If farmers wanted to increase the amount of naturally occurring nitrate in their farm fields so that they would have to apply less of it as fertilizer, what would be the best thing for them to do?

Alternate their normal crops with crops of legumes.

Explanation:

Certain plants, known as nitrogen fixing plants, can draw nitrogen from the air and store it in their roots. They do so through a bacteria called rhizobium. This bacteria infects legume plants in particular, and converts nitrogen gas to store it in the roots of these plants. In this way, a farmer could increase the amount of naturally occurring nitrate in his farm field.

when it's short and dry.