Soil test - increase calcium with affecting pH?

andrew_rvaMay 20, 2014

I've read that your calcium/magnesium ratio should be 7, and that low calcium can cause of a lot of different weeds (we had a ton of dandelions a few weeks ago). I got my soils tests back a few weeks ago, and my Ca/Mg ratio is 5 in the back yard and 4 in the front, but I think my pH was pretty good - 6.9 and 6.7. I'd like to add some calcium without affecting the pH, what would be a good source for that? Gypsum?

On the other hand, my test listed my pH buffer at 6.35, and my soil is rather clayey. Maybe calcitic limestone would be ok anyway?

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If you don't want to raise your PH, gypsum has been shown to be a PH neutral additive. Keep in mind that it takes vast amounts of any amendment (sand, gypsum, OM etc.) to significantly change soil structure.
It is common to mistake the characteristics ("hard soil") caused by high Mg as clay. A jar test can determine if the soil is actually clay or not.
There are two schools of thought/theories on this. One is that you should have the proper ratios, the other is that as long as you have sufficient mineral amounts to supply the necessary nutrients, don't worry about the ratios.
Additional OM and applying a surfactant (shampooing) have been used successfully to counteract high Mg and establish and maintain turf.

    Bookmark   May 21, 2014 at 10:17AM
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If he soil test was done by a good lab there should have been a recommendation about what, and how much, to use to correct any deficiency. If the test was done by your state universities Cooperative Extension Service your county horticultural agent should be able to advise you.

    Bookmark   May 21, 2014 at 11:25AM
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A word of caution. The recommendations given by SOME extensions and agencies are targeted for agricultural use. Verify that the recommendation you receive is for lawn surface application and not for incorporation through tilling.

    Bookmark   May 21, 2014 at 12:11PM
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If you supply the soil testing lab with the proper information you will get recommendations for the information you supply. If you tell the lab this is for a lawn you will not get recommendations for a vegetable garden, even though they are not all that different.

    Bookmark   May 22, 2014 at 7:51AM
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Do you post for the sole purpose of being contrary or do you not know the difference between amendment recommendations for surface application vs application by incorporation? The difference can be hundreds of pounds and over the years people have posted on lawn care sites of making that mistake. Particularly with lime.

    Bookmark   May 22, 2014 at 12:44PM
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I post to provide people with good reliable information, not myths and conjecture.

    Bookmark   May 23, 2014 at 7:25AM
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How ironic for you to say.

    Bookmark   May 23, 2014 at 11:26AM
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Johnny late to the fair here, but yes, gypsum will work. It will take some years to get the Ca:Mg ratio to where you want it.

Ratios around 5 aren't that bad if your calcium saturation is reasonably high and if the lawn is performing well. Mine are also off the optimal in the other direction--9 in the lawn, 10 in the gardens, but performance in both is stellar and I'm not inclined to pour magnesium just to correct a ratio.

    Bookmark   June 8, 2014 at 1:18PM
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Looking into this concept I find that while once thought to be of importance, and still hawked by people with vested interests in selling you stuff to correct a perceived problem, few people that study soils think it is something to think about.

Here is a link that might be useful: about soil Ca to Mg ratio

    Bookmark   June 10, 2014 at 6:54AM
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kimmsr, your link doesn't work.

This post was edited by yardtractor1 on Tue, Jun 10, 14 at 23:48

    Bookmark   June 10, 2014 at 10:13AM
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Works for me just fine. However, it is a .pdf so if you do not have Adobe Reader it may not open.

    Bookmark   June 11, 2014 at 11:40AM
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