Recently received an email from a customer that was inquiring about amending the soil for her roses with oyster shell.  Here is my reply (looking for other thoughts here):
“Yes, Oyster Shell Flour (or crushed oyster shell) adds calcium and helps raise soil pH.  Research I have examined shows that there is a limit to the soil pH effect so adding “too much” should not be a concern.  However, I do not have a specific recommendation of how many pounds to use per rose.  It would depend on existing soil pH and other liming you have done in past seasons.  But, I would hazard to guess that a cup worked into the soil around each rose’s drip line should be a good starting point.  But, I have to recommend you test your soil pH first.  If you are in the local Seattle area in King County, you can get up to five free laboratory soil tests done per lifetime at your address. Otherwise, you can use a pH meter for a rough reading.  Or there are pH test kits you can purchase as well.  We sell the meters at our shop.
Walt”