What is this yellowing on the top leaves?

if internodes at top are super short OR you recenlty lowered the light... could be light burn. A bit lackin on info to poperly answer.. so take what you can from below.. nothing is meant as an exact suggestion but ways to deduce what is wrong given your greater access to relevant info. Soilless only info below, though if you know how your soil depletes through familiarity, there may be a tidbit of useful info, but it will requie translation/conversion for a soil context... in the end must provide similar amounts (a rate is better understanding than an 'amount') of each for a healthy plant: Is this a soilless grow? should feed evey time you water, if so. plug GHE guaranteed label %'s into https://manicbotanix.com/calculators/ppm-in-solution-calc.php Add up each individual molecules ppm, as well as the total. Compare to my ppm screenshot in any recent diary.. minor differences are expected... HUGE differences are going to point toward what is out of balance. ** Ca and Mg might vary by up to 30-50ppm because i have hard water. if you use RO or filtered in some way etc, you will need more of Ca/Mg than i use, for example. The N,P,K molecules should be very similar thought. 5-10% different you can probably ignore. if nothing stands out... environment, pH, light are more likely causes. in this case the light and pH are something to check on. e.g. a QB at 12" could burn a plant in center of coverage... looks like a boron deficiency, but that'd be crazy rare in soil. any rusty/corky-ness at bottom of trunk? i doubt this is the case.. just covering bases.

Ich tippe auf Magnesium Defizit.