Best thing to do is test the runoff. Just water like normal the nest time you do, but collect the water that drips from the soil. You will need to feed it slowly to get an accurate reading, and the first bit of water that comes out will be what you want to test. With that reading you should be able to adjust the soil to the correct pH level. If the runoff water is 6.5 you are good to go; higher or lower you will need to adjust the soil. Hope that helps.
I can tell u Medi is right. I have learned first hand from nute lock ups.