The six-member patrol was escorting teachers on the Ban Duwa - Ban Thanon road when it was informed by villagers that two cloth banners insulting soldiers had been hung by the roadside.
Messages written on the banners condemned soldiers for serving as the "running dogs" of Tawee Sodsong, secretary-general of the Southern Border Provinces Administrative Centre.
The soldiers went to the scene to investigate. They attempted to remove one of the banners, but a home-made bomb went off.
The explosion injured Sgt Chaisit Yingdamnoon, who was hit by shrapnel in the leg. He was admitted to Mayo Hospital.