Fire crews remain at the scene after they were called to the blaze on Southend Road, between East Hanningfield and Rettendon, at 4.50pm on Friday, although it has now been put out.
Fire chief Rick Hylton said: “The incident is now under control, although firefighters will remain at the scene, dealing with the remaining hotspots and supporting the transition into the recovery phase.”
He added: “Firefighters arrived quickly and were faced with a large and rapidly developing fire involving several units and surrounding farmland.
“At 5.46pm, following an explosion involving fertiliser within one of the industrial units, Essex County Fire and Rescue Service declared a major incident.
“We took that decision because of the scale and complexity of the incident, the significant resources required to bring the fire under safe control, and the need to co-ordinate a multi-agency response.”
People living near the site were told to keep their windows and doors closed, and a “major incident” was declared as thick black smoke engulfed the surrounding area. Nearby residents had been evacuated.
The service’s area manager Dave Bond said on Friday: “The incident will be ongoing into the weekend, so please stay away from the area and allow us to continue operations. We apologise for any inconvenience this caused, and we thank you for your patience.
“The advice we’re still giving to the public is that if you do smell smoke, then close your doors and windows. But please bear with us as we continue our operations to extinguish the fire.”
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Colin Neale Jr was driving along the A13 near Dagenham when he saw the plumes of smoke.
He told Sky News earlier that, as he drove closer, he realised how big the fire was and decided to send his drone up over the fields.
Mr Neale said the drone showed “multiple fields” had caught fire and it appeared two industrial complexes were affected.
Local councillor Danielle Belton told Sky News: “The incident has really shook the community, the explosion was heard and felt for miles surrounding the site.
“As always, our fire and rescue service have shown an incredible display of bravery and dedication to getting the situation under control and to protect those closest to the incident.”
A spokesman for East of England Ambulance Service said its hazardous area response team had gone to the scene.
