Theres usually at least a couple small bags of organic time release fertilizer kicking around at most garden shops. You can use osmocote but just know its a chemical fert and will work against the microbes in your soil instead of with them, you also would probably need to scrape off any of it's residue ontop of the soil if you plan on flushing before harvest. Your native soil looks very rich I would continue with straight water until they show you that they need more food. Personally I would just assume water in all fertilizer just for more precision but if your not checking in regularly a time release fertilizer may be more up your alley. Last and final thing is I strongly recommend you look into brewing a compost tea I think it would be the best bet in your situation, just my 2 cents.