It takes a minimum of 64% calcium BS  in the soil for the gypsum to remove mag and replace it with calcium. Mag has 2 + charges and isn't easy to leach out. It takes good drainage 1st. Without enough calcium the gypsum is a waste of $. Gypsum isn't a product that works on all soils.

I guess I don't follow you....

if you have to have at least 64% calcium before using gypsum, what's the point of using gypsum?  

Who came up with the 64% number? Do you have some research articles to back up that number?  or is this someone's rule-of-thumb?

In that post below, the I didn't think the pH levels were too bad - at least in most of the samples, but the % Ca was a bit low, at least compared to the targets that that most toss around.  So, I guess I don't understand why adding gypsum would not help to improve calcium levels in those kinds of situations.

Can you elaborate?

The point of gypsum is it contains cheap sulfur.

64% is Kinsey's number and general rule of thumb.

It takes high calcium lime to raise the calcium base sat

Calcium effects the ph 1.0 x

Mag effects the ph 1.6 x 

The point is gypsum contains CALCIUM and sulfur.  So, if you need to raise calcium levels,  why can't you use gypsum as part of the program to raise calcium, and supply sulfur as an extra benefit? 

It seems to me that if your calcium levels are in the 40-50% range,  adding gypsum should help raise that level.  However, what I hear you say is that gypsum will not work until the calcium level is above 64%.  It does not make sense to me.

What am I missing?

P.S.  in your post above, you say that Mag is +2.  Well, so is calcium.

Lime is calcium carbonate. 

Gypsum is calcium sulfate.

Totally different chemistry.  

What you are saying is that you don't know the answers to my questions.

Otherwise, you would explain it.

"I've seen it on my soils where a field that varied from 4.5 to 6.0 pH received flat rate lime and AMS applications. (the worst grid spot might have been close to 4, don't remember for sure) After a few years the pH was 5.9-6.2 uniformly across the field. That was 5 ton of lime on a 10-15 CEC soil over 8 years, not sure how much AMS but at least 100 lbs every other year."

You are talking about a completely different animal here.  Here you are beginning with pH levels from 4.5-6.0.  Well, OF COURSE, you are going to apply lime to raise pH.   That was not the situation presented in the Agronomy 101 thread.  In THAT situation, the water pH ranged from 6.2- 7.9, with the % calcium saturation ranged from the 40's to 70% range.  The comment was made that gypsum will not work below 64%.  I am asking WHY NOT?   It should be basic chemistry.   If you add calcium to a soil, it should raise the % calcium, regardless of the source. 

I am beginning to believe that the people spouting this don't have a grasp on basic chemistry.

"The real answer to your question is probably mulit-factorial. However, from what I've seen, below 60-64% calcium saturation on midwest corn belt soils with a high Mg saturation, the calcium in gypsum tends to leach on through the soil profile without displacing the Mg from the soil colloids."

It seems to me that the higher your soil CA levels are, the more likely you are to be flushing CA down thru the soil profile.  THAT would make sense to me.