A fallow field is land that a farmer plows but does not cultivate for one or more seasons to allow the field to become more fertile again. The practice of leaving fields fallow dates back to ancient times when farmers realized that using soil over and over again depleted its nutrients. A three-field rotation system was used in medieval times in which one field was always fallow.
Agricultural experts debate whether the practice of fallow fields is necessary in modern farming and, if it is, how often a farmer needs to let a field go fallow. Most, however, agree that the practice at some interval or another is beneficial, and for dryland farming, it is particularly useful. All other factors being equal, fields that lie fallow do tend to produce better crops the next year. Other farmers choose to rotate crops to retard soil depletion, arguing that planting different crops in different years in a particular field is successful in keeping the soil healthy. Jewish farmers are diligent about the practice because it is commanded by the Torah. Modern farmers who do not want to "waste" a field by allowing it to go fallow plant companion crops or use fertilizers to rejuvenate the soil.