Why is it important that farmers plant crops during the winter that improve the levels of nitrogen in the soil?
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.
these crops all use nitrogen in their specialized photosynthesis process.

These crops can fix nitrogen into usable nitrates.  

Explanation:

There are certain crops which are called cover crops, specifically legumes like peas, vetch,clover and winter field beans . Farmers grow these crops during winter before the growth of actual target crops like cabbage etc which are thirsty for Nitrogen.

What is special about these cover crops is their ability to fix nitrogen into the soil from the atmosphere. There are various ways through which these leguminous crops can fix nitrogen into the soil:

Some legumes  like hairy vetch and crimson clover, may fix upto 100 pounds of nitrogen per acre for the utilization by the crop growing in same soil next season.Other legumes like red clover and bigflower vetch can fix from 30 to 80 pounds of nitrogen per acre .Now how the legumes fix nitrogen is through  a symbiotic relationship with soil-dwelling bacteria. These are the bacteria that dwell in soil and take nitrogen from atmosphere and provide this nitrogen to leguminous crops through roots which later fix this nitrogen into nitrates.In return, the bacteria can get food in the form of carbohydrates  from legumes.

This phenomenon confers the leguminous crops with an ability to fix  nitrogen in the soil that can be used by the nitrogen requiring crops like cabbage when grown next season in same soil.

Hope it help!

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