Skip to main content

Creating a Soil Mix for Blueberries

Karl Foord, UMN Extension Educator

Exhibit 1, click to enlarge.

Plants have a soil pH range within which they function well (Exhibit 1); however outside this range plants can show signs of stress. This stress is often in the form of a pH induced nutrient deficiency. In blueberries an out of the range high soil pH will induce an iron deficiency (Photos 2 & 3). To get a sense of pH, a number of commonly encountered materials and their respective pH values are listed in Exhibit 1.

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.

Photo 2: Iron-deficient plants have chlorotic leaves with green veins.

Symptoms develop first on the young leaves at the shoot tips.  It will be a significant challenge to modify the pH 7 clay soil to the point where it will be amenable to blueberries. Clay soils tend to resist changes in pH and dramatic changes will be necessary to achieve a soil pH of pH 4.5. A pH of 4 is not three points away from 7 it is 1000 because the pH scale is logarithmic. Rather than attempt this change, the plan is to abandon efforts to change the soil and start over with a new soil. The soil mix proposed is as follows:

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
20% Perlite

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.

Exhibit 4: click to enlarge.

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.