I put Gypsum down on Sunday because our calcium levels were pretty low from all the rain that we have got this month.  This was a new kind of gypsum that I was able to get from a vendor on short notice.  Usually one rotation of the irrigation heads will make gypsum disappear - not this time.  Even though I put down a heavy rate, the fertilizer didn't water in all that easy.  It is just now pretty much gone, and needless to say...that will be the last time I purchase that brand of gypsum.