I planted a long row of rabbiteye blueberry plants about three years ago, alternating ‘Tifblue’ and ‘Woodard’ for cross-pollination. The upper half of the row looks great and those plants have begun producing well, but the plants on the lower stretch look about the same size as when they were planted. The color is pale, and they do not produce well. I’m surprised, because these are just alongside my garden where they get extra water. I know the soil is acid because I had my garden tested and needed to add lime to it, since the pH was 5.5 to 6.0. Is the extra water a bad thing?
Answer: The extra water may be a problem if the soil is not well drained. This is easily tested by digging a hole, about a foot deep and wide. Fill it with water and check it a day later. If any water is still standing in the hole, the drainage is not good enough for blueberries.
However, I think your real problem is the soil pH. The range you cited is acid, but not acid enough for blueberries, which prefer a pH from 4.2 to a top range of 5.5. Since you added lime to your garden and water can move the lime, I would bet the blueberries are in soil that is much sweeter than they like.
You often read that you can acidify the soil by adding sulfur or other soil amendments, and you can—for the short term. My experience has been that the native soil pH begins to reassert itself, sometimes slowly, but often quickly, and you will find yourself continuously in a struggle. The blueberries have already told you where they want to grow, so if room allows, dig up the bottom half and move them up the hill alongside the others.
If you doubt that the pH is so different just a few feet away, let me relate my family’s first foray into blueberries. In 1980, we planted a few acres of blueberries on our farm for a u-pick operation, and while parts of the field prospered, other parts, just yards away, did not. It was uncanny how these areas related exactly to the soil map of the field. We knew those areas were a little higher in pH, but thought that adding lots of pine bark and peat moss to the planting holes would make up the difference. Over time, it did not. Also, the field made a slight valley and all the blueberry plants in the lowest area failed.
Other fruit crops have flourished in those areas where the blueberries failed. The blueberries planted in the “happy spots” still produce reliably with no care except for supplemental water during hot, dry summer months.
Moving them may seem like a lot of work, but you will find the root systems to be shallow, and fall a good time to do it, once the plants have gone dormant.