Four crews were called to Earls Ditton, Hopton Wafers, near Cleobury Mortimer, just after 6.05pm.
Shropshire Fire and Rescue Service said crews successfully worked to remove a potential explosive risk and to protect the environment.
Both the tractor and trailer were destroyed in the fire.
Breathing Apparatus, a hose reel jet, main jet and thermal imaging camera were used to tackle the blaze.
Adam Matthews, hazardous materials officer for the fire authority, said: "We believe it was an electrical or mechanical fault.
"We had concerns regarding the fertilizer.
"There was potential for it to react violently coming in contact with fire.
"We managed to avoid this risk. It probably took an hour to get it under control."