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You can use a proper mixtureYou can use a proper mixture of white vinegar and water to boost the acidity of your soil or you can add pure sphagnum peat which has a very low ph. Blueberry bushes thrive in low ph and are suggested to be planted in pure peat. So this should remedy you ph issue for raspberry bushes..

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?