This is a website I came across a couple years ago. You may want to check it out before applying any gypsum. Might save you some money.
Gypsum Jar Test <<<<<been taken offlineHere's a copy via Wayback Machine http://web.archive.org/web/20070710063951/http://www.agric.wa.gov.au/content/LWE/LAND/CULT/FN057_1990.HTM