Karl Foord, UMN Extension Educator
Traditional blueberry soils are sandy with low organic matter and pH of 4.5 to 5. In addition blueberries plants do not tolerate waterlogged or droughty soils. Unfortunately, my soil is a clay loam soil whose two soil tests results indicated pH values of 6.8 and 7.2. In addition the soil was compacted during house construction so the depth of the hole for planting needs to be deeper than the compaction zone, or the blueberries should be planted on raised beds or both. If the planting hole is filled with water and takes more than four hours to drain, the hole is not deeper than the compaction zone.
Proposed blueberry soil mix:
50% Sphagnum peat moss - (pH of 3.6 to 4.2)
10% Original soil - (pH 7 clay loam)
10% Compost - (likely pH @ 7)
10% Sand Inexpensive, untreated sand used primarily in construction
Logic of proposed mix
The peat moss will provide organic matter and be the source of low pH for the mix. It will have to overcome the high pH characteristics of both the compost and soil components of the mix. Perlite and sand will add drainage qualities to the soil mix. The only way to truly know if the soil mix is at the proper pH is to have it tested.
High pH characteristics of compost
Although tree leaves tend to be acidic (Exhibit 4), compost made from tree leaves and most yard wastes will likely be alkaline (pH between 7.0 and 8.0) due to changes in compost components during the composting process. Most manures also have a liming-effect (increase in soil pH) as the pH of cattle and sheep dung is typically >7.0.
Planting and maintenance
To be extra cautious about pH and drainage, remove the soil creating an 18 inch deep by 3 foot wide bed. Fill the ditch and add enough mix to create a 12 inch high raised bed. Space the plants 3 feet apart to create a hedge. Because the blueberry planting is a low pH island in a high pH field with environmental conditions that will tend to increase the pH, fertilize with ammonium sulfate or urea to help maintain the proper pH. Amend any mulch you use with a small amount of elemental fertilizer. See the UMN Extension website for dosage recommendations.
Key summary statement
This particular mix will work as will many other combinations. Given the tendency for many of the soils in Minnesota to be alkaline (high pH), this method may also prove valuable for planting of other acid loving crops such as rhododendrons and azaleas. It bears repeating that the main thing to do is get your soil mix tested and be sure that it is at the right pH for blueberry and other acid loving plants.