"Liquid seaweed is particularly rich in trace elements and hormones. The benefit of the latter can be quite remarkable when applied as a foliar spray, with a marked greening effect and improvement in growth. It can be applied at any stage of development and usually increases yield and size of specimen plants."
Seaweed itself is low in nutrients and is used as general tonic, but mixed with fertilizer eg. in tomato feed it boost the absorbsion of other elements..