Hi, Thank you for taking time to answer my question. If Professor Bernadine Strik is available, it would be great if she could answer. Overview: I live in Vancouver B.C. and have 30 blueberry bushes for personal use and to unwittingly feed birds. I built a raised bed for them 12 years ago, I used peat moss, well rotted hemlock bark mulch and elemental sulphur. Native soil was not suitable. Unfortunately I was heavy handed with the elemental sulphur, the ph has been stable for the last 7 years at 3.5 in some areas and 4.0 in others. The plants in areas at a ph of 4 bear fruit quite well and are about 5’ tall. All plants under 3.5 ph are growing and fruiting, but not thriving, around 3-4’ tall. I have two questions: 1. Ignoring the ph for a moment, (as I intend to make a new bed, for more blueberries) is growing in a raised bed (there is no standing water issues) with a small amount of peat moss (20%) and (80%) well rotted hemlock bark mulch (“Mulch”) (ph is 5.4) fine, or should I add some sand, or other material? 2. Can you please comment on and correct the following proposal for Lowering ph: I intend to remove every 2nd blueberry bush and move to another bed (as they are only 2 or 3 feet apart) I will then dig out all loose material between plants in the existing bed, down to 1’ deep and scrape off all mulch around plants down to the roots. I will then mix 1 part Mulch, with 1 part existing 4.0 ph material and put it back around bushes. I will use 2 parts Mulch 1 part existing 3.5 ph material. Then I will check ph every 6 months. I was planning to do this in November.
Do you have better ways to do this, I thought I could use gypsum. But I suggested above, as blueberry bed is easy to get at, so moving Mulch is not a concern labour wise.
Outside United States
Unfortunately Dr Strik is away from the site. It sounds like you are very much in control of your situation. Since the bushes even in the lower pH section are fruiting and growing (3' is not a bad size... different varieties attain different sizes...), I don't believe the situation is so dire that you need to remove and replace the soil. Acid levels will return to background over time anyway: unless the base soil is also acidic it is surprising the beds have remained stable for so long (seeing as you say you overdid the acidification originally which would imply the raised beds are more acidic than the natural soil). In any case with only a moderate amount of change required, I would suggest careful treatment with a pH-raising amendment: such as lime (that is the go-to amendment and easily available in most garden centres). Gypsum would not raise the pH. If you are having professional soil tests for pH, the source should be able to recommend the precise amount of lime to adjust for the crop. I would not recommend home-test kits since these are ok for a general idea but are not accurate enough for making decisions about amendments.
Thank you Glen, I appreciate you taking the time to respond.