Yellow leaves, what can it be?

Lower your feed, too much potassium and boost will cause a lock out of micronutrients, calcium magnesium zinc and iron, wich will leave permanent damage and will progress untill its fixed, even after its fixed, it will only stop damaging new folliage because the older folliage will not go back to normal, so... What you want to do here is to start alternating the feedings with water while reducing the feed if you really are growing in soil. Give it some time and watch if the symptoms keep progressing or if they slow down. When they stop progressing you know you fixed it. Or you could up your feed instead, and if it progresses at double the rate, then you know its the other option. You should consider it can just be from the Boost too, because some boost nutrients out there can make plants do some weird sh*t. So, yeah... do that, and if u need get back at me in dm's cuz i'm nearly always available. Good luck and stay lit growmie.