You're gonna need to flush with water with a lower ph value than u want to achieve. 6.4 is good for soil so if your water is ph 7.4 you need to adjust it down to about ph 6. Flush through keeping an eye on the runoff ph value. When you reach the desired ph... adjust the water you use for feeding to the desired ph value every feed. I keep a large container of water so Im not messin around with the ph too often