I've never understood why anyone should want to retain their search history at all. (That's what bookmarks are for!)
However that nice Mr Google suggests that what you're seeking to do simply isn't possible in Safari. e.g. here:
https://discussions.apple.com/thread/4587197
(There are other threads littered around the web which seem to confirm that information).
So you might want to try a different browser. Opera Mini is extremely popular
https://www.opera.com/mobile/ios
Dolphin is excellent on Android devices, so it's probably worth a look on iOS too
https://itunes.apple.com/us/app/dolphin-mobile-browser/id634693702?mt=8