Question

Why is it important that farmers plant crops during the winter that improve the levels of nitrogen in the soil?

Answer 1

These crops all use nitrogen in their specialized photosynthesis process.
These crops can fix nitrogen into usable nitrates.
These crops have root nodules that contain bacteria.
These crops produce fruit that quickly decomposes into usable nitrogen.

Answer 2

http://en.wikipedia.org/wiki/Nitrogen_fixation
The answer I believe is the second option, as some microorganisms (some of which symbiotically thrive on the root nodules of certain crops) can perform biofixation which converts atmospheric N2 into an important nitrate, NH3 (ammonia).