Question

why are fields sometimes allowed to remain fallow​

Answer 1

Answer:

In the past, not planting anything (also called leaving the field fallow) allowed the land to rest and replenish its nutrients..... so that the land can get converted into fertile land

Answer 2

ANSWER:

Sometimes, Fields are made to remain fallowed so that the soil could gain back its nutrients.

EXPLANATION:  

Fallowing the land refers to the act of leaving the field unplanted for a certain period. After a regular series of cultivation and harvesting a field with crops, the field tends to lose its moisture and nutrient content.

So that the farmland is left with no crops in it, to make the soil recover its fertility.