you can still use it to test the ph of your solution before going into the soil, as well as the runoff after it goes through the soil. Comparing the 2 numbers can help you determine what is causing the problem. Spotting like that is generally a Ca / Mg lockout due to ph being off, I've had it myself quite a number of times. I doubt they're over fed. They look good though so it may have corrected itself, but I'd still test the ph just to be safe and so you know what they're getting.,