runoff ph way to low

I flushed 1 plant 7 times with 6.5 ph to 7.5 ph water and each time the runoff ph stayed at 4 with and average ppm of 1000, how do i get my ph to rise, i ran many gallons of water through the soil to no avail, do i just have to keep doing it till the runoff ph reaches 6-6.5?