In a separate attack on Sunday, a bomb exploded in Narathiwat province, wounding four of 12 soldiers who were on a foot patrol, police Lieutenant Kanchit Keenor said.
The incidents followed one of the bloodiest days in the more than three-year-old uprising in which 2,200 people have died since early 2004.
On Thursday, 19 people died in separate attacks, including a roadside bomb which killed 11 paramilitary troops.
The bomb on the soccer field took place in the same district of Yala province as Thursday's roadside bomb.
Hanirut said the bomb may have been triggered by a digital watch, as mobile telephone signals have been cut since Thursday for security reasons.
Meanwhile, Muslim protesters in Pattani occupied the province's central mosque for the fourth day on Sunday, demanding the government withdraw soldiers from the region and lift a curfew and a state of emergency in the region.
Thailand's current, military-backed government has sought negotiations with the fighters and adopted a "hearts and minds" approach to ending the violence, reversing the security-oriented policy of Thaksin Shinawatra, the former prime minister .
But the response from the armed groups has been an intensified campaign of violence.