I have a different problem. My runoff from the pot is 7.2, but I'm pouring in 6.5, and I can't lower the pH. I tried flushing the soil with a lower pH; for a 15-liter pot, I poured in 1/3 of the water with a pH of 5.8 to balance it out, but the runoff is again 7.2 and rises to 7.6.