Sugar, grains, legums (beans), and dairy.
Since the air around you is about 78% nitrogen gas, you might think it would be easy for organisms to retrieve nitrogen easily. However, most organisms cannot use nitrogen gas. Nitrogen gas is called "free" nitrogen because it's not combined with other atoms. Most organisms can use nitrogen only when it has been combined with other elements to form nitrogen-containing compounds. So, is that the answer you were looking for (possibly for homework)?