How to acidify soil to grow blueberries
Last year, I bought 5 blueberry plants (3 varieties involved) at the Arboretum sale, added peat moss to half the prospective blueberry patch (estimate 1 cu. ft/15 sq ft) and as an afterthought got the soil tested. Result: pH 7.1, organic matter 5.1%, phos 26 ppm, K+ 68 ppm. Recommendation was to acidify by adding sulfur 1.9 lbs./100 sq ft, which I did. All 5 blueberries survived winter, 4 of the 5 flowered, berries are now forming despite the small size of the plants, which are growing little if any. They get several hours of sun per day. Repeat soil test this year (I probably did a better job this year of submitting a sample from the top 12", rather than a shallower sample) is pH 7.6, organic matter 4.8%, phos 13 ppm, K+ 41 ppm. Report includes the comment "soil acidification with sulfur is not recommended for this soil." Because I have leftover sulfur, I am tempted to use it anyway. Questions: 1) Why isn't sulfur recommended this time; what would happen if I used it? 2) Is there anything that is recommended instead?
If you got your soil test done at the University of Minnesota Soil Testing Laboratory, you should direct your specific questions to them at (612) 625-3101.
But I will make a guess about what is happening. My understanding is that the lab does not recommend growing blueberries if the soil pH is greater than 7. Your first test at 7.1 maybe came in just under the wire. The second test was too high.
The reason for this is that the soil acidification process becomes quite extreme at high soil pH and many folks don't follow through, resulting in failing plants. However, it can be done. Here is an excellent reference:
You will see that the recommendation is 0.8 to 2.4 pounds S per 100 square feet for lowering pH one unit. You are starting above 7 and want to get to below 5, so you want to drop pH by 2.5 to 3.0. The 1.9 pounds would be at the low end of this recommendation.
Also, you will note that elemental sulfur acts slowly, taking at least one season to begin to act, and that it should be incorporated to a depth of at least 6 inches (which is why we recommend adding the S before planting the plants). My recollection from my own garden (starting at 7.5) is that it took two full years before the pH began to drop. This may be why your soil pH hasn't changed yet.
I would use the leftover sulfur (within the guidelines in the above reference). Furthermore, keeping the soil at a low pH will be an ongoing process, since the soil will tend towards its native pH. (This is why we often recommend growing acid-loving plants in raised beds.) On a yearly basis, you should fertilize with acid fertilizers and incorporate peat moss into the top two inches of soil. You should also monitor the soil pH yearly - if you are not already, please use the University of Minnesota's reliable soil testing laboratory:
The blueberries you saw this year were probably the result of energy already stored in the plant before transplant time. I suspect it may be two or three years before you get regular crops.
Thank you for your detailed answer, it seems very sensible and thoughtful, and I'll follow it. I had had the same hypothesis as you regarding getting a crop already this year. FYI the lab which did the testing is the UMN lab, we live about a mile north of the St. Paul campus.