they allow 20% of the fields to lie fallow each year

fallow

1

adjleft unplowed and unseeded during a growing season

“fallow farmland”

Synonyms

unbroken, unploughed, unplowed

(of farmland) not plowed

adjundeveloped but potentially useful

“a fallow gold market”

Synonyms

undeveloped, unexploited

undeveloped or unused

ncultivated land that is not seeded for one or more growing seasons

Type of:

cultivated land, farmland, ploughland, plowland, tillage,tilled land, tilth

arable land that is worked by plowing and sowing and raising crops

lie fallow 

1. Lit. [for farmland] to exist unplanted for a period of time. The fields lay fallow under the burning sun. It had been too wet to plant last spring.

2. Fig. [for a skill and talent] to remain unused and neglected. You should not let your talent lie fallow. Practice the piano before you forget how to play it. His writing had lain fallow for so long that he could hardly write a proper sentence.

See also: lie