hi, I do not really grow any more brugmansias but I do grow a few iochromas. Every year, I have a huge problem with leaf yellowing. The youngest ones start first and it does look like a deficiency as the veins are more yellow than the rest of the leaves. Eventually, the leaves stop yellowing but, by then, it has lost so many and it also looks atrocious next to the small white or blue hanging blooms. I know it is not a brug but it is in the same family - solanaceae. Have tried PH altering, epsom salts - would a general nitrogen boost help?