The company's health and safety handbook stated that employees should never climb into the hopper when the chain is going, but there were no prominent signs on the truck warning of the dangers of doing so.
WorkSafe's chief investigator, Keith Stewart, said Wealleans Groundspread Ltd failed to protect the safety of workers.
"The footholds and handholds on the side of the truck were inadequate to allow Mr Old to safely lean over the side wall of the hopper and push any fertiliser that was stuck down on to the chain conveyor.
"The company could also have provided a tool with a reach long enough to reach any residual fertiliser instead of the standard spade that was made available," Mr Stewart said.