A fire destroyed 100 tonnes of straw and spread to a neighbouring field in the Fens.
Crews spent Sunday night and yesterday morning at the scene of the fire in Cross Drove, Tydd St Giles, near Wisbech.
They used hose reels and beaters to battle the flames after the alarm was raised at 10.30pm on Sunday.
Today Cambridgeshire fire service said the cause was deliberate.
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