Willie Nelson says he can't stop writing songs.

The 85-year-old country music legend has been in the music business for decades but isn't even thinking about retiring, because even though he has moments where he doesn't want to ''write another song'', ideas keep coming to him.

He said: ''I think there's some things that can only come out in songs. You can write a beautiful book, but take verses out of it and put a melody to it and you've got another dimension.

''I wrote something the other day that said, 'I don't want to write another song, but tell that to my mind!' I just throw them out there and try to make them rhyme.' I write everywhere, anywhere. I write a lot at home at night.''

The 'Always On My Mind' hitmaker thinks in lyrics first and then adds the music afterwards, as he believes songs are just ''poems with a melody''.

He added: ''Usually it starts as a poem. At some point I'll get up and go get the guitar and see what kind of melody those words suggest.''

Willie - who released his 68th studio album 'My Way' earlier this month - is known for his activism and often politically charged lyrics, and although he doesn't mention current US president Donald Trump by name, he has voiced his frustration and the divisiveness caused by the plans to close the Mexican border.

He said: ''I thought everything that happened there was unforgivable. We have a statue that says: 'Y'all come in.' I don't believe in closing the border. Open them suckers up!

''We need those folks. I used to pick cotton and pull corn and bale hay and I'm lucky to play guitar now, but we have to have the people who want to work, and take care of them.''

The 'On the Road Again' singer averages 150 days on the road in a year, and says he loves travelling the country as it lets him soak in the differences between each US state.

Speaking to The Guardian newspaper, he said: ''I've moved around a lot in 85 years. And I went through a lot of political spaces in our country - four years of this, eight years of that.''