A lightning strike ignited a stack of 9,000 tonnes of straw in Chatteris.
Emergency services were notified at 12.40pm and five fire crews fought the blaze at Langwood Fen.
One fire engines came from Chatteris, two crews from March, one crew from Ramsey and a crew from Ely.
Firefighters used two light portable pumps, pumping water from a nearby open water source through five jets, one hose reel and four ground monitors (a hose reel fixed to the ground) to extinguish the fire.
Crews created a fire break to stop the fire from spreading to about 17 hectares of standing crops.
The cause of the fire is believed to be due to a lightning strike.