Gypsum to amend clay

melissa668May 20, 2009

I have been reading many of these posts regarding how to amend clay. I have also been seeking advise from my fellow local gardners.

Today i was told to add gypsum to amend the clay.

Gypsum can be bought relatively cheap at any hardware store, i was told to sift it and then sprinkle to the soil and work through, i would see results quickly as the gypsum has a chemical reaction with the clay, it prevents the molecules from sticking together.

Has anyone had any success with this?

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the_virginian(Zone 7 NoVA)

Yes, to an extent, but I would also recommend adding lots of organic matter in the mix too for best results.

    Bookmark   May 20, 2009 at 9:58AM
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It helps if you have the right kind of clay. I think the clay it helps is called sodic clay. I've got clay, but gypsum doesn't help it at all because it's not sodic clay.

    Bookmark   May 20, 2009 at 10:12AM
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Gypsum will help sodic clays, the soils you would find in places like Utah, California, New Mexico, Arizona, etc. where too little rain falls to wash the salts out of the soils. Gypsum will not help the clay soils in New Jersey. Most of the people that will tell you that you need to add gypsum to your soil are selling that gypsum. If you talk to your people at the Rutgers USDA Cooperative Extension Service office near you they wil also tell you that gypsum will not help your clay soil.

    Bookmark   May 20, 2009 at 12:14PM
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emmers_m(9a/Sunset 7 N Cal)

I'd like to run an experiment to test this. I'm inclined to believe that gypsum won't help my clay (possibly heavy silt?) soil here in NJ, but my SO grew up not far from here and says he's seen it work. (I already have some gypsum because I'm trying to plant peanuts.)

I think I saw a thread where someone proposed a way to test the drainage w/gypsum and without, but I can't find it right now. So I'm thinking to cut the bottom off of two plastic quart containers, cover the bottom with cheesecloth or similar, fill one with dirt and one with gypsum amended dirt, add a measured amount of water to each, and record the time it takes to drain out.

Any flaws in my experimental design?


    Bookmark   May 20, 2009 at 5:21PM
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the_virginian(Zone 7 NoVA)

emmers_m: Take it from a resident of a peanut growing state, you will have one heck of a time growing peanuts in clay soil almost to the point of not being able to harvest them unless you amend the soil well. In clay soil you will need to add lots of sharp or "builder's sand" NOT PLAY SAND, to your clay to grow peanuts, so they won't rot in the ground and so you can get them out of the ground when you want to harvest them. Your growing season is almost too short, but if you plant now and the weather holds out you might get some by the Fall. Adding lots of organic matter will help, but gypsum won't really do much to help the soil for peanut growing or as we call'em "goobers."

    Bookmark   May 20, 2009 at 11:56PM
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To make any kind of difference that is beneficial someone adding sand to clay soils needs to add somewhere between 45 percent (most all Ag Schools including Virginia Tech) to 75 percent (Cornell University) sand to that clay. Aside from the expense it would be difficult to till that much sand into that clay soil.
What any soil needs, whether clay or sand, is organic matter, enough to be about 5 to 8 percent of the top 6 inches which is about as deep as necessary to go for any soil test.

    Bookmark   May 21, 2009 at 7:44AM
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the_virginian(Zone 7 NoVA)

For growing peanuts you may need to add that much sharp sand to have success. I guess it would depend on the size of the area you want to plant with peanuts to determine if it is worth the effort. How large an area are you planning on planting with peanuts?

    Bookmark   May 21, 2009 at 10:20AM
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rhizo_1 (North AL) zone 7

I can't think where gypsum wouldn't be added to the soil if one were growing peanuts. Not as a magic bullet for a clay soil, but to provide the very high amounts of calcium required to 'make' high quality seed. Gypsum is the calcium source used as it won't raise the pH unacceptable levels.

    Bookmark   May 21, 2009 at 4:24PM
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hortster(6a, southcentral KS)

Wow, what a great thread. So many people misunderstand the value of gypsum. Gypsum is sold and promoted by the companies that produce it as a "clay modifier." However, kimmsr is correct several times here. Soils with high mineralization, especially sodium (therefore sodic) soils structurally collapse from the excessive salts. Gypsum tends to reverse this collapse. kimmsr is also correct in that in order to improve soil percolation, sand particles must be added in huge quantities, enough that the sand particles are in constant contact with each other to do any good. I must say that my approach is to add organic material vs. sand. I have seen many cases where sand was added only to produce a nice brick hard adobe for plants to struggle against.
I have no knowledge of peanut growing but the virginian and rhizo 1 may have a good point about adding calcium since gypsum is calcium sulfate, albeit it DOES raise pH. No one can go wrong by adding heavy amounts of organic material to clay, again, kimmsr's right.
Back to the original question. GET A SOIL TEST for N, P, K, soluble salts and pH. Add organic material as well as the amendments that will revise your soil condition. If you have high soluble salts or need the calcium (another test), add the gypsum, otherwise, take the advice of the tester.

