Hi Prairie_north: Definition of plaster of paris: "a white powder, calcium sulfate, that when mixed with water forms a quick-hardening paste. It is used in the arts for sculpting and making casts, and in medicine for molding casts around broken limbs." Plaster of Paris is DIFFERENT from gypsum. Plaster of Paris is made by being heated up, making it into an unstable form ... if you use plaster of paris it will harden your clay further.

If you don't have access to SOLUBLE gypsum ... I would use Garden sulfur (has sulfur plus gypsum) to lower your tap water. Only a small amount (few teaspoon) is enough to lower your tap, to release nutrients in heavy clay. The down-fall of sulfur is it's caustic, best diluted in water, before giving that to plants.

If you want to try a small amount of SOLUBLE gypsum, Amazon sells that for $8 per 1 lb. http://www.amazon.com/Calcium-Sulfate-Dihydrate-Gypsum-Fertilizer/dp/B008LETYN6

Kelp4Less is where I get my SOLUBLE gypsum, at $16 per 5 lbs., free shipping in USA, but I'm not sure about Canada. Msgirl in Hawaii ordered from Kelp4Less before.

https://www.kelp4less.com/shop/soluble-gypsum-powder/

Check your local feed-store, they sell granular gypsum cheap, 25 lbs. bag for $10. Pet store or local feed-store carry cracked corn, plus alfalfa hay & timothy hay. Those break up heavy clay well, so nutrients can be released. I have a wet bed with layers of leaves on top, but underneath is compact & heavy clay. I put lots of gypsum in a hole to plant Duchess de Rohan .. it gave many blooms for spring flush. But the place is too shady, so I moved Duchess away, and planted a summer phlox. That summer phlox is the tallest compared to others, plus darkest-green leaves, plus bigger blooms. Gypsum does both: lowering pH of alkaline tap water, plus supplying calcium. Calcium is very important in growing roses: it's an anti-fungal agent, plus making blooms bigger, plus making stems stronger to support heavy blooms.

The granular gypsum doesn't dissolve well in water, it leaves a sticky & clayish residue .. best used to break-up clay. I prefer SOLUBLE gypsum to use on top since it easily dissolved when watered-in, rather than gunking up.

Thank you straw for the excellent information! My previous neighbor said he used gypsum in his yard, it must have been the granular kind as he said it was to break up the clay. It is good to know the soluble kind can be used for lowering pH and prividing calcium.

Unfortunately the feed store the other day did not have either granular or soluble gypsum, I will keep looking around. I was confused when researching gypsum as some pages said there was some kind of connection to plaster of paris.

Hi Prairie_north: I post info. on salty manure versus less salt organics (alfalfa hay, cracked corn, red-lava rock, pea pebble) in your other thread (red cabbage juice).

For the planting hole, I get the best result with the no-salt & low-salt stuff: coarse sand, pea pebble at the bottom, cracked corn, alfalfa hay, Rose-Tone or Tomato-Tone, humus & compost. The higher-salt stuff like rotted-manure is best on top. I bought Charlie Compost (organic chicken manure plus corn stalk) on Amazon. That stuff is composted more than 2 years, no smell, yet the result WAS NOT AS GOOD as gypsum, cracked corn, crushed red lava, and Tomato-Tone in the planting hole.

Rotted manure is best on top. Manure has many minerals added to their feed. Chicken has zinc, copper, and boron, antibiotics (bad) added to their feed. Cow has antibiotics, salt and phosphorus added to their feed. Horse has the highest quality diet, less antibiotics, and more alfalfa hay & timothy hay. I tested all 3 types of manure in the planting hole. My conclusion: manure is best on top in the spring.