I have seen some magnificent home blueberry plants in the past few weeks, just dripping with the dusty blue globs of fruit we love in the summer. Blueberries are fun to grow, but they are a little tricky in that they do prefer soil with a low pH, which is not found in every yard. That means they like acid soils with a pH between 4 and 5. Luckily we can amend our soils for the immediate future. The pH will tend to migrate back toward its original state unless the grower continues to manipulate it. Always get a soil test before you begin to amend it and again at least every 3-4 years.
A quick refresher on pH: soil pH is a measurement of the soil’s acidity. Acidity affects the availability of a soil’s nutrients for plant intake. For example, there might be significant amounts of iron present in a soil, but if the soil’s pH is too high, plants won’t be able to access that iron. The pH scale runs from 0 to 14; 7 is a neutral pH reading. A pH below 7 is acidic, while a pH above 7 is alkaline.
Soils with a higher pH will cause light green or reddish leaf discoloration on blueberry plants. The plants will grow very slowly, hardly producing any new shoot growth, and they might eventually die.
Your soil test will give you a precise pH reading, as well as an estimated soil texture. Soil texture will be estimated as “coarse”, “medium” or “fine”. Both these readings will help you decide on the proper course of action for soil amendment. If the desired soil has a pH higher than 7, or its soil texture is “fine”, your best option is to build a raised bed (at least 18 inches tall) and bring in new soil altogether.
For coarse to medium soils in the 5.5 to 7 pH range, you can mix 4 to 6 inches of sphagnum peat moss into the top 6 to 8 inches of soil. Peat is a good option because it also increases the soil’s organic matter, which is also desirable for blueberries. As the soil decomposes over time, you will need to continue adding peat moss or another acidifying agent.
Elemental sulfur (also sold as “soil sulfur”) is another product that will lower soil pH over time. Sulfur is less expensive than peat moss, but it also requires more precision and time to be effective. If you have the time to plan ahead, it is best to apply sulfur the year before you plant your blueberry shrubs. Incorporate the sulfur into the top 6 inches of soil. Depending on your soil pH and texture, you will need between 0.4 to 5.8 pounds of sulfur per 100 square feet of soil (lower for coarse soils, higher for medium soils). Three to four months after your initial sulfur application you should test the soil again. You might need to reapply. To help maintain your newly acidic soil over time, use a fertilizer that is specifically for acidic plants. Fertilizers that use ammonium sulfate or urea as their nitrogen source will help keep the soil pH low. Do not apply any fertilizer after July, as this will cause new growth and increases the potential for winter injury on blueberries.
For best success, continue to monitor the new growth and leaf color of your blueberries. Watch for symptoms of high pH, and test your soil every few years to determine if you should add more peat or sulfur.