Since starting growing outdoors, it was speculated that elephant dung was the best fertiliser for growing cannabis, so any time the circus was in town I'd be down there every day collecting it.
When that wasn't convenient, or a lack of elephants, I just relied on my soil and if the fade started early which it often would I'd suppliment with a feed of kelp extract, once a fortnight was all that was typically needed to once a week on occasion, but be careful, it's often high in N.