Recipes for homemade fertilizer for flowering shrubs?

Bananas are good as they contain high levels of potasium, essential for flowering. They worked really well with my girlfriends rose bush, literally turned a dead plant into a beautiful flowering bush! Still going now into October, keep in mind I live in Wales! I dry the skin and scrape the inside of the skin of and crumble it onto the top soil and water it through. My girlfriend puts the entire thing on it as slugs will eat the banana rather than her plants, I cant find a naturally occurring form of pottasium. Theres more than likely some in bananas but I don't know!!! I came here because I was looking for a better answer than mine not really to leave one! Hope it helps!