Well, seaweed is rich in iodine ( some types more than others) and seaweed is well known as a soil additive. Seaweed, dried and extracts thereof are commercially available and contain all manner of valuable trace elements as well as iodine, so I can see that iodine might well be a usefull additive to potting compost.
Just wondering if the Japanese translation of iodine might not refer to a seaweed or seaweed product, rich in iodine, rather than "straight" iodine ?