I will be planting blueberries in 18 inch tall 4x8 raised beds. I plan to use this as my basic soil: (pH 6-6.5) 30% Native Screened Sandy Loam 40% Garden Compost 5% Power Mulch 5% Mushroom Compost 20% Horticultural Pumice. How can I transform this basic soil to make it perfect for blueberries? I am thinking of taking the basic soil and combining it with equal parts Douglas fir bark. Or do you think I should leave the basic soil as it is and just add sulphur? Or something else... what is the perfect soil to place in the raised beds?
Be aware that over a few years time, the organic matter in your bed will decompose naturally. The soil level may drop several inches. This will make the blend in your bed more dense--less aeration for the blueberry roots. The pumice will alleviate this somewhat, but the bed’s current content remains 50% organic matter.
Adding uncomposted fir bark to the mix risks depletion of any nitrogen present (or added) as the decomposers suck up the nitrogen to balance the high carbon of the bark. In addition, you would be increasing the percentage of organic matter in the bed.
The addition of peat moss could lower the pH into the range you would like. You would be adding organic matter, but peat is somewhat resistant to decomposition.
Because your blend is essentially a container mix or potting soil, there is little information to go on when contemplating adding sulfur to adjust the pH. It may reduce it precipitously. If you do decide to go that way, do so slowly, by increments.
Going forward, the type of fertilizer you choose will also have an impact. Acid-forming fertilizers, those with ammonium sulfate or urea, will help reduce the media’s pH or maintain it at a lower level.
Testing your soil with pH papers or laboratory tests will help you monitor the pH as you make changes or additions.