Gypsum is one of the oldest soil amendments used in agriculture.
1) Gypsum is proven to help flocculate soils. If you don't believe it put gypsum in a cloudy pond (if the result of silt intrusion) and watch it turn very clear in a matter of only a few days. Farmers have don't this for years with barley bales. Straight gypsum works well also.
2) Purchase the cheapest fine textured gypsum you can. The finer the texture the quicker it works. I've used gypsum for clearing up lakes and applying on fairways and roughs for years, if you see a cloud behind the spreader your probably using something that will work quickly. "Rock" products may take years to fully break down and get into the soil. Sure they are less dusty to spread but you also may not be alive when they are fully available to the soil. Some companies sell crushed sheetrock (yes what your walls are made of) and that works ok also. Sheetrock is primarily gypsum.
3) Have realistic expectations. If my PH was incorrect, or I needed Nitrogen, or Phosphorus etc... I'd spend my money on those products 1st. Gypsum is helpful in tight, remediated, or de flocculated soils but it can only do so much. The good news is its generally inexpensive.
4) Consider construction suppliers like US Gypsum for larger orders. They generally have fine textured gypsum and they don't have a mark up.
Good luck. Gypsum is a great product. As with all products remember there is no Superintendent in the bag.
