Why Plants Thrive In Deep-Substrate

I tried this deep substrate approach too many times & always end up with failed system. I noticed dark areas on front of glass in sand substrate. I move a rock to suddenly notice a black area right under rock. I noticed small pebbles or shells have black in them. Has rotten egg smell. Probably sulfur. Very bad for aquariums. Aquariums do not want anaerobic areas within them. Plant roots die & turn black when they are planted in that area with zero oxygen. Unless one tries to use very "Coarse sand" mulm will never make its way passed the compacted sand substrate to reach the soil to benefit plants. I choice small pebbles/ gravel instead. There one has a chance to achieve what you talk about. Anoxic areas with low oxygen is better than zero oxygen.