It is more accurate to test the pH of your run off than using a soil probe
The soil probes are notoriously inaccurate in reading soil pH
Speaking from experience
Check your PPM like @TDubWilly states, it just tells you if you plant is eating just right or you may be giving to much, but really they look good is it only the bottom leaf with the problem? @Fool1977 you do PH the water after nutes added before feeding? what is the reading mate?
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I always pH my water after nutes and keep it between 6.5 and 6.8. Tonight I measured the pH of the run off and it was 5.6. I’m not sure how that happens. To be honest, my pH tester is probably a POS so I’m going to buy a new one that is slightly more expensive with a lot of good reviews. I also have a PPM meter coming tomorrow.
Great idea, a cheap PH pen can cause problems.
Do what you gotta do, beg, borrow, ( don’t steal though lol ) and get a ph pen and ppm meter. I was doing best I could just using drops for ph going in and had no idea what was coming out ph or ppm wise. It’s my first grow. My buds are looking good but I developed some leaf issues at about week 3 of flower. Finally broke down and bought ph and ppm found out I am getting a 5.2 ph run off and the ppm run off on one plant is high. On week 6 now flushed once, trying to raise soil ph and praying 
I can nurse them to the end, think I will be okay but for 40 dollars would have saved me a bunch of head ache 
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Most leaf issues seem to happen mid flower, when the plants are pushing to “survive”.
Believe it or not I grew for years, (outside) without ever testing pH. Indoor growing didn’t go so well for me without knowing pH.
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I believe it. My moms brothers did the same for years. I did as well when I was in my 20’s twenty years ago 
lol this is my first foray in decades and first ever indoors. I am learning everyday though. I updated my post if you get a chance to check it out. Thanks to all you guys for the support and help. Grow grow grow 
So I measured the PPM of my runoff and it was 1380. No idea what that means. Maybe somebody could enlighten me. Also, I got my new pH meter and I don’t think it works because it measures every liquid exactly the same. Water, distilled water, vinegar, whatever, it’s the same pH. I might have messed up its calibration, but it only came with 3 packets of buffer so now I’m screwed until I can either re-calibrate it or finally break down and get a truly decent meter.