From the North Carolina State University site:
http://www.ces.ncsu.edu/cabarrus/staff/dgoforth/limefaq.html#L20
What is gypsum?
Gypsum, is calcium sulfate. It doesn't change the soil pH. Applying gypsum to an acid soil (pH less than 5.5) can have adverse effects on certain crops by displacing soil aluminum, which is toxic to plant roots. Use lime until your pH is at the desired level. Then use gypsum to add more calcium.
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Can gypsum be used instead of lime?
No. It doesn't change the soil pH. Use lime until your pH is at the desired level. Then use gypsum to add more calcium.
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Does gypsum improve the soil?
On high sodium soils, gypsum will improve the structure of the soil without changing the pH. Most soils in North Carolina don't contain high sodium levels.
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What is landplaster?
The same thing as gypsum.
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Why is landplaster used on peanuts?
To provide calcium without changing the pH. Peanuts don't have the ability to translocate adequate calcium to the fruit so the pegs must touch calcium to form properly. Sandy soils don't hold calcium so land plaster (gypsum) puts the calcium there when it is needed. Clay soils have the ability to hold calcium so landplaster isn't needed on peanuts grown in clay soils.
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When I was growing peanuts we used agricultural gypsum extensively. The best way to loosen up heavy clay soil is to add organic matter. you can do this by applying and plowing in some type of manure. We use chicken litter. Also, you can grow some kind of cover crop, winter or summer, and plow it down. This is best done with a moldboard plow.