South Australia is set to extend its controversial ban on the growing of genetically-modified crops until 2025 after a bill put forward by the Greens passed the Upper House by a single vote.

The current ban will expire on September 1 in 2019 and was due to be debated later next year, but the Greens surprised the State Parliament with its motion to extend it for another 6 years.

The bill is also expected to pass the Lower House, and Greens leader Mark Parnell said when that happens the State's farmers will be the big winners.

"There are a lot of farmers in South Australia who are nervous about the (GM) technology, and what the marketing evidence shows is that there is a price premium for not growing GM crops," he said.