A method used by horticulturalists when they rotate the use of fields-planting crops on a particular piece of land for a period of time, then letting it go fallow as they move on and begin planting again in a different area-is called:

The correct answer will be - Crop rotation

Explanation:

When the growth of a type of crop which uses soil nutrients in abundance and then growing another crop in the same filed led to the decrease in the production of the next crop.

Thus farmers adapted to avoid such farming practices and developed a new practice which allowed the land to become fertile again so that the growth of next crop will not be affected.

The practice involved the planting of a crop on a piece of land then leaving the soil ploughed and unsown for a longer period of time and growing corp on another piece of land allowing the time to restore the nutrient content. This practice is known as "crop rotation".

Thus, crop rotation is the correct answer.