Keeping the pH Down in Containers

by Jeff Gillman

Keeping the pH down in container production is tough, there’s no getting away from it. Much of the irrigation water that we use here in Minnesota is over 7 and quickly renders our media too alkaline for production of many acid loving plants such as azaleas, rhododendrons, and blueberries. Injecting acid into irrigation water is an option, but this is expensive to set up and expensive to run. Adding amendments to soil is the preferred way to control pH whenever possible. The most commonly used amendments are iron and aluminum sulfate. These produce a quick result, but they tend to decrease in potency over time. Adding elemental sulfur is not commonly done in container production, however, it could produce the pH conditions that are desirable for acid loving plants.

In the spring of 2002 blueberries were planted in 1 gallon containers using a pine bark and peat media. Amendments were added that included iron and aluminum sulfate as well as a few different forms of elemental sulfur (prilled, flaked, and mixed). Over the course of the spring summer and early fall irrigation water was added that had a pH of about 7.5. Readings of the container pH were taken every two weeks so that charts could be drawn to predict pH changes. At the conclusion of the experiment some surprising results were discovered. Iron and aluminum sulfate both decreased pH in the containers, but by the end of the experiment pH’s were rising. Elemental Sulfur, however, continued to decrease the containers pH throughout the course of the experiment. The best treatment that we found for decreasing pH was a mixed sulfur product containing 90% sulfur and was mixed with media at a rate of 4.5 lbs / cubic yard. The mixed product include various sizes of sulfur including some flaked sulfur and sulfur powder. Although the other sulfur products produced acceptable results, as indicated by the figures included here, the mixed sulfur products, called agri-sul and solu-sul, were the products that we feel most comfortable recommending. It is important to keep in mind that we are recommending these products for acid loving species and not for general production.

This experiment was eye opening for us. We expected the iron and aluminum sulfate to do much better than it did. In the end though, it seems that elemental sulfur was much more effective at keeping the pH down than either of these compounds. The figures included with this article provide the breakdown of how all of the products that we tested performed over time.