Firefighters are appealing for the public and farmers to be on high alert following the latest field fire in Northampton on Tuesday (July 26).
Investigations are under way into what caused a blaze in Kings Heath at around 10am.
Up to seven crews from across the county battled flames covering around 400 square metres of crops, hedgerow and grass.
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A Northamptonshire Fire & Rescue Service spokesman said that some crews will remain at the scene dampening down all day.
It follows a number of similar incidents in the wake of a record-breaking heatwave and weeks with no rain.
Two fires in Chapel Brampton and Earls Barton are thought to have been accidental, caused by heat and sparks from farm machinery.
But investigators say at least three — one in Bradlaugh Fields, Northampton, on Monday (July 25) plus two in Billing Brook Road and Burton Latimer on Sunday — are all believed to have been started deliberately.
Sunday's fire at Burton Latimer destroyed seven acres of crops worth around £10,000 and also forced the closure of the A6 for more than four hours.
A spokesman said: “Setting fires deliberately puts everyone at risk and these crop fires are devastating to our farming communities on which we rely on.
“Grass and crops are tinder dry. Even small outdoor fires can be dangerous and place additional stress on our limited resources.
“If you have any info about deliberately started fires, big or small, please call Fire Stoppers 0800 159 5558.”