I spent years nurturing and building the soil in those raised beds and I did not experience any detrimental effect of the cedar on the soil contained in those raised beds. The healthiest, most delicious and best looking plants I've ever grown were in those raised beds. I am not a scientist and have not gathered data from tests beds along side control groups, but based on my experience I tend to have my doubts about cedar having a negative effect on the microbiome as a whole in the soil contained in raised beds made from cedar. Hope this helps!
"Study books and observe nature; if they do not agree, throw away the books." ~ William A. Albrecht
I've had a raised bed of eastern white cedar (6-8" logs) for a decade or so. Mine is a bit tall at 24" so it seems to dry out a bit more than I would like but I don't think the growth is retarded in that bed compared to the ground. Certainly the stuff around the base of the bed (right up against the logs) grows just fine.
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