Answer:
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.
