Ten tonnes of baled straw were deliberately set on fire in a barn in Werrington.
Fire crews from Stanground and Dogsthorpe were called to Hurn Road at 3.51pm yesterday.
The firefighters used one jet and one hose reel to extinguish the fire and had returned to their stations by 6.15pm.
Anyone with information should contact police on 101 or by visiting www.cambs.police.uk to submit an online report.
Alternatively, call Crimestoppers on 0800 555111.