You feed with every watering? If thats the case yeah, you definitely have build ups in the soil. I would skip fertigation, cut back to every other or even every 3rd watering. I only fertigate when my plants start to become pale. Im on day 50 of a photoperiod grow and Ive only fed 2 times. And photos are much hungrier than autos
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This is week 3 in flower, only one veg feed and one flower feed so far. I know biobizz light is a pretty baron soil but feeding with every watering will cause build ups over time which is what you are seeing now.