The historic Camino de Santiago route was travelled often on foot by 238,000 people last year from all parts of Europe and beyond.
They were making their way to the holy city of Santiago de Compostela in the north-west region of Galicia.
Many believe the apostle, St James, is buried in the cathedral there.
But some of the villages along the Way of St James are becoming increasingly upset by the human waste which pilgrims are dropping and leaving.
One village has posted pictogram signs asking people not to defecate in the area. The squatting figure is in a red circle and crossed with a bar.
The anti-poo sign accompanies others such as asking people not to drop litter.
A spokesman for the Federation of Friends of the Camino de Santiago said that it had not received specific complaints from anyone in Lastres village, but did say that farmers from other points along the route had reported that their field was being used as an open-air lavatory by a large number of pilgrims.
"Some olive groves or other fields just seem to be strategically positioned and offer the right kind of cover," the spokesman said.
"When you are out walking and you have that problem, you have to find a solution. What we can't control is how each pilgrim deals with the mess. Obviously, we hope people won't be beasts and leave it all there in full view," she explained.
"Maybe that's what some pilgrims lack: a little bit of care and conscientiousness. But there's no way we can put a toilet every two miles along the route."
I don't mind the [ahem] fertilizer, but I wish they wouldn't leave toilet paper behind. That lasts for up to a year, and I find it unsightly and disgusting.