    Bookmark   May 21, 2009 at 8:45PM
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coffeehaus(7a Central VA)

I must agree with horster. Gypsum is used to add calcium without significantly raising pH but does not greatly improve non-sodic soils. I have always argued that money spent on soil amendments should be used only for material that will not only improve drainage but also increase cation exchange capacity, water holding capacity and impart nutrients. Sand does only one of those. Organic material like compost or a composted manure product does them all.

    Bookmark   May 21, 2009 at 8:59PM
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the_virginian(Zone 7 NoVA)

As to what is being grown, peanuts like well draining soil and to get them out of the ground you will need to add lots of sharp sand to the clay. Gypsum may add some benefit too and adding organic matter is another must to improve the soil.

    Bookmark   May 22, 2009 at 2:47PM
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Here's some information about the many benefits of gypsum.

Here is a link that might be useful: gypsum

    Bookmark   May 24, 2009 at 11:32AM
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Dan Staley

Spam from the gypsum industry notwithstanding, to reiterate, gypsum is best used to amend sodic soils. If your soil is not sodic then you are wasting your money, despite what the link spam from the gypsum industry wants you to believe.


    Bookmark   May 24, 2009 at 12:57PM
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emmers_m(9a/Sunset 7 N Cal)


I am using the gypsum for calcium for the peanuts, just like rhizo said, on the recommendation of Ed Smith's book.

I know I might not have much luck with them, but I'm just trying it for fun! I'd be happy if I got enough to try boiled peanuts for the first time. I did amend with lots of compost, so I guess we'll see.

No comments on my experiment? I'd like to test the gypsum/clay thing personally, if only to discourage SO from buying 1000 #s to dump on the lawn. But it's hard to argue with his 'personal experience' without firsthand evidence.

    Bookmark   May 24, 2009 at 2:35PM
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i have tried gypsum on clay soils and found it very lacking, in fact it was negative afer a few years.
i found the information from; and
far better and results on my farm has been terrific.

this involves using lime on clay soils and it will not create a Ph problem for you.
i suggest you read the web pages i posted and seek further advice from kinseyag.

    Bookmark   July 6, 2010 at 12:53AM
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Lime on clay soils will create a Ph problem if the soil is alkaline to begin with which some clay soils are. I would never add lime to our Ph 7.5-8 clay soils.

    Bookmark   July 7, 2010 at 1:04AM
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toxcrusadr Clay Soil(Zone 6a - MO)

emmers: I missed this thread earlier or I would have commented on your experiment. It would be very difficult to do that in a controlled manner, because once you disturb the soil to put it in the container, it is nearly impossible to ensure that both containers are recompacted to the same extent, and the variation that causes in the speed of drainage would far outweigh the subtle effects of the gypsum. Drainage tests like that are done by using a large machine to extract a soil core intact, and that would have to be done after a year or several years of gypsum application. Not something you could do at home.

The original posts stated that gypsum works by preventing the clay molecules from sticking together. Actually the opposite is true. Clay particles are very small, which is why they pack so tightly. The calcium in gypsum becomes Ca++ which then competes with Na+ sticking to the surface of the clay. The double charge can potentially grab two clay particles (like using your two hands outstretched), causing agglomeration into larger particles, thus increasing permeability. See, I was paying attention in my one soil chemistry course. :-]

    Bookmark   July 7, 2010 at 10:46AM
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emmers_m(9a/Sunset 7 N Cal)

Thanks, toxcrusadr, that makes a lot of sense (to me), but doesn't give me any firsthand evidence to point to to dissuade SO from throwing tons of gypsum on our drainage problems. I'll just have to content myself that we can't afford it right now :)

And I ended up with a very small but respectable peanut crop!

    Bookmark   July 25, 2010 at 11:25AM
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New Jersey is rich in experienced old timers who used to grow vegetables for the New York market. That's why it's called the Garden State. Most of those still above ground are growing vegetables in their back yards. It's in their blood. If you can find one of them and get him talking you can learn a lot. The Italian community is a good place to start looking. Many of them were Italian.

    Bookmark   July 25, 2010 at 9:42PM
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rhizo_1 (North AL) zone 7

Again, if you have clay soils that are SODIC, gypsum can help improve the soil's ability to form natural aggregates. In good, hands-on terms, gypsum can improve tilth in soils with excess sodium. It will NOT help improve clay soils that are not sodic.

Sometimes coastal soils can develop problems, too, due to salt water intrusion, aerosol sodium, etc. Gypsum can help in those issues. I've done extensive work in locations where salt in the well water was a real problem for all kinds of nursery crops, sod, and other crops.

Farmers are the primary users of gypsum. So many agricultural crops have a high calcium requirement and some also require sulfur for their unique qualities. It would be a good amendment to add to your vegetable garden on an annual basis. This use is strictly nutritional, remember, and has absolutely nothing to do with the structure of the soil.

Here is a link that might be useful: click here

    Bookmark   July 26, 2010 at 12:12PM
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You may find this piece from the University of Illinois extension service helpful.

    Bookmark   July 26, 2010 at 3:10PM
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