Banteay Meanchey Province police and a team from the Cambodian Mine Action Center uncovered mortars, rockets and anti-personnel landmines Friday in Sisophon after being alerted by the farmer who came across them while inspecting an irrigation canal, The Phnom Penh Post reported.
Officials said they believed the weapons were buried by Khmer Rouge guerrillas and were exposed when the canal's water level dropped several yards.
The Khmer Rouge ruled the country in the late 1970s but was not officially disbanded until 1999.
"We found 13 mortars, 21 rockets and 13 landmines. Most of them were rendered useless by the dirt and water," said Saem Bunnareay, a manager at the mine center.
The center's deputy-director general, Heng Ratana, said the discovery was not an uncommon occurrence, with 3,200 items found in one section of Banteay Meanchey alone last year.
"We've found many items like this, at the back of a house, at the back of a barn. Sometimes even the owner doesn't know it at all," he said.
"Many hectares of land have unexploded ordnance, but we do not have a project to determine just how many."
A hectare is 2.47 acres.
Meanchey said his office received more than 400 tips last year.