How big are the pots?

This will bring down the pH temporarily but it will bounce right back. It would take a long time to permanently lower the soil pH. A much better solution is to re-pot the plants with good soil. If they are in small pots now then great. If the pots are larger then you will need to use a trowel to cut a ball around the plant. This is a bit more delicate. Another approach is to let them dry out a bit, remove the soil mass from the pot and then cut out a ball around the plants using a large sharp knife.