yellowing of leaves

It probably won't be insufficient watering, the plants receive 15 minutes on and 30 minutes off drip irrigation in expanded clay. The new growth doesn't stretch much anymore, I slightly increased the nitrogen level today (micro from Terra Aquatica) and added a new dressing, I'll see what that does..... EC of the dressing is 1.8-2.2, so there shouldn't be a lack of feeding either .

This is lack of run off or built up salts. Flowering npk ratio is 1-2-3. How is ph maintained. What is run off ph. I leaning towards lack of proper irrigation and feed.

Looks to be showing some key macro deficiencies

I think Auto is probably on the right track here or it could be something as simple the plant is cannibalizing some of the old growth leaves because flower nutes are super low in N and sometimes you don't get enough for the stretch period. Nitrogen is bad for flowering but your plants will use some to fuel the stretch. Your new growth looks healthy. Is it stretching a lot? After the stretch I tend to defoliate a lot of those older leaves anyway. If pH checks out I'd probably wait to see if it continues to worsen or moves to new growth. I think it will be fine when it's done stretching.

If i were you, i would check my "hydroponic soil" and see just what exactly the pH of your substrate is. I'm guessing it's not 6.2 like you think it is. Either pH isn't accurate, your soil is toxic with salts causing lockout, of your nutrients are shit, or you are under feeding them. It's gonna be one of those more than likely.

Add grown nutrients to you mix she wanting nitrogen look at my grow la sunset she did that and with a little grow nutes she snapped back go 2 part bloom 1 part grow

Probably gonna need more info to get help. Judging by the crispy edges on some of the edges of the leaves and from the bottom, I would say it's potassium deficiency. TA makes a micro, grow, and a bloom for tripart. Not sure which or how much your using. Plus I've only grown in soil. More info will probably get a better response from someone who uses hydro grows.

The pH is automatically maintained at 6.2 (hydroponics) through the fact that calcium should be best absorbed there and magnesium. I use the tripart from Terra aquatica.