Answer: It sounds like a great question to test all on its own: Do over-the-counter antacids have the same effect on soil pH as agricultural limestone (CaCO3)? But it sounds like you might be pressed for time, so I?ll cut to the chase.
According to David R. Hershey, author of Plant Biology Science Projects, you can indeed adjust the pH of soil, or a growing medium such as sphagnum moss, using over-the-counter antacids. It?s best to use those containing calcium carbonate rather than sodium carbonate, because concentrated sodium can harm plants. (I checked a Rolaids label; the active ingredients ? calcium carbonate and magnesium hydroxide ? are similar to the makeup of an agricultural dolomitic limestone ? calcium carbonate and magnesium carbonate.) Grind the tablets so you can distribute the material evenly throughout moistened soil. Take pH measurements before and after adding the material to document the amount of change that occurs, and adjust the amounts as necessary.
Antacids dissolve easily, but limestone does not. Also, remember that soils in climates with high rainfall tend to be more acidic than those in arid areas. Based on these two facts, you should check the pH of your growing medium after several waterings to see if it has changed.
Have fun, and may the marigolds thrive (or do whatever you hypothesize they should)!
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