Nitrogen deficiency yellows the old leaves at the bottom of a CBD plant, but when the yellowing starts on the young growth at the top and the leaves stay small and stiff, sulphur is the more likely gap. It is common in coco and in soil mixes built from compost, since neither carries much of it. How Epsom salt or gypsum corrects it quickly, and how to avoid confusing it with iron, is explained on https://calmcbdseeds.com. Most bottled feeds cover it, most organic mixes do not.