Emergency services were called to the crash site near Naas Road and Sunshine Road, south of Tharwa, at 3.47pm.

An ACT Policing spokeswoman said the 49-year-old pilot, who was the only person on board the 1974 G164B Grumman AG Cat, lost altitude after take-off and crashed.

The pilot was spraying fertiliser onto a field off Sunshine Road.

"The pilot was unable to gain altitude after take-off, he dumped a load of fertiliser then hit a 'sink hole' of air pressure, still could not gain altitude and crashed," she said.

"The landholders and grounds crew witnessed the plane take off and crash. They drove out to the plane and took the pilot to Calvary Hospital."

The spokeswoman said police believed the man sustained a minor knee injury.

The pilot was treated at Calvary Hospital and discharged late Thursday evening, a hospital spokesman said.

Richardson resident Kym Bradley briefly spoke to the pilot before the crash after stopping to photograph his plane. She was on her way to photograph images of birds in the area.

"I asked the man if I could take some photos of his plane - he was a lovely guy," she said.

"He was getting ready to get in it and take off. He went up and dropped a load of fertiliser. I saw him go up again and gave him a big wave."

Ms Bradley said she heard a noise in the distance a couple of hours later but had no idea what it was.

On the way back to Canberra she came across emergency services crews investigating the plane wreckage.

"I had no idea what happened until heading home," she said.

"I saw it in the paddock and said, 'oh my god'. We saw the fire brigade on the road.

"It's a bit of a shock, really. It's the last thing you expect. I just hope the poor guy is alright."