Had it not been for classical music, and opera in particular, Walt Whitman’s poetry would probably never have been written.

It sounds a ridiculously bold, even nonsensical claim to make, yet Whitman himself appeared to make it.

“But for opera I would never have written ‘Leaves of Grass,’ ” he once said, referring to the seminal collection that sealed his reputation as a great American poet.

The precise nature of Whitman’s relationship to classical music is examined at the Source Song Festival in Minneapolis this week, in a Walt Whitman Wednesday marking the 200th anniversary of the poet’s birth.

The influence of music on Whitman’s poetry goes deep, said Ed Folsom, co-director of the online Walt Whitman Archive, who will give the keynote lecture at the all-day event.

“There’s a long history of Whitman talking about his own poetry as being very much operatic,” Folsom said. “Critics have seen in his poetry a kind of continual interplay between something you might think of as recitative, and something you might think of as aria.”

Recitative and aria are the basic building blocks of 19th-century opera, and involve characters first singing in a manner close to normal speech, then switching to a more intensified style of expression. That template directly affected the way Whitman shaped his own poetry.

“He moves between a roughly prosaic poetic line to moments of soaring beauty that become the aria,” Folsom said. “Poems like ‘Out of the Cradle Endlessly Rocking’ or ‘When Lilacs Last in the Dooryard Bloom’d’ very easily yield to that kind of formal analysis.”

Whitman was not always an aficionado of opera. When he first heard the operas of Bellini, Donizetti and Verdi, he hated what he called “the trills, the agonized squalls, the lackadaisical drawlings, the sharp ear-piercing shrieks, the gurgling death-rattles.”

But a spell of employment as a music critic in New York City changed all that.

“As a journalist in the 1840s and ’50s, Whitman was continually covering music and writing about it, as all of the work that would become ‘Leaves of Grass’ was percolating,” Folsom said.

Whitman’s attachment had been to what he labeled “heart song,” the simple melodies performed by popular entertainers such as the Hutchinson Family Singers (who founded the central Minnesota city named for them).

But as he attended operas, recitals and orchestral concerts, the “art song” he was hearing gradually took over, and his conversion was total. Opera could make his very soul tremble, he wrote, and often left him crying.

He’s inspired the most music

If classical music had a profound effect on Whitman, the poetry he wrote has returned the compliment in spades by proving a fertile source of inspiration for generations of classical composers.

“Of all the American poets, he’s overwhelmingly the one who more composers set to music than any other,” Folsom said.

He estimated that well over 500 composers have published songs or vocal works to texts by Whitman, and the number keeps rising.

“There’s rarely a week goes by that I don’t hear in my e-mail from a composer writing a piece of music based on a Whitman poem. It’s really a nonstop phenomenon that begins early in the 20th century and just builds and builds.”

So what draws composers inexorably to Whitman’s poetry? Why do so many want to add their music to it?

“His poetry is so immersed in the intricacies and details and the muck of American culture in the 19th century, and he discovered new idioms in language to express it,” Folsom said. “And composers are challenged to find new idioms in music to match him.”

Twin Cities-based composer Libby Larsen is one who has taken up the Whitman challenge. Her 2007 choral piece “Whitman’s America” sets lines from the poem “Song of the Open Road,” and she will share her experience of composing it Wednesday at the Source Festival.

“Normally I want a text to suggest music that’s not in the text — that makes the text different,” Larsen said. What she found in Whitman’s text, however, was poetry filled to the bursting point with the writer’s outsize personality, and his all-encompassing views on love, loss, war, democracy and the destiny of America.

Finding music fit to conjure Whitman’s visionary intensity wasn’t easy, Larsen said. “He is more difficult for me to set than almost any other composer.”

Difficult, yet irresistible — Folsom argues it’s the very boldness and ambition of the subjects Whitman writes about that draw classical composers to him like a magnet.

“Some of the most powerful Whitman-inspired music emerges out of his Civil War poetry,” Folsom said — Paul Hindemith’s Lincoln requiem “When Lilacs Last in the Dooryard Bloom’d,” for instance, or John Adams’ “The Wound-Dresser,” which sets a poem describing Whitman’s experiences as a volunteer nurse in Union Army hospitals.

The homoeroticism of Whitman’s poetry, especially the “Calamus” poems in “Leaves of Grass,” has been a powerful inspiration to American composers. Ned Rorem’s “Three Calamus Poems” and Leonard Bernstein’s “To What You Said” are two outstanding examples of songs inspired by Whitman’s revolutionary attitude toward same-sex love, a strand of interest which Folsom says expanded exponentially in the post-Stonewall generation of composers.

Jazz, opera — even Iggy Pop

A rich selection of Whitman songs will feature in an evening recital Wednesday, where the performers will be drawn from the Source Festival’s MNDuo workshop for singer-pianist partnerships.

Festival co-director Mark Bilyeu marvels at the wide variety of musical responses Whitman’s poems have elicited from classical composers.

“The styles and idioms of Whitman song settings are about as wide-ranging as they come,” he said. “The two most distinctive settings I can think of are George Crumb’s ‘Apparition’ for voice and amplified piano, and jazz pianist Fred Hersch’s album ‘Leaves of Grass,’ both of which should be heard more frequently.”

And the range of musical responses to Whitman’s poetry keeps widening further, Folsom said.

“Just recently we’ve had ‘Crossing,’ a much heralded new opera by Matthew Aucoin based on Whitman’s Civil War experiences, and Iggy Pop performing Whitman’s poems to electronic music. I cannot think of a corner of the realm of music that has not embraced Whitman at some key point.”

What would Whitman have made of all this?

Folsom pointed out that Whitman positively invited those who came after him to take his mold-breaking experimentations further.

“Poets to come! orators, singers, musicians to come!” he wrote in one poem. “Arouse! for you must justify me.”

In the same poem, he said he had written just “one or two indicative words for the future,” leaving it to others to “prove and define” them.

“I’m always struck by how the balance of that opening line leans toward singers and musicians,” Folsom said. “I think Whitman would be surprised and delighted by how they have responded to the lines he left them.”

Terry Blain is a freelance classical music critic for the Star Tribune. Reach him at artsblain@gmail.com.