try and sum it all up.

First, thank you for the link. I actually found another post by the author discussing dolomitic lime and verifying what I learned. Here it is:

RE: Fertilizer Program for Containerized Plants II clip this post email this post what is this?
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� Posted by tapla z5b-6a MI (My Page) on
Wed, May 27, 09 at 23:26
Well, you would think that because garden lime (dolomitic lime) has a solubility of about 1/3 oz/100 oz of water and is comprised of both CaCO3 and MgCO3 (calcium and magnesium carbonate) that they should become available in the same ratio as the dolomite dissolves. The fact of the matter is that the CaCO3 fraction of the lime has a solubility of about .0015, while the MgCO3 fraction is soluble at a rate of about .175, or roughly about 125X more soluble than the Ca fraction.
With dolomite being about 20% Ca and 10% Mg, you would also think that they would be released in about a 2:1 ratio, but what actually occurs is: at first, during the first few months, the release rate is about 1:1 or a little greater, then slowly shifting to around a 2:1 release at some point in the soil/planting's life, and then after 9 months to a year to 2:.05 or even less. This issue can be very acute for plants grown in the same soil for more than a single growth cycle.

I couldn't figure out an easier way to insert this other than cut and paste. Is there another way? How does one message the author as I would like to ask some questions directly?

As I stated I grow using organic amendments and am trying to replicate the outdoor environment in a container. I do understand there are many soluble cal/mag fertilizers available but I don't like to use synthetics (personal preference).

Anyhow the reason I was suggesting adding more calcium carbonate to dolomitic lime was to increase the cal/mag ratio. This was directly due to the fact of the magnesium being more soluble (in the soil solution as you asked). Also due to the role calcium plays in buffering the pH of a soil. This is where the CEC and base saturation rate come into play. Now dolomitic lime can certainly be used and I'm not trying to say there is anything wrong with it, just suggesting this as another way.

Greentiger suggested that the high CEC of container media would handle the buffering capacity but it is my understanding that calcium is the key cation in this situation. Therefore having more of it in the soil would be beneficial (within reason). Even if one is not trying to hit a certain base saturation rate it seems that it is suggested to have more calcium than magnesium and more magnesium than potassium in soil (please correct me if I am wrong). Now here is where I once again get confused! I see in many plant tissue analyses that potassium is present in a much higher rate than calcium. Probably why potassium is considered a primary and calcium a secondary nutrient. So why is it that much more calcium is suggested to be in soil? Is this because of the role it plays in buffering the pH? Or do the molecular weights have something to do with this difference? Or am I misunderstanding this all?

I'm not trying to challenge the tried and true just simply trying to learn as much as I can. Thanks

    Bookmark   September 7, 2012 at 11:09PM
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I think I'll defer to Al on this one, I'm sure there's empirical evidence for the availability ratios that wipe out my armchair theory.

I do want to not though, that true dolomite is not just a mixture of calcium carbonate and magnesium carbonate.. it has a different integrated crystal structure that is much more stable to dissolution. Of course, many products labelled as dolomitic lime may just be combinations of separately procured magnesium and calcium carbonates. They can also contain MgO and CaO.

    Bookmark   September 8, 2012 at 5:54PM
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