People living near the fire – which happened close to the village of Rettendon, south of Chelmsford – were urged to keep their windows closed as smoke spread.
As of Friday evening, it was not clear whether there had been casualties, but in an update that night, Dave Bond of Essex County Fire and Rescue Service said: “While on the scene, fertiliser inside one of the units exploded and residents across the county have reported hearing and feeling it.
“This is why we declared a major incident, and I want to reassure our residents that there is no wider risk to the public at this time.”
He said “everybody at the scene is safe” but that residents have been evacuated.
The fire service said the response was scaled down, with about 50 firefighters on scene as well as specialist officers and advisors to extinguish the rest of the blaze.
East of England Ambulance Service said its hazardous area response team and the police were also in attendance.
Earlier on Friday evening, Bond said there was still “significant smoke” at the scene, but he believed that the fire had been “surrounded”.
