Perhaps you should try some spelling variations:
ref history » reef history, brief history, bief history
2f germany » 20 germany, 21 germany, 25 germany
2 germany » 7 germany, 2 german, 8 germany
1990 20th » 1995 20th, 1999 20th, 1994 20th
90 20th » 20 20th, 94 20th
4 20th » 94 20th, _ 20th, a 20th

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %22agriculture AND state %((2f germany) OR (2 germany)) %ref history %((((90 20th) OR (1990 20th))) OR (4 20th)) century%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22agriculture and state %((2f germany) or (2 germany)) %ref history %((((90 20th) OR (1990 20th))) or (4 20th)) century%22".
• Adding a wildcard symbol may retrieve word variants: %22agriculture and state %((2f germany) or (2 germany)) %ref history %((((90 20th) OR (1990 20th))) or (4 20th)) century%22*.