as your plant uses the nutes in the medium and salt is added your pH of the medium will be changed. The flushing of the medium will remove those bad intoxicants and you can see that by the run-off water. This is more of a personal pref to do soil or water. Most people find that the water run-off is the best way. I personally have a couple of testers that i use. I have one that is water and i have another that will test the pH of the soil as well as test for N, K, and potash. This will tell me more of what the plant needs.

Hope that helps