You can usually tell if they are ripe if the tendril is dry on the stem of the watermelon. It looks like a curled pigs tail. That is not always a sure thing though. The large one I posted a picture of was not completely ripe and it was dry on it. The best way is to count the days from when they sprouted and see if the number of days are up for that particular variety. It usually varies by variety, anywhere from 80 to 100 days after the plant is sprouted. As far as fertilizer , I water them every day for 30 minutes on a trickle irrigation system and time clock. I have it teed off of our fall mums field so it gets the same feed as they do, from early June till they are harvested. Its an odd fertilizer type, but I think the home gardener could use any 20-10-20 type with good results. The cut one pictured is a crimson sweet in case anyone was wondering.