At 5.15pm on Monday (26) a crew from Cambridge was called to a fire in the area of Trumpington Meadows, Hauxton.
A passer-by had noticed a fire in fields adjacent to the M11 and when crews arrived they discovered a fire involving about 40 tonnes of waste straw.
Firefighters used two hose reels to prevent the fire from spreading and worked with farmers on site using agricultural machinery to safely spread the straw and dampen it down.
The crew left the site at 3am and have returned every few hours to inspect it.
The cause of the fire is believed to be deliberate.